When writing the chemical formula of a covalent compound, you need to satisfy the valency of that element. Valency is the combining power of the element. It is the number of bonds that elements need to form in a compound.
Figure 14.4.4 shows the combining powers of each group of elements on the periodic table. From this, you can see that carbon has a combining power of 4. This means carbon needs to form four bonds.
1 2
H Li
Na K
Rb Cs Fr
Be Mg Ca Sr Ba Ra Ac
3 4 3 2 1 0 B
Sc Ti V Cr Mn Fe Co Ni Cu Zn Y Zr Nb Mo Tc Ru Rh Pd Ag Cd La Hf Ta W Re Os Ir
Pt Au Hg
Al Ga In Tl
C Si
Ge Sn Pb
N P
As Sb Bi
O S
Se Te Po
F
Cl Br I
At
He Ne Ar Kr Xe Rn
Rf Db Sg Bh Hs Mt Ds Rg Cn Uut Fl Uup Lv Uus Uuo
For example, when carbon bonds with another element, it needs to create four bonds with that element. Oxygen has a combining power of two. The building blocks in Fig. 14.4.5 show how carbon and oxygen combine to form carbon dioxide.
14.4
z
Fig. 14.4.4
Combining powers of element groups on the periodic table
z
Carbon needs to form four bonds
Oxygen needs to form two bonds
The formula for carbon dioxide is CO2
Constructing the chemical formula of covalent compounds
Using Fig. 14.4.4, construct the chemical formula for each of these compounds:
zz zz zz zz zz
Water (compound of oxygen and hydrogen). Methane (compound of carbon and hydrogen). Ammonia (compound of nitrogen and hydrogen). Hydrogen sulfide (compound of hydrogen and sulfur). Carbon tetrafluoride (compound of carbon and fluorine).
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Fig. 14.4.5
Building blocks used to model the covalent bonding in carbon dioxide (CO2)
