Which of the following atoms / ions contain(s) exactly 18 electrons?
1 18O
8
2 34S
16
3 35Cl
17
4 37Cl–
17
5 40Ca2+
20

Answer:

In an atom, the number of electrons equals the atomic number (the number of protons). None of the
atoms (options 1, 2 and 3) have atomic number 18.
Negative ions have additional electrons. For example, a 2– ion has two extra electrons compared to
the atom it is formed from. Positive ions have fewer electrons. For example, a 2+ ion has two fewer
electrons than the atom it was formed from. Option 4 has 17 protons and as a 1– ion has 17 + 1
electrons, i.e. 18 electrons. Option 5 has 20 protons and so as a 2+ ion has 20 – 2 electrons, i.e.
18 electrons.
The answer is E