Matter, Atoms and Periodic Table Lesson Test

Answer: A

Correct answer: Light from a flashlight. Matter must have mass and take up space. Light is a form of energy with no mass to weigh and no volume to fill, so it is the only choice that is not matter.

• A (correct): Light carries energy but has no mass of its own and does not fill a container, so it is not matter.
• B: Water vapor is a gas made of water molecules, which have mass and occupy space.
• C: Dust particles are tiny bits of solid material with mass and volume, so they are matter.
• D: Air has mass and fills the tire, so it is matter even though it is invisible.

Answer: A

Correct answer: Condensation. The water starts as vapor (gas) in the humid air and ends as liquid droplets on the glass. Gas to liquid is condensation.

• A (correct): Water vapor in the air cools on the cold glass and turns from gas to liquid, which is condensation.
• B: Melting is solid to liquid; the droplets come from the air, not from ice passing through the glass.
• C: Deposition goes straight from gas to solid, as with frost, but these droplets are liquid.
• D: Evaporation is liquid turning to gas at a surface, the reverse of what happens on the glass.

Answer: B

Correct answer: Electron. An atom has three main particles. Protons are positive and neutrons are neutral; together they form the nucleus. Electrons are negative and occupy the space around the nucleus. In a neutral atom the number of electrons equals the number of protons, so the charges cancel. Electrons are also far lighter than protons, about 1/1836 of the mass. A nucleus is tempting, but it is positively charged because it holds the protons.

• A: A nucleus contains protons and often neutrons; it is not the negatively charged particle.
• B (correct): Electrons carry one negative elementary charge.
• C: Neutrons have no net electrical charge.
• D: Protons carry positive charge.

Answer: A

Correct answer: Its number of protons. The number of protons, called the atomic number, defines which element an atom is. Every carbon atom has 6 protons, and every oxygen atom has 8; change the proton count and you have a different element. Neutron number can vary, producing isotopes of the same element, and electron number can vary, producing ions. Neither changes the element. That is why the periodic table is arranged in order of atomic number.

• A (correct): Proton count is atomic number and uniquely identifies the element.
• B: Different structures and conditions affect density.
• C: Shell occupancy does not uniquely set element identity.
• D: Changing neutrons changes isotope, not element.

Answer: D

Correct answer: 8. The atomic number, written Z, is defined as the number of protons in the nucleus, so an atom with eight protons has atomic number 8 and is oxygen. Whether the atom is neutral affects only the electron count, which would also be eight here. Doubling the proton count is tempting because oxygen-16 has a mass number of 16, but the mass number counts protons plus neutrons, not just protons.

• A: Four is half the proton number, not Z.
• B: This incorrectly doubles the protons.
• C: This value has no basis in the given nucleus.
• D (correct): Atomic number always equals proton count, here eight.

Answer: D

Correct answer: Plasma. When a gas is given enough energy from heat or strong electric fields, electrons are torn off its atoms, leaving free electrons and positive ions. That electrically conducting state is plasma, the fourth state of matter.

• A: A liquid has close particles that move past one another; it is not made of free electrons and ions.
• B: A solid forms when particles are locked in place, which requires removing energy, not adding a huge amount.
• C: Vapor is just the gas form of a substance; its atoms still hold all their electrons.
• D (correct): Plasma is an energized, glowing mix of free electrons and positive ions, found in lightning and stars.

Answer: A

Correct answer: An iron gate slowly rusting. A chemical change produces a new substance. Rusting turns iron metal into iron oxide, a different material, while melting, dissolving and splitting leave the substance the same.

• A (correct): Rust is a new substance formed when iron reacts with oxygen and water, so this is a chemical change.
• B: Melting is a change of state; the liquid is still water, so the change is physical.
• C: Splitting changes only the size and shape of the wood; it is still wood, so the change is physical.
• D: Dissolving spreads sugar molecules through the liquid without making a new substance, so it is physical.

