Answers · Isotopes

Two waters — answer key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. IQ1 · Practice

Model answer. Isotopes have the same number of protons, so they are the same element, but different numbers of neutrons. Only the pair 6 protons + 6 neutrons and 6 protons + 8 neutrons fits: the same protons (6), different neutrons (6 and 8). The pair 6 protons + 6 neutrons and 7 protons + 6 neutrons are different elements. The pair 6 protons + 6 neutrons twice is the same atom twice. The pair 6 protons + 8 neutrons and 7 protons + 7 neutrons have the same mass number, 14, but different protons, so they are different elements.

A full answer includes

The pair with 6 protons each but 6 and 8 neutrons.
I checked that isotopes have the same number of protons and a different number of neutrons.

Also accept

Atom 1: 6 protons + 6 neutrons; atom 2: 6 protons + 8 neutrons.

Watch for

“Atoms with the same mass number are isotopes.” — The mass number counts protons and neutrons together. With 6 and 7 protons they are different elements, even though both mass numbers are 14.
“Atoms with the same number of neutrons are isotopes.” — The neutrons are what differ between isotopes. Atoms with different numbers of protons are different elements.

2. IQ2 · Practice

Model answer. The proton number is 35, so there are 35 protons. The mass number 81 counts protons and neutrons, so there are 81 − 35 = 46 neutrons. A neutral atom has 35 electrons, the same as its protons.

Checked automatically

Protons: 35 (exact value)
Neutrons: 46 (exact value)
- Common wrong answer 81 : 81 is the mass number. Take away the 35 protons.
- Common wrong answer 116 : You added the numbers. The mass number already includes the protons: 81 − 35.
Electrons: 35 (exact value)
- Common wrong answer 46 : That is the number of neutrons. An atom has as many electrons as protons.

A full answer includes

Protons: 35, the proton number.
Neutrons: 81 − 35 = 46.
Electrons: 35, because an atom has as many electrons as protons.

Also accept

35 protons, 46 neutrons, 35 electrons.

Watch for

“Bromine-81 has 81 neutrons.” — 81 is the mass number, protons and neutrons together. Take away the 35 protons: 46 neutrons.

3. IQ3 · Practice

Model answer. The proton number is 16, so there are 16 protons, and 34 − 16 = 18 neutrons. The charge is 2−, so the ion has two more electrons than protons: 16 + 2 = 18 electrons. Check: 16 − 18 = −2.

Checked automatically

Protons: 16 (exact value)
Neutrons: 18 (exact value)
- Common wrong answer 34 : 34 is the mass number. Subtract the 16 protons.
Electrons: 18 (exact value)
- Common wrong answer 14 : A 2− charge means two extra electrons, not two fewer. Add them to the protons.
- Common wrong answer 16 : That would be a neutral atom. The 2− charge means two more electrons.

A full answer includes

Protons: 16.
Neutrons: 34 − 16 = 18.
Electrons: 18, because the charge is 16 − 18 = −2.

Also accept

16 protons, 18 neutrons, 18 electrons.

Watch for

“The ion has 14 electrons, because the charge is 2−.” — A negative charge means extra electrons, which carry the negative charge. 16 protons and 18 electrons give 16 − 18 = −2.
“The ion has 16 electrons, the same as its protons.” — That is true for an atom. An ion has a charge, so its electrons and protons are different.

4. IQ4 · Practice

Options as numbered on the pupil sheet: 1 — ²⁴₁₁Na; 2 — ²⁴₁₂Mg²⁺; 3 — ²⁶₁₂Mg; 4 — ²⁴₁₂Mg; 5 — ²⁶₁₂Mg²⁺; 6 — ²²₁₂Mg.

Model answer. For each particle the proton number gives the element (12 is magnesium, 11 sodium), protons + neutrons give the mass number, and protons − electrons give the charge. 12 + 12 = 24 and no charge: ²⁴₁₂Mg. 12 + 14 = 26: ²⁶₁₂Mg, another isotope of magnesium. 12 + 12 = 24 with 12 − 10 = +2: ²⁴₁₂Mg²⁺. 11 + 13 = 24 with 11 protons: ²⁴₁₁Na, a different element with the same mass number.

A full answer includes

12 p, 12 n, 12 e: ²⁴₁₂Mg.
12 p, 14 n, 12 e: ²⁶₁₂Mg.
12 p, 12 n, 10 e: ²⁴₁₂Mg²⁺.
11 p, 13 n, 11 e: ²⁴₁₁Na.

Also accept

²⁴₁₂Mg; ²⁶₁₂Mg; ²⁴₁₂Mg²⁺; ²⁴₁₁Na.

Watch for

“The particle with 11 protons and mass number 24 is magnesium-24.” — Only the proton number decides the element. 11 protons is sodium, whatever the mass number.
“The ion with 10 electrons has mass number 22.” — Electrons are not counted in the mass number. It is still 12 + 12 = 24; the missing electrons give the charge.

