The Periodic Table and Chemical Formulae 23 Methane (CH4 ) is a covalent compound. Each carbon atom wants to share four electrons so that
it has a full outer shell. Each hydrogen atom shares just one. Therefore four hydrogen atoms are needed to make methane.
Fig. 12 The Bohr diagram for HCl.
Fig. 13 The Bohr diagram for CH4
.
Predicting the ratio of atoms in two element compounds The above examples of ionic and covalent compounds show how compounds form. Sodium and chlorine combine in a 1:1 ratio to form NaCl when a sodium atom gives one electron to a chlorine atom. Magnesium and fluorine combine in a 1:2 ratio to form MgCl2
l l l l
when a magnesium atom
gives one electron to each fluorine atom. Hydrogen and chlorine combine in a 1:1 ratio to form HCl by each sharing one electron with the other. Carbon and hydrogen combine in a 1:4 ratio to form CH4
when each hydrogen atom shares
one electron with a carbon atom and the carbon atom shares one electron with each hydrogen atom.
Activity
Use the first 20 elements on the Periodic Table to make 15 different two-element compounds. The table below and examples which follow will help you. Note the purple background for non-metals.