Answer: D

Correct answer: Carbon dioxide, CO₂. A compound is two or more different elements chemically bonded in a fixed ratio. CO₂ has carbon and oxygen bonded together; O₂ and gold contain one element only, and brass is a mixture.

• A: Brass is an alloy, a homogeneous mixture whose copper and zinc are not bonded in a fixed ratio.
• B: Pure gold contains only gold atoms, so it is an element.
• C: O₂ is a molecule, but both atoms are oxygen, so it is an element rather than a compound.
• D (correct): Carbon dioxide is carbon and oxygen chemically bonded in a fixed 1 to 2 ratio, which makes it a compound.

Answer: B

Correct answer: Noble gases. Helium, neon and argon sit in group 18. This family, the noble gases, has full outer electron shells, so its members are the least reactive elements.

• A: Alkali metals are the soft, reactive group 1 metals such as sodium and potassium.
• B (correct): Helium, neon and argon are in group 18, the noble gases, which have full outer shells and rarely react.
• C: Alkaline earth metals are the group 2 metals such as magnesium and calcium.
• D: Halogens are the group 17 nonmetals such as fluorine and chlorine, which are highly reactive.

Answer: D

Correct answer: Ag. Most element symbols come from English names, but several come from Latin, so they do not match the English word. Silver is Ag from argentum, gold is Au from aurum, iron is Fe from ferrum, lead is Pb from plumbum, and sodium is Na from natrium. Symbols always start with a capital letter, followed by a lowercase letter if needed. The trap is Au, since gold and silver are often paired; think Au for aurum, gold.

• A: Au comes from the Latin aurum and stands for gold, not silver.
• B: Sv is not the symbol of any element; silver uses a symbol based on its Latin name.
• C: Si is the symbol for silicon, a metalloid, even though it starts with the same letters as silver.
• D (correct): Silver's symbol is Ag, from its Latin name argentum.

Answer: B

Correct answer: 8. The number in an isotope name, such as carbon-14, is its mass number, which counts protons plus neutrons in the nucleus. To find neutrons, subtract the atomic number from the mass number. All carbon atoms have six protons, so carbon-14 has eight neutrons, while ordinary carbon-12 has six. Isotopes of an element differ only in neutron count. Do not mistake the mass number itself for the neutron count.

• A: Twenty incorrectly adds the two given numbers.
• B (correct): Mass number 14 minus proton number 6 equals eight neutrons.
• C: Fourteen is the total nucleon count.
• D: Six is the proton count, not the neutron count.

Answer: A

Correct answer: 2.7 g/cm³. First find the volume:

• A (correct): The volume is 4 × 3 × 2.5 = 30 cm³, and 81 g divided by 30 cm³ gives 2.7 g/cm³.
• B: This divides 81 by 9.5, the sum of the dimensions, instead of by their product.
• C: This divides 81 by 3, using only one dimension instead of the full volume.
• D: This comes from dividing volume by mass (30 ÷ 81) instead of mass by volume.

Answer: B

Correct answer: 12. Protons carry a +1 charge and electrons a −1 charge of exactly equal size, so an atom is neutral only when the two counts match. The proton count, or atomic number, is fixed for an element (12 means magnesium), but electrons can be gained or lost to form ions. A common slip is to bring neutrons into it; neutrons are uncharged, so they never affect the electron count of a neutral atom.

• A: Twenty-four would make a strongly negative ion.
• B (correct): Neutrality requires as many electrons as protons.
• C: Ten would leave a +2 charge.
• D: Six would leave a +6 charge if protons were twelve.

Answer: D

Correct answer: Different neutron counts. An element's identity is fixed by its proton count, the atomic number, so every carbon atom has 6 protons. Neutrons add mass but no charge, so isotopes differ in mass number while behaving almost identically in chemical reactions. Carbon-12 and carbon-14 are a familiar pair: same symbol, same place on the periodic table, but 6 versus 8 neutrons. Changing the proton count would create a different element, not an isotope.

