See the image below.
Here are the points to remember.
1. The number of protons (atomic number) is the same whether an atom is neutral or ionic.
2. In a neutral atom, the number of electrons = the number of protons.
3. The number of electrons in each shell is 2, 8, 8, 18, 18, …
4. The number of electrons in an atom at the end of each period (a noble gas) is 2, 10, 18, 36, 54, …
5. An atom will gain or lose electrons to achieve a noble gas configuration.
For example, a potassium atom has 19 protons and 10 electrons. It will form an ion by losing one electron to get to 18 electrons.
