2.1.2 - Compounds, Formulae and Equations

Ions

A stable compound must have no overall charge, because if it had an overall charge, it would react or combine with oppositely charged compounds.

However, a neutral compound can be comprised of charged particles called ions. They are formed by a disparity between the number of negatively charged electrons and positively charged protons. More electrons than protons results in an overall negative charge, whereas fewer electrons than protons results in a positively charged ion.

For example, a chlorine atom normally has 17 protons and electrons, but can gain one more electron to have a total of 18 electrons. One more electron than protons means an overall charge of -1. Conversely, a sodium atom can lose an electron to form a positive ion with 11 protons and 10 electrons, resulting in a charge of +1.

Formulae

Since sodium ions are +1, denoted by Na$^+$, and chloride ions are -1, denoted by Cl$^-$; they can form an ionic lattice, forming NaCl (table salt). This formula does not show that there are particles of Na$^+$ and Cl$^-$ together, it just shows that - because it is a lattice - that the ratio of sodium and chloride ions is 1:1.

Predicting Charges

The position of an element in the periodic table is now dependent on the atomic number, i.e., the number of protons in its atoms’ nuclei. As a result it can also be viewed as being arranged by electronic number also, since the numbers of protons and electrons in an atom are equal.

As a result, moving left to right in a period (row), the number of electrons increases. In addition to this, moving top to bottom in a group (column) increases the number of electron shells. Atoms in the same group have similar properties, since they have the same number of electrons in their outermost electron shells, and thus can produce ions of the same charge.

Three rules are required to predict charges:

  • Metals produce cations (positively charged ions).
  • Non-metals produce anions (negatively charged ions).
  • Atoms (excluding transition metal atoms) are most stable when electron shells are full or empty.

Cations (pronounced cat-ions) are positively charged, so contain fewer electrons than protons. Therefore, in their formations, metal atoms lose electrons to empty their outermost electron shell. The opposite is true for non-metals, gaining electrons to fill the shell.

For example, a magnesium (metal) atom is in group 2, so has two electrons its outer shell; so to empty it, two electrons must be lost, and the ion’s charge is 2+.

Chlorine (non-metal) is in group 7, so has 7 electrons in an outer shell that can hold 8; so to fill it, one electron must be gained, and the ion’s charge is 1-.

Ammonium Ions

Ammonia, NH$\small_{3}$, is a non-metal compound, but because of the nature of nitrogen, forms an ammonium 1+ (NH$small_{4^+}$). This is because a lone pair (pair of electrons not involved in a covalent bond) in the nitrogen atom of ammonia can be used to form a dative covalent bond (a covalent bond where both electrons in the shared pair come from one atom) with a proton ($H^+$). This results in a compound with one more proton than electrons, so has an overall charge of 1+.

Other Known Molecular Ions

With OCR A Chemistry, certain ions must be known as predicting their charges is perhaps too difficult:

  • Nitrate: $\text{NO}_{3} \ ^{-}$
  • Carbonate: $\text{CO}_{3} \ ^{2-}$
  • Sulphate: $\text{SO}_{4} \ ^{2-}$
  • Hydroxide: $\text{OH}^{-}$

Constructing Balanced Equations

Equations are useful in chemistry as they show the stoichiometric ratio between the moles of each reactant used and product formed.

Reaction equations must be balanced by charge and mass, since no atoms must be lost, no electrons.

Simple equations to balance include the complete combustion of propane:

$$ C_{3}H_{8}+O_2\longrightarrow CO_2+H_2O $$

Since there are no charged particles, mass can now be considered. On the left hand side, there are 3 carbons; but on the right, there is only one. Therefore we must ‘multiply’ the carbon on the right by three to match. Similarly, 8 hydrogens on the left must be matched by 8 hydrogens on the right:

$$ C_{3}H_{8}+O_2\longrightarrow 3CO_2+4H_2O $$

Finally, there are 10 oxygen atoms on the right, and only 2 on the left. Multiplying the reactant oxygen by 5 will balance the equation fully.

$$ C_{3}H_{8}+5O_2\longrightarrow 3CO_2+4H_2O $$

From this fully balanced equation, we know that for one molecule of propane to completely combust, 5 molecules of oxygen must also react. Except stoichiometric ratios (the coefficients of the equation) are considered in moles, or $mol$ as the unit.

We also know that from one mole of propane, we can get 3 moles of carbon dioxide and 4 moles of water, although 100% success is essentially impossible, and these are instead the theoretical maxima.

Note: harder examples where further knowledge is required will be available in later sections.