• A: Different protons would mean different elements.
• B: Each element has one position on the table regardless of isotope.
• C: The element symbol stays the same for isotopes.
• D (correct): Isotopes share proton count but have different neutron counts.

Answer: B

Correct answer: M₂X. A group 1 atom has 1 valence electron and loses it to form M⁺. A group 16 atom has 6 valence electrons and gains 2 to form X²⁻. Two M⁺ ions are needed to cancel one X²⁻, giving M₂X; sodium oxide, Na₂O, is a real example.

• A: MX₂ fits a +2 metal with a −1 nonmetal, which reverses the charges these groups actually form.
• B (correct): Two M⁺ ions (+2 total) balance one X²⁻ ion (−2), so the formula is M₂X.
• C: MX would pair a +1 ion with a −1 ion, but a group 16 atom gains two electrons and forms a −2 ion.
• D: M₂X₃ would need ions of +3 and −2; a group 1 metal forms only a +1 ion.

Answer: C

Correct answer: 17. The subscript 3 multiplies everything inside the parentheses:

Aluminum has its own subscript of 2. Total =

atoms.

• A: This multiplies every atom, including aluminum, by 3, but the 3 applies only inside the parentheses.
• B: This is the number of oxygen atoms alone, not the total of all atoms.
• C (correct): Al: 2; S: 1 × 3 = 3; O: 4 × 3 = 12. The total is 2 + 3 + 12 = 17 atoms.
• D: This ignores the subscript 3 outside the parentheses and counts only 2 + 1 + 4.

Answer: C

Correct answer: Lower air pressure lets water boil at a lower temperature. A liquid boils when its vapor pressure matches the air pressure above it. At high altitude the air pressure is lower, so water boils at a lower temperature, near 95 °C in some mountain towns, and food in that cooler water cooks more slowly.

• A: The tiny change in gravity at altitude has no meaningful effect on cooking time.
• B: Water's density barely changes with altitude, and density does not set its boiling point.
• C (correct): With less air pressure above it, water boils below 100 °C, so the boiling water is cooler and food cooks more slowly.
• D: The stove flame is not the issue; the limit is the temperature at which the water itself boils.

Answer: D

Correct answer: A 90 g object with a volume of 120 cm³. An object floats if its density is less than the fluid's. Dividing mass by volume gives 1.2, 0.75, 1.2 and 1.5 g/cm³. Only the 0.75 g/cm³ object is less dense than water, so only it floats.

• A: Its density is 300 ÷ 250 = 1.2 g/cm³, so it sinks even though its volume is large.
• B: Its density is 60 ÷ 50 = 1.2 g/cm³, greater than water, so it sinks.
• C: Its density is 45 ÷ 30 = 1.5 g/cm³, so it sinks even though it is the lightest object.
• D (correct): Its density is 90 ÷ 120 = 0.75 g/cm³, less than water, so it floats.

Answer: D

Correct answer: Paper chromatography. Ink is a mixture of dyes dissolved together. Chromatography separates them because each dye moves a different distance along the paper, a difference in physical properties that needs no chemical reaction.

• A: Decanting pours a liquid off a settled solid; dissolved dyes do not settle out.
• B: A magnet only pulls out magnetic materials such as iron; ink dyes are not magnetic.
• C: Filter paper holds back solid particles by size, but dissolved dyes pass straight through it.
• D (correct): Each dye travels a different distance along the paper, so chromatography spreads the ink into its separate colors.

Answer: C

Correct answer: They can reflect isotope masses and natural abundances. Most elements occur in nature as a mixture of isotopes, and the listed atomic weight is an average of their masses weighted by how common each one is. Chlorine, for example, is roughly three-quarters chlorine-35 and one-quarter chlorine-37, giving about 35.45. Even a single isotope's mass is not an exact whole number. Protons and atomic numbers are always whole numbers, so they cannot explain the decimals.

