Page 79 - Theory and Problems of BEGINNING CHEMISTRY

68 CHEMICAL BONDING [CHAP. 5

EXAMPLE 5.1. How many H atoms and how many P atoms are there per formula unit of (NH 4 ) 3 PO 3 ?
Ans. There are three NH 4 groups, each containing four H atoms, for a total of 12 H atoms per formula unit. There is only
one P atom; the final3defines the number of O atoms.

In summary, chemical formulas yield the following information:
1. Which elements are present
2. The ratio of the number of atoms of each element to the number of atoms of each other element
3. The number of atoms of each element per formula unit of compound
4. The fact that all the atoms represented are bonded together in some way
You cannot tell from a formula how many atoms of each element are present in a given sample of substance,
because there might be a little or a lot of the substance present. The formula tells the ratio of atoms of each
element to all the others, and the ratio of atoms of each element to formula units as a whole.

EXAMPLE 5.2. (a) Can you tell how many ears and how many noses were present at the last Super Bowl football game?
Can you guess how many ears there were per nose? How many ears per person? (b) Can you tell how many hydrogen and
oxygen atoms there are in a sample of pure water? Can you tell how many hydrogen atoms there are per oxygen atom? per
water molecule?
Ans. (a) Since the problem does not give the number of people at the game, it is impossible to tell the number of ears
or noses from the information given. The ratios of ears to noses and ears to people are both likely to be 2 : 1.
(b) Since the problem does not give the quantity of water, it is impossible to tell the number of hydrogen atoms or
oxygen atoms from the information given. The ratios of hydrogen atoms to oxygen atoms and hydrogen atoms
to water molecules are both 2 : 1.

The atoms of many nonmetals bond together into molecules when the elements are uncombined. For example,
a pair of hydrogen atoms bonded together is a hydrogen molecule. Seven elements, when uncombined with other
elements, form diatomic molecules. These elements are hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine,
and iodine. They are easy to remember because the last six form a large “7” in the periodic table, starting at
element 7, nitrogen:
NOF
Cl
Br
I

5.3. THE OCTET RULE
The elements helium, neon, argon, krypton, xenon, and radon—known as the noble gases—occur in nature
as monatomic gases. Their atoms are not combined with atoms of other elements or with other atoms like
themselves. Prior to 1962, no compounds of these elements were known. (Since 1962, some compounds of
krypton, xenon, and radon have been prepared.) Why are these elements so stable, while the elements with
atomic numbers 1 less or 1 more are so reactive? The answer lies in the electronic structures of their atoms. The
electrons in atoms are arranged in shells, as described in Sec. 3.6 and in greater detail in Chap. 4.

EXAMPLE 5.3. (a) Arrange the 19 electrons of potassium into shells. (b) Arrange the 18 electrons of argon into shells.
Ans. The first two electrons fill the first shell, the next eight fill the second shell, and the following eight occupy the third.
That leaves one electron left in potassium for the fourth shell.
Shell Number
Detailed Configuration
1 2 3 4 (if you studied Chap. 4)
2
6
2
6
2
(a)K 2 8 8 1 1s 2s 2p 3s 3p 4s 1
2
6
2
2
(b)Ar 2 8 8 1s 2s 2p 3s 3p 6