# 5.4 Writing formulae  (Page 2/2)

 Page 2 / 2

In the case of ionic compounds, the valency of an ion is the same as its charge (Note: valency is always expressed as a positive number e.g. valency of the chloride ion is 1 and not -1). Since an ionic compound is always neutral , the positive charges in the compound must balance out the negative. The following are some examples:

Write the chemical formula for potassium iodide.

1. Potassium iodide contains potassium and iodide ions.

2. Potassium iodide contains the ions $\mathrm{K}{}^{+}$ ( $\mathrm{valency}=1$ ; $\mathrm{charge}=+1$ ) and $\mathrm{I}{}^{-}$ ( $\mathrm{valency}=1$ ; $\mathrm{charge}=-1$ ). In order to balance the charge in a single molecule, one atom of potassium will be needed for every one atom of iodine.

3. The chemical formula for potassium iodide is therefore:

$\mathrm{KI}$

Write the chemical formula for sodium sulphate.

1. Sodium sulphate contains sodium ions and sulphate ions.

2. $\mathrm{Na}{}^{+}$ (valency = 1; charge = +1) and $\mathrm{SO}{}_{4}^{2-}$ ( $\mathrm{valency}=2$ ; $\mathrm{charge}=-2$ ).

3. Two sodium ions will be needed to balance the charge of the sulphate ion. The chemical formula for sodium sulphate is therefore:

$\mathrm{Na}{}_{2}\mathrm{SO}{}_{4}$

Write the chemical formula for calcium hydroxide.

1. Calcium hydroxide contains calcium ions and hydroxide ions.

2. Calcium hydroxide contains the ions ${\mathrm{Ca}}^{2+}$ ( $\mathrm{charge}=+2$ ) and $\mathrm{OH}{}^{-}$ ( $\mathrm{charge}=-1$ ). In order to balance the charge in a single molecule, two hydroxide ions will be needed for every ion of calcium.

3. The chemical formula for calcium hydroxide is therefore:

$\mathrm{Ca\left(OH\right)}{}_{2}$

Notice how in the last example we wrote ${\mathrm{OH}}^{-}$ inside brackets. We do this to indicate that ${\mathrm{OH}}^{-}$ is a complex ion and that there are two of these ions bonded to one calcium ion.

## Chemical formulae

1. Copy and complete the table below:
 Compound Cation Anion Formula $\mathrm{Na}{}^{+}$ $\mathrm{Cl}{}^{-}$ potassium bromide $\mathrm{Br}{}^{-}$ $\mathrm{NH}{}_{4}^{+}$ $\mathrm{Cl}{}^{-}$ potassium chromate $\mathrm{PbI}$ potassium permanganate calcium phosphate
2. Write the chemical formula for each of the following compounds:
1. hydrogen cyanide
2. carbon dioxide
3. sodium carbonate
4. ammonium hydroxide
5. barium sulphate

## Chemical compounds: names and masses

In Giving names and formulae to substances the names of chemical compounds was revised. The relative molecular mass for covalent molecules is simply the sum of the relative atomic masses of each of the individual atoms in that compound. For ionic compounds we use the formula of the compound to work out a relative formula mass. We ignore the fact that there are many molecules linked together to form a crystal lattice. For example $\mathrm{NaCl}$ has a relative formula mass of $58\phantom{\rule{3pt}{0ex}}\mathrm{g}·{\mathrm{mol}}^{-1}$ .

## Summary

• A chemical bond is the physical process that causes atoms and molecules to be attracted together and to be bound in new compounds.
• Atoms are more reactive , and therefore more likely to bond, when their outer electron orbitals are not full. Atoms are less reactive when these outer orbitals contain the maximum number of electrons. This explains why the noble gases do not combine to form molecules.
• When atoms bond, electrons are either shared or exchanged.
• Covalent bonding occurs between the atoms of non-metals and involves a sharing of electrons so that the orbitals of the outermost energy levels in the atoms are filled.
• The valency of an atom is the number of electrons in the outer shell of that atom and valence electrons are able to form bonds with other atoms.
• A double or triple bond occurs if there are two or three electron pairs that are shared between the same two atoms.
• A dative covalent bond is a bond between two atoms in which both the electrons that are shared in the bond come from the same atom.
• Lewis and Couper notation are two ways of representing molecular structure. In Lewis notation, dots and crosses are used to represent the valence electrons around the central atom. In Couper notation, lines are used to represent the bonds between atoms.
• An ionic bond occurs between atoms where the difference in electronegativity is greater than 1,7. An exchange of electrons takes place and the atoms are held together by the electrostatic force of attraction between oppositely-charged ions.
• Ionic solids are arranged in a crystal lattice structure.
• Ionic compounds have a number of specific properties , including their high melting and boiling points, brittle nature, the lattice structure of solids and the ability of ionic solutions to conduct electricity.
• A metallic bond is the electrostatic attraction between the positively charged nuclei of metal atoms and the delocalised electrons in the metal.
• Metals also have a number of properties, including their ability to conduct heat and electricity, their metallic lustre, the fact that they are both malleable and ductile, and their high melting point and density.
• The valency of atoms, and the way they bond, can be used to determine the chemical formulae of compounds.

## End of chapter exercises

1. Explain the meaning of each of the following terms
1. Valency
2. Covalent bond
2. Which ONE of the following best describes the bond formed between an $\mathrm{H}{}^{+}$ ion and the $\mathrm{NH}{}_{3}$ molecule?
1. Covalent bond
2. Dative covalent (coordinate covalent) bond
3. Ionic Bond
4. Hydrogen Bond
3. Which of the following reactions will not take place? Explain your answer.
1. $\mathrm{H}+\mathrm{H}\to {\mathrm{H}}_{2}$
2. $\mathrm{Ne}+\mathrm{Ne}\to {\mathrm{Ne}}_{2}$
3. $\mathrm{Cl}+\mathrm{Cl}\to {\mathrm{Cl}}_{2}$
4. Draw the Lewis structure for each of the following:
1. calcium
2. iodine (Hint: Which group is it in? It will be similar to others in that group)
3. hydrogen bromide ( $\mathrm{HBr}$ )
4. nitrogen dioxide ( $\mathrm{NO}{}_{2}$ )
5. Given the following Lewis structure, where X and Y each represent a different element...
1. What is the valency of $\mathrm{X}$ ?
2. What is the valency of $\mathrm{Y}$ ?
3. Which elements could $\mathrm{X}$ and $\mathrm{Y}$ represent?
6. A molecule of ethane has the formula $\mathrm{C}{}_{2}\mathrm{H}{}_{6}$ . Which of the following diagrams (Couper notation) accurately represents this molecule?
7. Potassium dichromate is dissolved in water.
1. Give the name and chemical formula for each of the ions in solution.
2. What is the chemical formula for potassium dichromate?

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