• A: Atomic number defines an element and does not vary between isotopes.
• B: A proton count is always a whole number.
• C (correct): Naturally occurring isotopes and precise mass values produce weighted atomic weights.
• D: Protons determine identity, so the proton count cannot vary within a given element.

Answer: C

Correct answer: Salt dissolved in water. A homogeneous mixture, or solution, has the same makeup throughout and contains substances that are not bonded, in any proportion. Salt water fits; pure water and oxygen are pure substances, and sand in water has visible separate parts.

• A: Oxygen gas contains only oxygen atoms, so it is an element, not a mixture.
• B: Pure water is a compound of hydrogen and oxygen in a fixed ratio, a pure substance rather than a mixture.
• C (correct): Dissolved salt spreads evenly through the water and the amount can vary, so salt water is a homogeneous mixture.
• D: The sand grains are visible and settle out, so this is a heterogeneous mixture.

Answer: D

Correct answer: Chlorine. Atomic radius decreases left to right across a period and increases down a group. Sodium, magnesium and chlorine are all in period 3, and chlorine is farthest right, so it is the smallest of the three. Potassium is a row lower and larger still.

• A: Sodium is at the left of period 3, where atoms are largest in that row.
• B: Potassium is in period 4 and at the far left, so it has an extra shell and is the largest of these.
• C: Magnesium is in period 3 but near the left side, so it is larger than chlorine.
• D (correct): Chlorine is far right in period 3 and above potassium's row, so its extra protons pull electrons in tightly, making it smallest.

Answer: B

Correct answer: Fluorine. Seven valence electrons place an element in group 17, the halogens. The halogen in the second row (period 2) is fluorine; chlorine is the one below it in period 3.

• A: Neon is in period 2 but is a noble gas with a full set of 8 valence electrons.
• B (correct): Fluorine is in period 2 and group 17, so it has 7 valence electrons.
• C: Chlorine also has 7 valence electrons, but it is in period 3, not period 2.
• D: Nitrogen is in period 2 but is in group 15 with 5 valence electrons.

Answer: C

Correct answer: 88 g. By the law of conservation of mass, atoms are only rearranged in a reaction, so the products have the same total mass as the reactants: 56 g + 32 g = 88 g of iron sulfide.

• A: This counts only the iron and ignores the sulfur atoms that also end up in the product.
• B: This multiplies the masses; reactant masses add, they do not multiply.
• C (correct): Mass is conserved, so the product equals 56 g + 32 g = 88 g.
• D: This subtracts the masses, but atoms are not lost in a reaction, so the masses add.

Answer: A

Correct answer: Mass 20 kg; weight about 32 N. Mass is the amount of matter and stays 20 kg. Weight is

• A (correct): Mass stays 20 kg anywhere, and the Moon weight is 20 × 1.6 = 32 N.
• B: This swaps the two quantities; the mass stays 20 kg and the weight is 20 × 1.6.
• C: Mass does not change with location; only the pull of gravity, and so the weight, changes.
• D: 196 N is the crate's weight on Earth (20 × 9.8); its weight drops on the Moon.

Answer: A

Correct answer: Potassium bromide, KBr. Metal plus nonmetal suggests ionic bonding; nonmetal plus nonmetal suggests covalent. Only KBr pairs a metal (potassium, group 1) with a nonmetal (bromine, group 17).

• A (correct): Potassium is a metal and bromine a nonmetal, so potassium transfers an electron to bromine, forming an ionic bond.
• B: Carbon and oxygen are both nonmetals, so carbon dioxide is covalent.
• C: Nitrogen and hydrogen are both nonmetals, so ammonia is a covalent molecule.
• D: Carbon and hydrogen are both nonmetals, so they share electrons in covalent bonds.

Answer: C

Correct answer: Baking soda is stirred into vinegar and fizzes. Bubbles are only a clue. Ask where the gas came from: in baking soda and vinegar, a new gas is made when the two substances react, which is a chemical change. In the other cases the gas was already present or is the same substance in a different state.