5. IQ5 · Practice

Model answer. (a) The number after the name is the mass number. Each has 1 proton and 1 electron, so the neutrons are 1 − 1 = 0, 2 − 1 = 1 and 3 − 1 = 2. (b) Hydrogen-2 is ²₁H. (c) The element is decided by the number of protons. All three have 1 proton, so all are hydrogen. They differ only in their neutrons, which makes them isotopes.

A full answer includes

(a) Hydrogen-1: 1 p, 0 n, 1 e; hydrogen-2: 1 p, 1 n, 1 e; hydrogen-3: 1 p, 2 n, 1 e.
(b) ²₁H: mass number 2 at the top left, proton number 1 at the bottom left (or H, mass number 2, proton number 1).
(c) I explained that all three have 1 proton, and the number of protons decides the element; only the neutrons differ, so they are isotopes.

Also accept

(a) 1, 0, 1; 1, 1, 1; 1, 2, 1. (b) ²₁H. (c) All have one proton; they differ only in neutrons.

Watch for

“Hydrogen-2 has 2 protons.” — Then it would be helium. The 2 is the mass number: 1 proton and 1 neutron.
“Hydrogen-3 has 3 electrons.” — Electrons match protons in an atom, and every hydrogen atom has 1 proton. The extra mass is in neutrons.

6. IQ6 · Practice

Model answer. (a) Both isotopes have 17 protons, so their atoms have 17 electrons: 2,8,7. (b) In a reaction only the electrons take part; here each chlorine atom gains one electron to form a chloride ion. Both isotopes have the same electrons in the same shells, so they react in the same way. The extra neutrons in chlorine-37 add mass but are not involved.

A full answer includes

(a) Both 2,8,7.
(b) I explained that both isotopes have the same number of electrons (17) in the same arrangement, and reactions involve only the electrons, so both react in the same way.

Also accept

(a) 2,8,7 for both. (b) The same number and arrangement of electrons; reactions involve electrons, not neutrons.

Watch for

“Chlorine-37 reacts differently because it has two more neutrons.” — Neutrons stay in the nucleus and have no charge, so they take no part in reactions. The electrons decide how an atom reacts.
“Chlorine-37 is 2,8,9, because it has a bigger mass number.” — The mass number counts protons and neutrons. Both isotopes have 17 protons, so both have 17 electrons: 2,8,7.

7. IQ7 · Practice

Model answer. The isotopes in the two waters have the same electrons, so chemical tests give the same result: both turn anhydrous copper(II) sulfate blue. Their molecules have the same number of atoms, two hydrogen and one oxygen, and both liquids are colourless. The extra neutrons add mass, so properties that depend on mass differ: 10 cm³ of heavy water has a greater mass (about 11.1 g against 10.0 g), and it boils at 101.4 °C instead of 100 °C.

A full answer includes

The mass of 10 cm³ of each liquid.
The boiling point of each liquid.
I explained that both liquids turn the powder blue, because their hydrogen atoms have the same electrons, so the copper(II) sulfate test can’t tell them apart.

Also accept

The mass of 10 cm³; the boiling point.

Watch for

“Anhydrous copper(II) sulfate stays white with heavy water.” — Heavy water reacts as water does, because its hydrogen atoms have the same electrons. Both turn the powder blue.
“Heavy-water molecules have more hydrogen atoms.” — Both molecules have two hydrogen atoms and one oxygen atom. The extra mass is in the neutrons.

8. IT1 · Apply

Model answer. (a) Each has 6 protons, so the neutrons are 12 − 6 = 6, 13 − 6 = 7 and 14 − 6 = 8. (b) The pupil is wrong. All three isotopes have 6 protons and 6 electrons, arranged 2,4, and reactions involve only electrons, so they all burn to form carbon dioxide. (c) The mass of the atoms differs, so properties that depend on mass differ, such as the mass or density of the carbon dioxide made.

A full answer includes

(a) 6, 7 and 8 neutrons.
(b) I explained that the pupil is wrong: all three have 6 electrons (2,4), and reactions depend on electrons, so carbon-14 also burns to make carbon dioxide.
(c) The mass of an atom (or the density of the carbon dioxide made). Carbon-14 is also radioactive: true, but not part of this course.

Also accept

(a) 6, 7, 8. (b) Wrong: same electrons, so the same reaction, carbon dioxide. (c) Mass of an atom.

Watch for

“Carbon-14 has 14 electrons.” — Electrons match protons in an atom, and carbon always has 6 protons. 14 is the mass number.
“Heavier atoms react to make heavier products of a different kind.” — The product is still carbon dioxide; only its mass is a little greater. The electrons decide what forms.