In this video, we will focus on dot and cross drawing of calcium oxide ionic compound, CaO.

The Periodic Table

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.

Calcium

Calcium has 20 electrons. We will use dot to represent the electrons of calcium. The first shell holds 2 electrons, the second and third shell holds 8 electrons. The last 2 electrons are in the fourth shell. Calcium atom would love to donate these 2 electrons to a non-metal atom so that it can achieve an octet configuration.

Oxygen

Now the non-metal in this compound is oxygen. Oxygen has 8 electrons. Let’s use cross to represent the electrons of oxygen. The first shell holds 2, second shell holds 6. Oxygen has only 6 electrons in its outermost shell. In order to achieve the octet configuration, it needs to take in 2 more electrons.

So, what will happen now is, calcium atom will donate both the electrons to oxygen, forming calcium ion and oxide ion. Let’s work out the charges of the ions.

Calcium ion

Calcium has 20 protons, which gives it a charge of +20, while 20 electrons, which gives it a charge of -20. Hence, the overall charge of this atom is 0. When calcium gives away its electron to the non-metal, the number of protons remained unchanged. Hence, the positive charge is still +20. However, there are only 18 electrons left. Hence, the negative charge is -18. Overall charge in this ion is +2. We will need to represent this charge in the dot and cross diagram. Put the bracket, and the charge.

Oxide ion

For oxygen, the atom has 8 protons, which gives it a charge of +8, while 8 electrons, which gives it a charge of -8. The overall charge of this atom is 0. When oxygen takes in electron from calcium, the number of protons remained unchanged. Hence, the positive charge is still +8. However, there are 10 electrons now. Hence, the negative charge is -10. Overall charge in oxide ion is -2. Put the bracket and the charge in the dot and cross diagram.

Sometimes, in the examination questions, they will ask you to draw only the outermost electrons. Hence, we can omit the inner shells, and this is what you should draw.

In a nutshell, a metal loses its outermost electrons to form positive ion while a non-metal gains the electrons to form negative ion. The oppositely charged ions are held together by strong electrostatic forces of attraction, which we call the ionic bond.

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