• A: The carbon dioxide was already dissolved in the soda and is simply escaping, a physical change.
• B: Shaking traps air that was already present; no new substance forms, so the change is physical.
• C (correct): The gas is a new substance produced when the two materials react, so this is a chemical change.
• D: The bubbles are water vapor, still water in a new state, so boiling is physical.

Answer: B

Correct answer: Co. Element symbols are one or two letters: the first is always capitalized and any second letter is always lowercase. That rule separates Co, the element cobalt, from CO, where two capital letters mark two elements, carbon and oxygen, joined in carbon monoxide. When reading symbols, treat each capital letter as the start of a new element. Many symbols begin with C, such as Cl for chlorine and Ca for calcium, so the second letter matters.

• A: C alone denotes carbon.
• B (correct): Co with lowercase o is the case-sensitive symbol for cobalt.
• C: Cl denotes chlorine.
• D: CO with capital C and O denotes a carbon-monoxide formula.

Answer: C

Correct answer: Freezing. Melting, vaporization (including evaporation) and sublimation absorb energy because particles are being pulled apart. Freezing, condensation and deposition release energy as particles pull together. Of the choices, only freezing releases energy.

• A: Evaporation is a form of vaporization, which absorbs energy so particles can escape the liquid.
• B: Sublimation takes a solid straight to a gas and absorbs energy to do it.
• C (correct): As liquid particles lock into a solid, they release energy, so freezing gives off energy.
• D: Melting absorbs energy to break particles out of their fixed positions.

Answer: A

Correct answer: 16. The mass number counts every particle in the nucleus, protons plus neutrons, so subtracting the neutrons leaves the protons. The atomic number is defined as the proton count, which makes it 31 minus 15, or 16, the element sulfur. A handy check is

The usual traps are reporting the neutron count itself or adding the two given numbers instead of subtracting.

• A (correct): Protons = 31 − 15 = 16, so Z = 16.
• B: Thirty-one counts all nucleons.
• C: Fifteen is the neutron count, not proton count.
• D: Forty-six incorrectly adds the given values.

Answer: B

Correct answer: 12 protons, 13 neutrons, 10 electrons. Treat it as three separate questions. The atomic number fixes protons at 12 for every magnesium particle. The mass number, 25, is protons plus neutrons, so the neutrons number 25 minus 12, or 13. The charge describes only electrons: a positive charge means electrons were lost, so 2+ leaves 12 minus 2, or 10. Mixing up which number goes with which particle, or thinking positive means extra electrons, produces the usual errors.

• A: Mass number is not the proton count.
• B (correct): Z = 12, N = 25 − 12 = 13, and a 2+ charge means two electrons were lost, leaving ten.
• C: A 2+ ion has fewer rather than more electrons than protons.
• D: Mass number was misread as neutrons and the ion charge reversed.

Answer: D

Correct answer: 11.0 u. A weighted average multiplies each value by its fraction and adds the results, so the more common isotope pulls the average toward itself. Here 80% of 10 is 8 and 20% of 15 is 3, giving 11. A quick sanity check: the answer must lie between 10 and 15 and closer to 10, since most atoms have mass 10. A simple average of 12.5 would be right only if the two isotopes were equally common.

• A: 12.5 u is an equal-abundance average, not the weighted value.
• B: An average of ten would ignore the heavier 20% component.
• C: The masses must be averaged, not added.
• D (correct): 0.80 × 10 + 0.20 × 15 = 8 + 3 = 11 u.

Answer: D

Correct answer: 2.8 g/cm³; it sinks. The rock's volume is the water it displaces:

= 12.0 cm³.

Since 2.8 g/cm³ is greater than water's 1.0 g/cm³, the rock sinks.

