In this video, we will focus on dot and cross drawing of magnesium chloride ionic compound, MgCl2.

The Periodic Table

We can determine the number of electrons each atom has from the periodic table.

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.

Magnesium

Magnesium has 12 electrons. We will use dot to represent the electrons of magnesium. The first shell holds 2 electrons, the second shell holds 8 electrons. The last 2 electrons are in the third shell. Magnesium atom would love to donate this electron to a non-metal atom so that it can achieve an octet configuration.

Chlorine

Chlorine has 17 electrons. Let’s use cross to represent the electrons of chlorine. The first shell holds 2, second shell holds 8. It has 7 in the third shell. Chlorine has only 7 electrons in its outermost shell. It needs 1 more electron to achieve the octet configuration.

Drawing Magnesium chloride

Now, magnesium has to give away 2 electrons but chlorine can only take in 1 electron to form a chloride ion. Hence, we will need to have one more chlorine to take in both electrons from magnesium.

Now, magnesium atom will donate 1 electron to each of the two chlorine atoms, to form magnesium ion and chloride ion. Let’s work out the charges of the ions.

Magnesium atom has 12 protons, which gives it a charge of +12, while 12 electrons, which gives it a charge of -12. Hence, the overall charge of this atom is 0. When magnesium gives away its electron to the non-metal, the number of protons remained unchanged. Hence, the positive charge is still +12. However, there are only 10 electrons left. Hence, the negative charge is -10. 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.

For chlorine, the atom has 17 protons, which gives it a charge of +17, while 17 electrons, which gives it a charge of -17. The overall charge of this atom is 0. When chlorine takes in electron from magnesium, the number of protons remained unchanged. Hence, the positive charge is still +17. However, there are 18 electrons now. Hence, the negative charge is -18. Overall charge in chloride ion is -1. 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 electons 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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