Electronic Configuration

The ways in which atoms interact is determined by the arrangement of the electrons orbiting the nucleus. The first 20 elements in the periodic table follow a simple pattern:

• The first shell can hold 2 electrons
• The second shell can hold 8 electrons
• The third shell can hold 8 electrons
• The fourth shell can hold 2 electrons
  
  
• The electrons always fill up the lowest shell before any other

Because atoms are electrically neutral they have the same number of electrons as protons. So the number also tells us the number of electrons. We write down the electronic configuations using decimal points. For example Sodium has an atomic number of 11 and so has an electonic configuation of 2.8.1

The number of shells is the (rows) in the periodic table, and the number of electrons in the outer layer is the (columns) in periodic table