Chemistry - A Molecular Science

We can also predict the formula of a binary inorganic compound by using likely
oxidation states and the fact that compounds are electrically neutral;

i.e

., the sum of the

oxidation states of all atoms must be zero. There are only two elements in a binary compound, so one must have a positive oxidation state (OX

pos

) and the other a negative

oxidation state (OX

neg

). To determine the formula of the compound, we need to know the

ratio of the atoms with positive oxidation states (N

pos

) to the number with negative

oxidation states (N

neg

). If the sum of all oxidation states is zero, we can write

Npos

OX

pos

= -N

neg

OX

neg

Eq.

4.1

Equation 4.1 states that the total positive charge (ions or oxidation states) must equal the total negative charge. Use the followi

ng steps to predict the formula:

Step 1

Assign likely oxidation states to each

of the elements. Typically, the Group

number is used for OX

pos

and (Group Number – 8) is used for OX

neg

.

* The lowest common multiple (LCM) of two integers is the smallest

integer for which they are both factors. For example, the LCM of 4 and 6 is 12 because both 4 and 6 are factors of 12. Both 4 and 6 are also factors of 24, but 24 is no

t the LCM because it is not the

smallest integer for which they are both factors.

Step 2

Find the lowest common multiple (LCM)* of OX

pos

and OX

neg

. This is the total

positive or negative charge.

Step 3

Determine how many atoms of each are required to deliver the charge determined in Step 2 (LCM). N

pos

= LCM/OX

pos

and N

neg

= -LCM/OX

neg

.^

Example 4.7 a) What is the formula of the compound between A and B in Example 4.5?

The oxidation states of A and B

are +1 and -3, so LCM = 3.

Npos

= 3/1 = 3 A atoms and N

neg

= -(3/-3) = 1 B atom

Three A atoms are required for every B atom, so the formula is A

B. Note the A is written 3

first in the formula because it is less electronegative than B.

b) What is the formula of the compound between B and C in Example 4.5?

If B is +5 and C is -2, then LCM = 10

Npos

= 10/5 = 2 B atoms and N

neg

= -(10/-2) = 5 C atoms

Two B atoms are required for every five C atoms, so the formula is B

C 2

. 5

If B is +3, then LCM = 6 and the compound is B

C 2

. 3

A possible set of elements with the relative energies given in Example 4.5 would be A = H, B = N, and C = O, in which case the compounds would be NH

and N 3

O 2

or N 5

O 2

. 3

Chapter 4 The Ionic Bond

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State

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