Give and take
Cations and anions.
An electron changes hands
A sodium atom has 1 electron in its outer shell. A chlorine atom has 7 and needs 1 more.
If they meet, what might happen?
What this lesson covers
The idea
A metal atom loses its outermost electrons to leave a stable octet and becomes a positive ion (cation); a non-metal atom gains the electrons to complete its octet and becomes a negative ion (anion).
An electron changes hands
A sodium atom has 1 electron in its outer shell. A chlorine atom has 7 and needs 1 more.
If they meet, what might happen?
- Sodium gives its electron to chlorine
- Chlorine gives 7 electrons to sodium
- Nothing can change
Follow the electron
Press Next step.
Losing and gaining
A metal atom can lose its outermost electrons and be left with a stable octet. Then it has more protons than electrons, so it is a positive ion, a cation. A non-metal atom can gain electrons to complete its octet. It then has more electrons than protons, so it is a negative ion, an anion.
A metal atom loses its outermost electrons to leave a stable octet and becomes a positive ion (cation); a non-metal atom gains the electrons to complete its octet and becomes a negative ion (anion).
Notes
A metal atom loses electrons and becomes a cation; a non-metal atom gains electrons and becomes an anion.
Check yourself
When a sodium atom loses one electron it becomes ...
A chlorine atom that gains one electron becomes ...
How many electrons does a chloride ion Cl⁻ have? (Its nucleus has 17 protons.)
Answer: 18
The atom has 17 electrons; gaining one gives 18. Net charge: 17 − 18 = −1.
A magnesium atom (2, 8, 2) with 12 protons loses its two outer electrons. What is the size of the positive charge on the ion, Mg²⁺?
Answer: 2
Protons 12, electrons 10, so the net charge is 12 − 10 = +2.
- a negative ion, Na⁻. Losing a negative electron leaves the atom with more protons than electrons, so it is positive.
- a positive ion, Na⁺ — correct. Yes!
- a neutral atom again. Sodium has 11 protons and only 10 electrons after losing one, so it is a charged ion.
- a chloride cation, Cl⁺. Gaining an electron gives more electrons than protons, so the ion is negative.
- a sodium cation. Chlorine remains chlorine; it becomes the chloride ion.
- a chloride anion, Cl⁻ — correct. Yes!