How to Draw Dot and Cross Diagram of Sodium Oxide, Na₂O

Sodium oxide is an ionic compound. It forms when sodium atoms transfer electrons to an oxygen atom.

Why do atoms of elements form bonds?

Atoms form bonds to reach a stable noble gas configuration. If an atom has only one shell, it needs 2 electrons. This is a duplet. If it has more than one shell, it needs 8 electrons in the outer shell. This is an octet. Only the outer electrons are involved in bonding.

Electrons in each atom

We can determine the number of electrons each atom has from the periodic table. Refer to the key in the periodic table. The number at the top tells us the number of proton an atom has. Since number of protons and electrons are the same in an atom, this number also tells us how many electrons each atom has. The number at the bottom tells us the number of protons and neutrons each atom has.

Sodium has proton number 11, so it has 11 electrons.

First shell: 2
Second shell: 8
Third shell: 1

Each sodium atom can donate only 1 electron.

Oxygen has proton number 8, so it has 8 electrons.

First shell: 2
Second shell: 6

Oxygen needs 2 more electrons to reach an octet.

How the ions form

Oxygen needs 2 more electrons in its outermost shell, but each sodium atom can give only 1. Two sodium atoms are needed.

Each sodium atom donates its 1 outer electron to oxygen. Each sodium atom becomes a sodium ion, Na⁺.

Each oxygen atom receives 2 electrons from both sodium. Oxygen becomes an oxide ion, O²⁻.

The oppositely charged ions are held together by strong electrostatic forces of attraction. This is the ionic bond.

Charges of the ions

A sodium atom has 11 protons (+11) and 11 electrons (−11), so its charge is 0. After it loses 1 electron, it still has 11 protons (+11) but only 10 electrons (−10). The charge is +1.

An oxygen atom has 8 protons (+8) and 8 electrons (−8), so its charge is 0. After it gains 2 electrons, it still has 8 protons (+8) but now has 10 electrons (−10). The charge is −2.

Dot and cross diagram

Show two sodium atoms, each with 1 outer electron, using dots.
Show the oxygen atom with 6 outer electrons, using crosses.
Move one electron from each sodium atom into the outer shell of oxygen.
Draw square brackets around each ion.
Write the charge outside the bracket: + for each sodium ion and 2− for the oxide ion.

You can choose not to draw both draw both sodium ions. You may draw one sodium ion and put a 2 in front of it.

Note for exams: If the question asks you to draw only the outermost electrons, you can omit the inner shells. Only show the valence electrons involved in bonding.