Structure of the Atom, and the Periodic Table: Chemical Formulae

1.0 Structure of the Atom, and the Periodic Table

1.8 Chemical Formulae

In Section 1.7 (Ion Formation) we learnt that unstable atoms react by losing or gaining electrons to complete their energy levels and become stable. We can therefore easily determine a chemical formula by considering how energy levels can be completed. We will do this through examples.

1.8.1 Chemical formulae of reactive gaseous elements

Open the pencast on formulae of gaseous elements.

Study Figure 1.8.1 which shows hydrogen atoms and how they combine.

Single hydrogen atoms have incomplete energy levels; so they are unstable. When two such atoms come together, the energy level in each becomes complete, with 2 electrons so the product has the formula, H₂. A third hydrogen atom is not required.

Hydrogen therefore exists as pairs of atoms, called diatomic molecules. Many other gaseous elements such as nitrogen (N₂), oxygen (O₂), fluorine (F₂), and chlorine (Cl₂) exist as diatomic molecules.

Solid elements are represented by their chemical symbols as though they were single atoms. Examples are lithium (Li), carbon (C), sodium (Na), magnesium (Mg), aluminium (Al), phosphorus (P), sulphur (S), potassium (K), and calcium (Ca).

1.8.2 Chemical formulae of compounds

Study the following examples presented on diagrams to show how to arrive at chemical formulae of compounds.

Example 1: Formula of lithium hydride

One (1) atom of lithium combines with one (1) atom of hydrogen (H); so the chemical formula of the product is Li₁H₁, which is simplified as LiH. Note that the combining atoms have the same number of lone or unpaired electrons, which explains the ratio 1:1. Also, in compounds where there are metals, the metal comes first in the formula.

Where only non-metals are involved, atoms share rather than lose or gain electrons (Example 2).

Example 2: Formula of water

Oxygen atom has two (2) lone electrons while hydrogen has one (1). Two hydrogen atoms are therefore required to satisfy one oxygen atom. This leads to the familiar formula of water, H₂O. Notice that in the structure of H₂O, oxygen and hydrogen atoms have complete energy levels with 8 and 2 electrons respectively; so they are stable.

From these examples, we notice that chemical formula of a compound (made up of different elements)

1. Depends on number of lone electrons (valency) in the atoms
2. Take the form X_yY_x

(where X and Y are symbols of the elements; y is the valency of Y; x is the valency of X)

NB: In the formula X_xY_y, x and y must be in the simplest ratio. For example, X₂Y₄ becomes XY₂.

We have used Figures 1.8.1 to 1.8.3 to understand why reactive gaseous elements exist as paired atoms and how valency comes about. Valency is the combining power of an element or atom. It equals the number of lone electrons and normally ranges between 1 and 4.

Otherwise, if valency is known, we can use Figure 1.8.4 directly to arrive at a chemical formula. Table 1.8.1 shows the valencies of some common elements and ions.

Table 1.8.1: Valencies of some common elements and ions

NB: The charge on an ion equals its valency.

Following the guide in Figure 1.8.4, the formula of zinc nitrate, for example, is Zn(NO₃)₂.

NB: Remove the charge on an ion once it combines with another, because it becomes neutralized.

Questions 1.8

Use the valencies given in Table 1.8.1 to write the chemical formulae of the compounds named in Questions 1 and 2.

1. 1. Hydrogen chloride (same formula as hydrochloric acid)
   2. Potassium hydroxide
   3. Magnesium chloride
   4. Calcium carbonate
   5. Calcium chloride
   6. Aluminium oxide
   7. Sodium sulphate
   8. Potassium nitrate
   9. Ammonium nitrate
   10. Carbon (IV) oxide
   11. Zinc sulphate
   12. Copper nitrate
   13. Ammonium phosphate (V)
   14. Silver nitrate


2. 1. Lead (II) oxide
   2. Lead (II) nitrate
   3. Iron (II) oxide
   4. Iron (III) oxide
   5. Iron (II) sulphate
   6. Copper (II) oxide
   7. Copper (I) sulphide
   8. Manganese (IV) oxide
   9. Sulphur (VI) oxide
   10. Sulphur (IV) oxide

NB: The values given in Roman numbers are the valencies of the metals involved.

Answers to Questions 1.8