• A: This divides by the starting reading of 25.0 mL rather than the change in water level.
• B: This divides by the final reading of 37.0 mL instead of the 12.0 mL of water actually displaced.
• C: This divides the volume by the mass (12 ÷ 33.6), flipping the density formula.
• D (correct): The rock displaces 12.0 mL, so its density is 33.6 ÷ 12.0 = 2.8 g/cm³, which is more than water's 1.0, so it sinks.

Answer: C

Correct answer: P is neutral neon and Q is a +1 sodium ion. The proton count alone names the element: 10 protons is always neon and 11 is always sodium. Charge then equals protons minus electrons, so P has

and Q has 11 − 10 = +1. Particles with the same electron count, like Ne and Na⁺, are called isoelectronic. Matching electrons can make them seem alike, but different proton counts mean different elements, so they cannot be isotopes.

• A: Neutron counts were not specified and need not match.
• B: The 11-proton species with only ten electrons is positively charged.
• C (correct): Z = 10 is neon and neutral with ten electrons; Z = 11 is sodium and +1 with ten electrons.
• D: Different proton counts mean different elements, not isotopes.

Answer: C

Correct answer: 30. In one unit of (NH₄)₂SO₄, the subscript 2 multiplies everything in the parentheses: N = 2 and H = 8. Add S = 1 and O = 4 for 15 atoms. The coefficient 2 means two units, so

atoms.

• A: This applies the subscript 2 to nitrogen only and not to the hydrogens inside the parentheses.
• B: This ignores the subscript 2 after the parentheses, counting 10 atoms per unit and then doubling.
• C (correct): One unit has N 2, H 8, S 1 and O 4, which is 15 atoms; the coefficient 2 doubles this to 30.
• D: This is correct for one unit but ignores the coefficient 2 in front, which counts two whole units.

Answer: A

Correct answer: They are oxygen isotopes with mass numbers 15, 16 and 17. Atomic number 8 identifies oxygen, and because all three atoms share that proton count, they are the same element. Add protons and neutrons to get each mass number: 8 + 7, 8 + 8 and 8 + 9. Neutron differences change only mass, not identity or charge; charge comes from an imbalance between protons and electrons. Oxygen-16 and oxygen-17 occur naturally, while oxygen-15 is a short-lived radioactive isotope.

• A (correct): Oxygen has Z = 8, giving A = 15, 16 and 17 for 7, 8 and 9 neutrons.
• B: Atomic number 8 corresponds to oxygen, not nitrogen; mass numbers include protons.
• C: Proton number stays eight, so element identity does not change.
• D: Ion status depends on electron imbalance, not neutron number.

Answer: C

Correct answer: Al₂O₃. Aluminum has 3 valence electrons and loses them to form Al³⁺. Oxygen has 6 and gains 2 to form O²⁻. The smallest total charge both can reach is 6: two Al³⁺ give +6 and three O²⁻ give −6, so the formula is Al₂O₃.

• A: One Al³⁺ (+3) with three O²⁻ (−6) leaves a net charge of −3, so this is not neutral.
• B: This puts the ion charges in as subscripts the wrong way round, giving +9 and −4, which do not balance.
• C (correct): Two Al³⁺ ions give +6 and three O²⁻ ions give −6, so Al₂O₃ is neutral.
• D: AlO would require equal and opposite charges, but aluminum forms +3 and oxygen −2, which do not cancel one to one.

Answer: A

Correct answer: 8 protons and 10 neutrons. The nucleus holds only protons and neutrons, and gaining or losing electrons never changes it. Oxygen's atomic number fixes 8 protons, and mass number 18 leaves 18 minus 8, or 10, neutrons. The 2− charge tells you only that the ion has two more electrons than protons, 10 versus 8. The tempting mistake is to read the electron count of 10 as the proton count, which would turn the nucleus into neon's.

• A (correct): Oxygen has eight protons; neutron count is 18 − 8 = ten. The extra electrons produce the negative charge.
• B: Ten is electron count, not oxygen’s proton count.
• C: Eight protons plus eight neutrons would give mass number 16, not 18.
• D: This treats electrons as protons and produces the wrong nucleus.

