2.5 Covalent Bonding and Molecules
A molecule is a combination of at least two atoms in a specific arrangement held together by chemical forces (chemical bonds). A molecule may be an element or a compound. Some elements that naturally exist as molecules include Br2, I2, N2, Cl2, H2, O2, F2, C60 and S8. (BrINClHOF: “The super 7”)
Different samples of a given compound always contain the same elements in the same ratio. This is known as the law of definite proportions.
| Sample | Mass of O (g) | Mass of C (g) | Ratio O(g):C(g) |
|---|---|---|---|
123 g CO2 |
89.4 |
33.6 |
2.66:1 |
50.5 g CO2 |
36.7 |
13.8 |
2.66:1 |
88.6 g CO2 |
64.4 |
24.2 |
2.66:1 |
If two elements can form a series of different compounds, the law of multiple proportions tells us that the ratio of masses of one element that combine with a fixed mass of the other element can be expressed in small whole numbers. In addition to carbon dioxide, carbon also combines with oxygen to form carbon monoxide.
\[\dfrac{2.66~\mathrm{O}}{1.33~\mathrm{O}} = \dfrac{\mathrm{2~O~in~CO_2}}{\mathrm{1~O~in~CO}}\]
The law of multiple proportions tells us 2 O in CO2 and 1 O in CO.
2.5.1 Molecular Formulas
Diatomic molecules contain two atoms and may be either heteronuclear or homonuclear (BrINClHOF). Polyatomic molecules contain more than two atoms.
A chemical formula denotes the composition of the substance through element symbols and numbers, as well as parentheses, dashes, brackets, commas, plus, and minus signs.
A molecular formula shows the exact number of atoms of each element in a molecule.
\[\mathrm{H_2O}\]
Some elements have two or more distinct forms known as allotropes. For example, oxygen (O2) and ozone (O2) are allotropes of oxygen. C, S, and P also have allotropic forms
A structural formula shows not only the elemental composition, but also the general arrangements.
Molecular substances can also be represented using empirical formulas, the whole–number ratio of elements.
While, the molecular formulas tell us the actual number of atoms in the molecule (the true formula), the empirical formula gives the lowest whole–number ratio of elements (the simplest formula).
- Molecular formula: N2H4
- Empirical formula: NH2
The molecular and empirical formulas are often the same.
| Compound | Molecular Formula | Empirical Formula |
|---|---|---|
Water |
H2O |
H2O |
Hydrogen peroxide |
H2O2 |
HO |
Ethane |
C2H6 |
CH3 |
Propane |
C3H8 |
C3H8 |
Acetylene |
C2H2 |
CH |
Benzene |
C6H6 |
CH |
Practice
Write the empirical formulas for the following molecules:
A. glucose (C6H12O6), a substance known as blood sugar
B. adenine (C5H5N5), also known as vitamin B4
C. nitrous oxide (N2O), a gas that is used as an anesthetic (“laughing gas”) and as an aerosol propellant for whipped cream.
Solution
A: CH2O
B: CHN
C: N2O
Practice
True or False: If any coefficient (subscript) in the molecular formula is 1, then the molecular formula and empirical formula are the same.
Solution
True