COVALENT MOLECULAR STRUCTURE: How do atoms bond to make molecules?
One way atoms bond is by sharing electrons amongst their outermost shell with another atom.
These outermost shell electrons are called valence electrons. Their number of valence electrons
is important in determining what other atoms they will bond with. Notice that the Periodic Table
is organized in rows and columns. The rows (left to right), called periods, are related to how
many valence electrons an atom’s outermost shell can hold. The first row of the Table shows that
these elements can hold up to 2 electrons. The second and third rows of the Table show that up to
8 electrons can be held in these elements valence shell. How many electrons can be held in the
valence shells of elements in rows 4 and 5? ____ How about rows 6 and 7? ________
Did you say 8 electrons in all the other row (periods)? Recall that the metals only have 2 valence
electrons, with some exceptions, until we advance beyond the transition metals. Here is the
important rule to remember about how atoms bond by sharing their electrons with each other.
The Octet Rule: atoms will share their valence electrons with other atoms in order to fill all 8
electrons spots in their valence shell (with the important exception of the first period which is 2).
For example, H and He would each like to have 2 electrons in their valence shells. Elements in
the second row (Li-Ne) would like to have 8 electrons in their valence shell. How many electrons
would N like to have in its valence shell? ______. How many electrons will N have to find to fill
its outer shell____? How many electrons would K like to have in its valence shell? ________.
How many electrons will K have to find to fill its outer shell? ______Here’s how a molecule of
water forms. Oxygen, O, would like to have a total of 8 electrons and H would like to have 2
electrons in its valence shell, so 1 O will bond with 2 H to make water! (H2O)
Notice how in covalent molecules like water every atom is surrounded by 8 valence electrons (8
dots), with the lone exception of hydrogen which has two valence electrons. Remember H is
the one atom that only needs two electrons to fill it's one and only shell.
Try these problems: remember to represent your atoms now as electron dot models.
1. Draw the molecule HF on the back of this paper showing all the electrons in their
valence shells.
2. How many H atoms does C need to fill its valence shell? Draw methane (CH4) on the
back of this paper showing all the electrons in their valence shells.
3. How many O atoms does Si need to fill all the electrons in its valence shell? Draw a
molecule of SiO2 on the back of this paper showing all the valence electrons.
4. How many H atoms does N need to fill all the electrons in its valence shell? Draw a
molecule of ammonia (NH3) showing all the valence electrons.
5. How many Br atoms does As need to fill all the electrons in its valence shell? Draw a
molecule of astatine tribromide (AsBr3) showing all the valence electrons.
6. Now a tricky one. Draw a molecule of CO2. Hint: Carbon makes two double bonds in this
molecule (one to each oxygen atom)!
7. Another tricky one. Draw a molecule of N2 (nitrogen gas). Nitrogen makes a triple bond to
another N atom!