Summary of what I learnt (Equation Writing)

The formulae and names of some ions are shown in the picture above. Besides classifying ions into cation and anion, ions can also be classified into:

1) Simple ions

2) Polyatomic ions

Simple ions:

These ions are formed from single atoms such as magnesium atom and fluorine atom. Some elements form more than one type of positively charged ion. When naming such ion, Roman numerals are used to indicate the number of charges, and are placed after the name of the metal.

For example: iron forms iron (II) & iron (III) ions.

Polyatomic ions:

Polyatomic ions, such as ammonium ion (NH₄⁺), consist of more than 1 type of atom. For example, the ammonium ion consists of one nitrogen atom and four hydrogen atoms chemically combined together. The ammonium ion has a charge of 1+ because the total number of protons in all of the atoms is one more than the total number of electrons. This charge of 1+ is not localized on any particular atom but spread over all the atoms in the ammonium ion.

Formulae and names of ionic compounds:

Formulae of ionic compounds:

The negative and positive charges in an ionic compound must be balanced so that the ionic compound is neutral.

Example:

Potassium fluoride consists of K⁺ and F^- ions. Each F^- ion must have 1 K⁺ ion so that the negative and positive charges are balanced. Therefore, the formula of potassium fluoride is KF. Note that it is KF and not K₁F₁, as the “1” one can be omitted.

Example:

Magnesium fluoride consists of Mg²⁺ and F^- ions. Each Mg2+ ion must have 2 F^- ions so that the negative and positive charges are balanced. Therefore, the formula of magnesium fluoride is MgF₂. Note that is MgF₂ and not MgF2.

Example:

Copper (II) hydroxide consists of Cu²⁺ and OH^- ions. Each Cu²⁺ ion must have 2 OH^- ions so that the negative and positive charges are balanced. Therefore, the chemical formula for copper (II) hydroxide is Cu(OH)₂.

Naming ionic compounds:

The ammonium ion or metal ion is always written first in the formula and in the name.

Eg: NaCl not ClNa

It is sodium chloride and not chloride sodium

Eg: NH₄Cl not ClNH₄

It is ammonium chloride and not chloride ammonium

When there is more than 1 polyatomic ion in the compound, the ion is placed in brackets.

Correct: Cu(OH)₂

Wrong: CuO₂H₂

Wrong: CuOH₂

Constructing chemical equations:

During a chemical reaction, new substances are formed from some starting substances. An equation can be used to represent this formation. The equation will show the starting substances (reactants) and the new substances formed (products). When an equation is written in words, it is called word equation. When an equation is written with symbols and formulae, it is called chemical equation.

Example:

Word equation: hydrogen + oxygen -> water

Chemical Equation: 2H₂ + O₂ -> 2H₂O

Balancing Equation:

All chemical equation must be balanced. A balanced equation has the same number of atoms of each element on both sides of the equation.

Example: 2H₂ + O₂ -> 2H₂O

2H₂ + O₂ :

H atoms = 4

O atoms = 2

2H₂O:

H atoms = 4

O atoms = 2

Note that in 2H₂, the “2” in front of “H₂” means 2 molecules of hydrogen, whereas in “H₂”, the “2” below means each hydrogen molecule has 2 hydrogen atoms.