How Chemical Compounds Form
Chemical compounds form by sharing or swapping electrons between 2 or more atoms.
Covalent bonds form between atoms when electrons are shared. Ionic bonds form
between atoms when electrons are swapped. Water, H2O is an example of a covalent
compound and sodium chloride, NaCl is an example of an ionic compound. To
understand why atoms form bonds we need to look at the Periodic table. Look at the
right most family (column). How many valence electrons are in these atoms? _______
The octet rule says all atoms are most stable energetically when they have all 8 valence
electrons (except for the first 2 elements H & He which only have 2 valence electrons).
The elements of the far right family all have 8 valence electrons, and as a consequence
they are energetically stable and don’t generally form compounds. Look at the left most
family. These elements have 1 valence electron and are 7 short of being stable as stated
in the octet rule. Would it be harder for these elements to gain 7 more valence electrons
to get a total of 8 valence electrons or to lose 1 so that the shell below (which is full)
becomes their valence shell? _____________________________
Atoms that are not at the far right column will either gain or lose valence electrons
depending on which is the smaller number and thus easier. For example Be will lose 2
electrons rather than gain 6 electrons, and O will gain 2 electrons rather than lose 6
electrons. Draw the electron dot diagrams for the elements in the second period below:
The elements in the second period only (row) are the answers
For each element you just drew indicate how many electrons an atom from that element
will gain or lose based on the octet rule.
-1, -2, -3, -4 or +4 (carbon usually gains 4 electrons), +3, +2, +1, 0 (Neon already has all
its valence electrons)
Let’s see why water is H2O. As we now understand, H would like to lose an electron (it
could just as easily gain one since it’s in the first period). And O would like to gain 2
electrons. When energy is added to allow a reaction to occur two H atoms will each
donate an electron to one O atom and the compound water is formed. The 2 H atoms
have lost their 1 valence electron and O has gained 2 electrons. All 3 atoms have
satisfied the octet rule and reached energy stability. Here is the electron dot diagram for
a molecule of water. Notice the electrons from each atom are being shared between all
three atoms.
Notice that the octet rule is satisfied.
Now try drawing electron diagrams for the following compounds below. Use the back if
you need more room to draw!
1. NaCl (sodium chloride)
4. MgO (magnesium oxide)
2. CH4 (methane)
5. NH3 (ammonia)
3. 3. C2H4 (ethane)
6. CCl4 (carbon tetrachloride)
NaCl
CH4
C2H2 Ethane
MgO
NH3
CCl4