How Chemical Compounds Form
Chemical compounds form by sharing or swapping (giving up) electrons between 2 or
more atoms. A bond forms when 2 electrons are shared or swapped between 2 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 again at the Periodic table. Look at
the right most family 18 (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 1. These elements have 1 valence electron and are 7 short of being stable by 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? _____________________________ Elements to the left end of the
Periodic Table will try to lose their electrons and those to the right will try to gain them.
Elements in the middle can often donate or accept electrons. For example Be will lose 2
electrons rather than gain 6 electrons, and O will gain 2 electrons rather than lose 6
electrons. On the back of this page draw the electron dot diagrams for the elements in the
second period and for each element you just drew indicate how many electrons an atom
from that element will gain or lose based on the octet rule.
Can you see why compounds form? Atoms form compounds because they are sharing or
swapping electrons with other atoms to satisfy the octet rule and thus form compounds in
the process! As an example, let’s see why water is H2O. H will share an electron to fill
its shell because O will also share one. And O would like to gain 2 electrons (easier than
losing 6) which it gets from two Hydrogen atoms. When energy is added to allow a
reaction to occur, two H atoms will each share an electron with one O atom that shares 2
electrons and the compound water is formed. 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 2 electrons between atoms make a bond and that the octet rule is satisfied for
each atom. Now try drawing electron diagrams for the following compounds below on
the back of this paper.
1.
2.
3.
4.
NaCl (sodium chloride)
CH4 (methane)
BaO (Barium Oxide)
CO2 (Carbon Dioxide)
5.
6.
7.
8.
MgO (magnesium oxide)
NH3 (ammonia)
CCl4 (carbon tetrachloride)
C2H6 (ethane)