Ions, Cations and Anions
Every heartbeat and every thought depends on ions: charged atoms of sodium, potassium and calcium racing across the membranes of your cells. Lose the right balance of these ions and nerves and muscles stop working.
Printable Worksheets
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Q1 · A chlorine atom gains one extra electron. Do you think it is still chlorine? What do you think happens to its overall electrical charge?
Q2 · If a sodium atom loses one electron, what do you think happens to its overall charge?
● Know
- Ions = atoms that have lost or gained electrons
- Cation (+) and anion (−) are the two kinds of ion
- An ion keeps the same number of protons, so it is the same element
● Understand
- Why atoms form ions (to reach a more stable, full outer shell)
- Why losing electrons makes a positive ion, and gaining makes a negative ion
- Why an ion is the same element, just charged (not a new element)
● Can do
- Predict whether an atom is likely to form a cation or anion
- Work out the charge on a simple ion from its electron arrangement
- Write the charge on a simple ion (e.g. Na+, Cl−, Mg2+)
- Valence electrons
- Ion
- Cation
- Anion
- Stable
- Negative ion (gained electrons)
- Outer-shell electrons lost or gained
- Positive ion (lost electrons)
- Low-energy, unreactive arrangement
- Atom with an overall charge
A neutral atom has equal numbers of protons (+) and electrons (−), so the charges cancel out and the atom has no overall charge. An ion forms when an atom gains or loses one or more electrons. The number of protons never changes, so it is still the same element, but it now carries an overall electric charge.
Take sodium (Na). A neutral sodium atom has 11 protons and 11 electrons. If it loses the single electron in its outer shell, it still has 11 protons but only 10 electrons, so its overall charge is now +1. We write this as Na+, a sodium ion.
An ion forms when an atom gains or loses . The number of never changes, so it is still the same . Losing electrons makes a ion, and gaining electrons makes a ion.
Wrong: "An ion is a brand-new element." An ion still has the same number of protons as the atom it came from, so it is the same element, just carrying a charge.
Right: Same protons = same element. Only the electron count, and so the charge, changes when an ion forms.
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.
Both cations and anions are ions. The only difference is the direction of electron transfer:
| Cation | Anion | |
|---|---|---|
| Electrons | Lost | Gained |
| Overall charge | Positive (+) | Negative (−) |
| Usually formed by | Metals (Na, Mg, K) | Non-metals (Cl, O) |
| Example | Na+, Mg2+ | Cl−, O2− |
Tip: a cation is positive; an anion is a negative ion.
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 met the idea that a sports drink "replaces electrolytes" lost in sweat, and that your nerves and muscles run on ions. Now you know what an ion is!
Explain in your own words what an ion is, the difference between a cation and an anion, and why an atom like sodium loses an electron to become Na+ rather than gaining seven.
Q1. Define the terms ion, cation and anion. Give one example of a cation and one example of an anion. (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. A magnesium atom (2, 8, 2) and an oxygen atom (2, 6) each form an ion. Predict the charge on each ion, name each as a cation or anion, and explain why each forms that ion. (4 marks)
Answers
▾MCQ 1
C Electrons are negative, so losing one or more leaves the atom with more protons than electrons, giving an overall positive charge. A positive ion is called a cation.
MCQ 2
B Gaining an electron makes the atom negative. So Cl becomes Cl−, the chloride anion.
MCQ 3
B Magnesium has 2 electrons in its outer shell. Losing 2 (to leave 2, 8, a full shell) is much easier than gaining 6, so magnesium forms Mg2+.
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 ion is an atom that has gained or lost one or more electrons, so it has an overall electric charge. A cation is a positive ion, formed when an atom loses electrons (example: Na+, Mg2+). An anion is a negative ion, formed when an atom gains electrons (example: Cl−, O2−).
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: Magnesium (2, 8, 2) has 2 electrons in its outer shell, so it loses them to reach a stable full shell (2, 8). It now has 12 protons and 10 electrons, a charge of +2, forming the cation Mg2+. Oxygen (2, 6) has 6 outer electrons, so it gains 2 to reach a full shell (2, 8). It now has 8 protons and 10 electrons, a charge of −2, forming the anion O2−. Each atom gains or loses whichever number of electrons is closest to a full, stable outer shell.