Answer: B

Correct answer: Neon is a group-18 noble gas with a stable outer-electron arrangement. Reactivity is driven by how strongly an atom tends to gain, lose or share outer electrons. Neon's outer shell is already full, so it has little to gain by reacting. Chlorine, in group 17, is one electron short of a full shell and grabs electrons readily, making it one of the most reactive nonmetals. Avoid absolute claims, though: heavier noble gases such as xenon can form compounds.

• A: Neon’s Z = 10 is smaller than that of many elements.
• B (correct): Neon’s filled valence shell underlies its low reactivity in common conditions.
• C: Noble gases are relatively unreactive; the absolute claim is false for some heavier members.
• D: Chlorine is a group-17 halogen, not a transition metal.

Answer: B

Correct answer: Homogeneous mixture. Leaving a solid behind after boiling shows that something was dissolved, so the liquid is not a pure substance; the varying amounts confirm the proportions are not fixed. Because it looks the same throughout, it is a homogeneous mixture (a solution), like salt water.

• A: An element contains one kind of atom and could not split into a liquid that boils away and a solid left behind.
• B (correct): The liquid is uniform yet holds a dissolved solid in amounts that vary, which describes a solution, a homogeneous mixture.
• C: A heterogeneous mixture has visible parts or regions that differ, but this liquid looks the same throughout.
• D: A compound has a fixed composition, so every sample would leave the same proportion of solid; the varying amounts rule this out.

Answer: B

Correct answer: Sublimation. Name the start and end states. The snow starts as solid ice and disappears into the air as vapor with no liquid stage, and solid directly to gas is sublimation.

• A: Melting turns a solid into a liquid, but no liquid water forms here.
• B (correct): The ice crystals go straight from solid to water vapor, skipping the liquid stage.
• C: Evaporation is liquid turning to gas at the surface, and there is no liquid in this case.
• D: Deposition is the reverse change, gas going directly to solid, as when frost forms.

Answer: D

Correct answer: 36. Use electrons = protons − charge. Selenium has 34 protons and the charge is −2, so electrons =

The mass number 80 affects only the neutron count.

• A: This subtracts 2 from the proton count, which is what happens when an atom loses electrons to become positive.
• B: This is the electron count of a neutral selenium atom, ignoring the 2− charge.
• C: This is the number of neutrons, 80 − 34, not the number of electrons.
• D (correct): A 2− charge means two electrons were gained: 34 + 2 = 36.

Answer: D

Correct answer: 75%. Let x be the fraction of Rd-52. Then

and x = 0.75.

So 75 percent of the atoms are Rd-52. Check: 51.5 lies closer to 52, so the heavier isotope must be the more common one.

• A: This is the share of Rd-50, the lighter isotope, not Rd-52.
• B: Equal amounts would give an average of exactly 51 u, halfway between 50 and 52.
• C: With 60 percent Rd-52 the average would be 0.4 × 50 + 0.6 × 52 = 51.2 u, not 51.5 u.
• D (correct): Then 0.25 × 50 + 0.75 × 52 = 12.5 + 39 = 51.5 u, which matches the listed average.

Answer: A

Correct answer: 395 g. Rearrange

to

Then m = 0.79 g/mL × 500 mL = 395 g, which makes sense because the alcohol is a little less dense than water.

• A (correct): Mass equals density times volume: 0.79 g/mL × 500 mL = 395 g.
• B: This divides the volume by the density instead of multiplying, so the result does not even have units of mass.
• C: This assumes the liquid has the density of water, 1.0 g/mL, instead of 0.79 g/mL.
• D: This misplaces the decimal point; 0.79 × 500 is 395, not 39.5.

