Forming negative ions

Negative ions are called anions.

Oxygen is in group 6. An oxygen atom has six electrons in its outer shell. The atom is more stable if it has a full outer shell.

An oxygen atom can gain two electrons to fill its outer shell. It will still have eight positive protons but will now have ten negative electrons. This gives it an overall charge of -2.

This charge is added to the symbol for oxygen, O2-. It is always written as 2- and not -2. The name of oxygen changes when naming it as an ion. The ion is called an oxide ion.

How an oxygen atom becomes an oxygen ion when it gains two electronsAn oxygen atom gains two electrons to form an oxide ion

The name of a non-metal ion is different to the name of its element:

Atom nameIon name
OxygenOxide
FluorineFluoride
BromineBromide
IodineIodide
SulfurSulfide
Question

Fluorine is in group 7. It has atomic number 9 and mass number 19.

  1. How many outer electrons does a fluorine atom have?
  2. What is the charge of a fluoride ion and what is its symbol?
  3. How many protons does a fluoride ion have?
  4. How many neutrons does a fluoride ion have?
  • fluorine has 7 electrons in its outer shell (it is in group 7 and its electron arrangement is 2.7)
  • a fluoride ion has a charge of -1. Its symbol is F-
  • number of protons = atomic number, so there are 9 protons
  • number of neutrons = mass number - atomic number, so 19 - 9 = 10 neutrons

Explaining reactivity in group 7

Group 7 non-metals attract an extra electron to complete their outer shell. The outer electron shell gets further from the nucleus moving down the group. This means that the extra electron is less strongly attracted, so the reactivity of group 7 gets less going down the group.