Isotopes and Ions
In 1974, geologist Jim Bowler discovered human remains at Lake Mungo, NSW; carbon-14 dating showed the bones were approximately 42,000 years old — evidence for Australia's oldest known human burial site.
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Q1 · Two atoms each have 6 protons but one has 6 neutrons and the other has 8. Do you think they are the same element? Why?
Q2 · If a sodium atom loses one electron, what do you think happens to its overall charge?
● Know
- Isotopes = atoms of the same element with different neutron counts
- Ions = atoms that have lost or gained electrons
- Cation (+) and anion (−) are the two kinds of ion
● Understand
- Why isotopes are still the same element
- Why atoms form ions (to reach a more stable shell arrangement)
- Why losing electrons makes a positive ion, and gaining makes a negative ion
● Can do
- Identify an isotope from a Z and A pair
- Predict whether an atom is likely to form a cation or anion
- Write the charge on a simple ion (e.g. Na+, Cl−)
- Isotope
- Ion
- Cation
- Anion
- Stable
- Negative ion (gained electrons)
- Same element, different neutrons
- Positive ion (lost electrons)
- Low-energy, unreactive arrangement
- Atom with an overall charge
Pick up two carbon pencil leads that look completely identical — same colour, same hardness, same chemical behaviour. But some of those carbon atoms weigh slightly more than others, because they carry 1 or 2 extra neutrons in their nucleus. Those heavier versions are called isotopes: same number of protons (same Z), different number of neutrons (different A).
Carbon is the classic example. Every carbon atom has 6 protons, but the number of neutrons can vary:
| Isotope | Protons | Neutrons | Mass number (A) |
|---|---|---|---|
| Carbon-12 | 6 | 6 | 12 |
| Carbon-13 | 6 | 7 | 13 |
| Carbon-14 | 6 | 8 | 14 |
All three are still carbon, because all three have 6 protons. They just have different masses. Carbon-14 is slightly unstable and breaks down over thousands of years — this is what makes carbon dating work. Scientists measure how much carbon-14 is left in a sample (bone, charcoal, an old piece of wood) to estimate its age.
Isotopes of an element have the same number of but different numbers of . They have the same number but a different number. Carbon-14 is used in .
Wrong: "Isotopes are different elements." Isotopes have the same number of protons, so they are the same element. They just have a different mass.
Right: Same Z = same element. Different A = different isotope of that element.
Wrong: "A sodium ion is a different element to sodium." A sodium ion (Na+) has lost one electron, but it still has 11 protons — so it is still sodium, just charged.
Right: Ions have the same number of protons (same element). Only the electron count changes.
Wrong: "Losing electrons makes an atom negative." Electrons are negative, so losing them takes negative charge away — the atom becomes positive (a cation).
Right: Lose electrons → positive ion (cation). Gain electrons → negative ion (anion).
An ion is an atom that has gained or lost one or more electrons. Because protons (+) are now no longer balanced by electrons (−), the atom ends up with an overall charge.
| If the atom… | It becomes… | Name | Example |
|---|---|---|---|
| LOSES electrons | Positive | Cation | Na → Na+ (sodium ion) |
| GAINS electrons | Negative | Anion | Cl → Cl− (chloride ion) |
Sodium (Na) has 11 electrons (2, 8, 1). If it loses the single electron in its outer shell, it now has only 10 electrons but still 11 protons. Net charge = +1, so we write Na+.
Chlorine (Cl) has 17 electrons (2, 8, 7). If it gains one electron, it now has 18 electrons but still 17 protons. Net charge = −1, so we write Cl−.
Atoms with a full outer electron shell are very stable (think of the noble gases like neon and argon). Atoms with almost-empty or almost-full outer shells will try to reach that stable arrangement by losing or gaining electrons.
- Sodium (2, 8, 1) finds it easier to drop its single outer electron. Now its arrangement is 2, 8 — a full shell. Becomes Na+.
- Chlorine (2, 8, 7) finds it easier to grab one extra electron. Now it has 2, 8, 8 — a full shell. Becomes Cl−.
So Na+ and Cl− together make sodium chloride — table salt. The whole reason atoms react with each other is to chase a more stable electron arrangement.
These two words sound similar but mean very different things:
| Isotope | Ion | |
|---|---|---|
| What changes? | Neutrons | Electrons |
| What stays the same? | Protons (so same element) | Protons (so same element) |
| Mass number? | Different | Same |
| Overall charge? | Same (still 0) | Different (becomes + or −) |
| Example | C-12 vs C-14 | Na vs Na+ |
Tip: isotopes change the nucleus; ions change the (e)lectrons.
Magnesium has 12 electrons, arranged 2, 8, 2. Predict (a) whether magnesium will form a positive or negative ion and (b) what its charge will be when it does.
How close was your prediction?
Nice — atoms with few outer electrons usually lose them.
Good — the trick is to ask which is easier: lose 2 or gain 6?
At the start of this lesson you were asked: Scientists used a slightly heavier kind of carbon (carbon-14) to work out that the human skeleton "Mungo Man" is around 42,000 years old — how can that even work? Now you know what carbon-14 is!
Explain in your own words what makes carbon-14 an isotope of carbon, why it can be used for dating ancient things, and what would happen if one of those carbon atoms lost an electron.
Q1. Define isotope and ion. State one example of each. (3 marks)
Q2. A chlorine atom has 17 protons and 17 electrons. Explain what happens when it gains one electron. State the charge, name (cation or anion) and the symbol of the ion formed. (4 marks)
Q3. Compare an isotope and an ion of the same element. In your answer, refer to what is different about each, and why it does NOT change the element. (4 marks)
Answers
▾MCQ 1
C — Isotopes have the same number of protons (so the same element) but different numbers of neutrons.
MCQ 2
B — Gaining an electron makes the atom negative. So Cl becomes Cl−, the chloride anion.
MCQ 3
A — Both have 6 protons (same element) and 6 electrons (neutral). C-12 has 6 neutrons; C-14 has 8 neutrons.
MCQ 4
D — Sodium has only 1 outer electron (2, 8, 1). Losing 1 is much easier than gaining 7 to fill the shell, so sodium becomes Na+.
MCQ 5
C — Na+ has the same 11 protons (still sodium) but has lost one electron, so it has 10 electrons. Net charge = +1.
Short Answer 1
Model answer: An isotope is a version of an element with the same number of protons but a different number of neutrons (example: carbon-12 vs carbon-14). An ion is an atom that has lost or gained electrons, so it has an overall charge (example: Na+, Cl−).
Short Answer 2
Model answer: When chlorine gains one electron, it now has 17 protons and 18 electrons. Because there is one more negative than positive, the atom has a net charge of −1. This makes it a negative ion, or anion. Its symbol is Cl− (the chloride ion). The outer shell is now full (2, 8, 8), which makes it more stable.
Short Answer 3
Model answer: Both isotopes and ions still have the same number of protons, so they are still the same element — the atomic number does not change. The difference is what changes elsewhere: isotopes have different numbers of neutrons, so they have a different mass number but the same charge (still 0); ions have different numbers of electrons, so they have a different charge (+ or −) but the same mass number. As long as the number of protons stays the same, the element stays the same.