Before we discuss atomic bonding, it may be useful to review a simplified model of electrons in atoms. This is the Bohr atomic model presented schematically below:

Bohr

The main particles in an atom are electrons, protons and neutrons. The protons and neutrons sit in the centre of the atom in what is known as the nucleus.

Orbiting

Protons and Neutrons are very much larger than electrons. Protons have a positive(+) charge, neutrons have no charge and electrons have a negative(-) charge. Electrons exist in separate orbitals or energy states surrounding the nucleus.

The electrons can be considered to sit in energy levels or shells, dictated by the quantum energy of each electron. Thought of another way, the electrons orbit the atoms nucleus in different sized orbits, with some closer to the nucleus and some further away.

As you increase the atomic number of the atom, you increase the number of electrons in orbit around the nucleus. These electrons usually fill the lower shells (orbits) first. This is because the inner shells are of lower energy than the outer ones.

The first inner shell (known as the K shell) has two positions for electrons, the next shell (the L shell) has 8. Therefore hydrogen (whose atomic number is 1) will only have one electron in the first shell, or 1K electron. Helium will have 2 K electrons, lithium will have 2 K electrons and 1 L electron, beryllium will have 2 K electrons and 2 L electrons, boron will have 2 K electrons and 3 L electrons, and so forth. This periodic filling of the electron shells results in a periodic variation of properties of atoms as you increase in atomic number. This is the basis of the periodic table.