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How to draw dot and cross diagram of ionic compound
In this video, we will focus on dot and cross drawing of sodium oxide ionic compound, Na2O.
The Periodic Table
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.
Sodium
Sodium
Sodium has 11 electrons. We will use dot to represent the electrons of sodium. The first shell holds 2 electrons, the second shell holds 8 electrons. The last electron is in the third shell. Sodium atom would love to donate this electron to a non-metal atom so that it can achieve an octet configuration.
Oxygen
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. It needs 2 more electrons to achieve the octet configuration.
Now, oxygen needs 2 electrons, but sodium can only give away 1 electron. Hence, we will need one more sodium to help oxygen fulfil the octet configuration. So, we have 2 sodium, each donates 1 electron to oxygen. Sodium ions and oxide ion are formed. Let’s work out the charges of the ions.
Sodium ion
Sodium ion
Sodium atom has 11 protons, which gives it a charge of +11, while 11 electrons, which gives it a charge of -11. Hence, the overall charge of this atom is 0. When sodium gives away its electron to the non-metal, the number of protons remained unchanged. Hence, the positive charge is still +11. However, there are only 10 electrons left. Hence, the negative charge is -10. Overall charge in this ion is +1. 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 electrons from sodium, 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.
You realise there are 2 sodium ions present in this ionic compound. Instead of drawing 2 sodium ions, we can draw only one of them, and put a 2 in front of sodium ions.
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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