Identify whether longhand notation or noble-gas notation was used in each case below. Potassium (K) : 1s22s22p63s23p64s1

Neutral potassium has 19 electrons and has the following configuration:

1s2 2s2 2p6 3s2 3p6 4s1

A potassium ion (K+) has only 18 electrons (remember that electrons are negatively charged and so one must be removed in order to give an ion a + 1 charge) so its electron configuration looks like this:

1s2 2s2 2p6 3s2 3p6

Notice the loss of the electron in 4s.

Negative ions are caused by an atom gaining electrons. Neutral bromine has 35 electrons and the following electron configuration:

1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p5

The bromide ion (Br-) has 36 electrons:

1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6