Write the chemical formulas of the following compounds, given the valences and the oxidation states (between brackets) of their component elements, and name them.
- Compounds made of:
- Boron (3) and Hydrogen (1)
- Phosphorus (5) and Chlorine (1)
- Hydrogen (1) and Sulfur (2)
- Carbon (4) and Oxygen (2)
- Carbon (3) and Oxygen (3)
- Compounds made of:
- Aluminum (3+) and Oxide (2-)
- Sodium (+) and Sulphate (2-)
- Magnesium (2+) and Oxygen (2-)
- Among all chemical formulas in 1. and 2. above, which ones are molecular chemical formulas and which ones are formula units? Explain why.
Solution:
A pure chemical substance is represented and identified by its chemical formula, its name (traditional or scientific) and its properties.
- The guiding principle here is that the total valences of the two atoms must be equal; or use the “Cross-over method”. Their formulas indicate the different chemical elements (symbols) present in a molecule and their respective numbers.
BH3: Boron hydride; B (valence 3), 3H (3x1 =3).
PCl5: Phosphorus pentachloride; P (5), 5Cl (5x1 = 5)
H2S: Hydrogen sulfide; 2H (2x1 =2), S (2)
CO2: Carbon dioxide (:Ö=C=Ö:); C (4), 2O (2x2=4)
CO: Carbon monoxide (:C≡O:); C (3), O (3)
2. The guiding principle here is that the total charges of the two ions (cation and anion) must cancel each other, because the whole chemical formula must be neutral. Their formulas indicate the ratios of ions, cation: anion, in the crystal of the ionic compound.
Al2O3: Aluminum (III) oxide (Aluminum trioxide): 2Al (2x3+ = 6+), 3O (3x2- = 6-)
Na2SO4: Sodium sulphate: 2Na (2x1+ = 2+), SO4 (2-) in which S(+6), 4O (4x2-= 8-)
MgO: Magnesium oxide: Mg (2+), O (2-)
In writing the chemical formula of a compound, as shown above, the formula starts by the atom with lower EN (more metallic) for molecular compounds, and by the cation for ionic compounds.
Naming ionic compounds: starts by the name of the metal (cation) followed by the name of the anion or polyatomic anion. Simple atomic anions are named by replacing the last part of the name by the suffix -ide at the end of their chemical names (ex: Chloride, oxide). If the metal can have more than one oxidation state, the actual charge must be written, in Roman figures, between brackets; ex: Fe(II), Fe(III).
N.B: You have noticed that even in the main groups, some elements may have more than one valence or oxidation state. Examples C (3) and O(3) in CO; C (4) and O (2) in CO2; H(1) and O(2) in H2O; H(1-) and Na(1+) in NaH.
Naming covalent compounds: many covalent compounds are organic compounds and there are rules for nomenclature of organic compounds (see Introduction to Organic chemistry).
Hydrogen (1), Nitrogen (3), Oxygen (2) and Halogens (1) form diatomic molecules: H2(H-H), N2(N≡N), O2(O=O), X2(X-X), those molecules bear the same name as the elements.
Few covalent compounds classified as inorganic can be named following the same rules for naming ionic compounds. But most of them have traditional/historical names such as: H2O: water instead of hydrogen oxide, NH3: ammonia instead of nitrogen hydride.
3. All compounds in question 1. are made of combinations of non-metals; therefore they are covalent and exist as a collection of individual molecules where the number of each atom in the molecule is indicated in the chemical formula. Their chemical formulas are called molecular chemical formulas.
But in question 2. all compounds are made of combinations of metals and non-metals; therefore they are ionic, and ionic compounds do not exist as individual molecules. They exist as an infinite tridimensional network of alternating cations and anions. Their chemical formulas represent the cation: anion ratio in the crystal lattice of the ionic compound and are called Formula Units.
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