Answer: C

Correct answer: A bromine-80 ion with a −1 charge, in the halogen family. Atomic number 35 is bromine. Mass number =

and charge = protons − electrons =

Bromine is a group 17 halogen, which fits gaining one electron to form a −1 ion.

• A: The charge sign is reversed; with more electrons than protons, the particle is negative.
• B: Krypton has 36 protons; the element is set by the 35 protons, not by the 36 electrons.
• C (correct): 35 protons means bromine, 35 + 45 = 80 is the mass number, and 35 − 36 = −1 is the charge.
• D: This uses the mass number 80 as if it were the atomic number; the atomic number is the proton count, 35.

Answer: B

Correct answer: Calcium. The alkaline earth metals make up group 2, which includes beryllium, magnesium, calcium, strontium, barium and radium. Calcium is the only group 2 element listed.

• A: Sodium is in group 1, so it is an alkali metal, not an alkaline earth metal.
• B (correct): Calcium is in group 2, the alkaline earth metals, and forms a +2 ion.
• C: Chlorine is a group 17 halogen, a nonmetal.
• D: Iron is in the middle of the table, among the transition metals.

Answer: C

Correct answer: Distillation. Distillation boils a liquid and condenses the vapor in a separate place, separating substances with different boiling points. The water leaves as steam and is recondensed, while the dissolved salt stays behind.

• A: A filter holds back undissolved solids by size, but dissolved salt passes right through it.
• B: Decanting pours a liquid off a settled solid, but dissolved salt never settles out.
• C (correct): Boiling the water and condensing the vapor elsewhere separates it from the salt, which has a far higher boiling point.
• D: Chromatography separates dyes by how far they travel along paper, not salt from water.

Answer: B

Correct answer: Rubidium. Ionization energy decreases going down a group and increases going left to right across a period. The element lowest and farthest left, rubidium, loses an electron most easily.

• A: Lithium is in group 1, but it sits at the top, where outer electrons are close to the nucleus and harder to remove.
• B (correct): Rubidium is low in group 1; its single outer electron is far from the nucleus, so its ionization energy is lowest.
• C: Fluorine is at the upper right, where atoms hold their electrons tightly and ionization energy is high.
• D: Neon is a noble gas with a full outer shell, so removing an electron takes a great deal of energy.

Answer: A

Correct answer: 32. The number after the element name is the mass number, which counts protons plus neutrons. Neutrons =

• A (correct): Neutrons equal mass number minus atomic number: 60 − 28 = 32.
• B: This is the number of protons, given by the atomic number.
• C: This is the mass number, the total of protons and neutrons together.
• D: This adds the two numbers instead of subtracting the protons from the mass number.

Answer: A

Correct answer: It floats, because its average density is 0.8 g/cm³. Convert mass to grams: 2.4 kg = 2,400 g. Average density = 2,400 g ÷ 3,000 cm³ = 0.8 g/cm³.

Since 0.8 is less than 1.0, the canister floats, with about 80 percent of its volume below the waterline.

• A (correct): 2.4 kg is 2,400 g, and 2,400 g ÷ 3,000 cm³ = 0.8 g/cm³, which is less than water's 1.0 g/cm³.
• B: This divides volume by mass, 3,000 ÷ 2,400, which flips the density formula.
• C: Steel alone is denser, but floating depends on the average density of the whole sealed canister, including the air inside.
• D: This uses 240 g instead of 2,400 g, a slip in converting kilograms to grams.

Answer: C

Correct answer: Oxygen gas, O₂. A molecule is two or more atoms bonded together; a compound needs two or more different elements. O₂ has two bonded oxygen atoms but only one element, so it is a molecule that is not a compound.

• A: Water is a molecule, but it contains two different elements, so it is also a compound.
• B: Sodium chloride is an ionic compound that forms a lattice of ions rather than separate molecules.
• C (correct): O₂ is two bonded atoms, making it a molecule, but it contains only one element, so it is not a compound.
• D: Neon exists as single, unbonded atoms, so it is neither a molecule nor a compound.