IonsIB MYP Chemistry: Revision notes

An atom is neutral because it has equal numbers of protons and electrons. An ion is an atom (or group of atoms) that has gained or lost electrons, so it has an overall charge.

• Losing electrons gives a positive ion (more protons than electrons)
• Gaining electrons gives a negative ion (more electrons than protons)

Only the electrons change. The number of protons in the nucleus stays the same, so the element does not change.

Metal atoms have few outer electrons, and they lose them to form positive ions.

• Group 1 metals lose 1 electron: Na to Na⁺
• Group 2 metals lose 2 electrons: Mg to Mg²⁺
• Group 3 metals lose 3 electrons: Al to Al³⁺

The charge equals the group number. A sodium atom (11 protons, 11 electrons) becomes a sodium ion with 11 protons and 10 electrons.

Non-metal atoms have nearly full outer shells, so they gain electrons to form negative ions.

• Group 5 gain 3 electrons: N to N³⁻
• Group 6 gain 2 electrons: O to O²⁻
• Group 7 gain 1 electron: Cl to Cl⁻, F to F⁻

The charge is found by working out how many electrons are needed to make 8 outer electrons: 8 - group number. The names of simple negative ions end in -ide, for example chloride and oxide.

When atoms form ions they usually end up with a full outer shell, the same electron arrangement as the nearest noble gas.

• Na (2,8,1) loses 1 electron: Na⁺ is 2,8, the same as neon
• Mg (2,8,2) loses 2 electrons: Mg²⁺ is 2,8
• O (2,6) gains 2 electrons: O²⁻ is 2,8
• Cl (2,8,7) gains 1 electron: Cl⁻ is 2,8,8, the same as argon

This stable arrangement is why atoms form these ions.