# 5.4 Writing formulae

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## The formulae of covalent compounds

To work out the formulae of covalent compounds, we need to use the valency of the atoms in the compound. This is because the valency tells us how many bonds each atom can form. This in turn can help to work out how many atoms of each element are in the compound, and therefore what its formula is. The following are some examples where this information is used to write the chemical formula of a compound.

Write the chemical formula for water

1. A molecule of water contains the elements hydrogen and oxygen .

2. The valency of hydrogen is 1 and the valency of oxygen is 2. This means that oxygen can form two bonds with other elements and each of the hydrogen atoms can form one.

3. Using the valencies of hydrogen and oxygen, we know that in a single water molecule, two hydrogen atoms will combine with one oxygen atom. The chemical formula for water is therefore:

$\mathrm{H}{}_{2}\mathrm{O}$ .

Write the chemical formula for magnesium oxide

1. A molecule of magnesium oxide contains the elements magnesium and oxygen .

2. The valency of magnesium is 2, while the valency of oxygen is also 2. In a molecule of magnesium oxide, one atom of magnesium will combine with one atomof oxygen.

3. The chemical formula for magnesium oxide is therefore:

$\mathrm{MgO}$

Write the chemical formula for copper (II) chloride.

1. A molecule of copper (II) chloride contains the elements copper and chlorine .

2. The valency of copper is 2, while the valency of chlorine is 1. In a molecule of copper (II) chloride, two atoms of chlorine will combine with one atom of copper.

3. The chemical formula for copper (II) chloride is therefore:

$\mathrm{CuCl}{}_{2}$

## The formulae of ionic compounds

The overall charge of an ionic compound will always be zero and so the negative and positive charge must be the same size. We can use this information to work out what the chemical formula of an ionic compound is if we know the charge on the individual ions. In the case of $\mathrm{NaCl}$ for example, the charge on the sodium is $+1$ and the charge on the chlorine is $-1$ . The charges balance ( $+1-1=0$ ) and therefore the ionic compound is neutral. In $\mathrm{MgO}$ , magnesium has a charge of $+2$ and oxygen has a charge of $-2$ . Again, the charges balance and the compound is neutral. Positive ions are called cations and negative ions are called anions .

Some ions are made up of groups of atoms, and these are called compound ions . It is a good idea to learn the compound ions that are shown in [link]

 Name of compound ion formula Name of compound ion formula Carbonate $\mathrm{CO}{}_{3}^{2-}$ Nitrite ${\mathrm{NO}}_{2}^{-}$ Sulphate ${\mathrm{SO}}_{4}^{2-}$ Hydrogen sulphite ${\mathrm{HSO}}_{3}^{-}$ Hydroxide ${\mathrm{OH}}^{-}$ Hydrogen sulphate ${\mathrm{HSO}}_{4}^{-}$ Ammonium ${\mathrm{NH}}_{4}^{+}$ Dihydrogen phosphate ${\mathrm{H}}_{2}{\mathrm{PO}}_{4}^{-}$ Nitrate ${\mathrm{NO}}_{3}^{-}$ Hypochlorite ${\mathrm{ClO}}^{-}$ Hydrogen carbonate ${\mathrm{HCO}}_{3}^{-}$ Acetate (ethanoate) ${\mathrm{CH}}_{3}{\mathrm{COO}}^{-}$ Phosphate ${\mathrm{PO}}_{4}^{3-}$ Oxalate ${\mathrm{C}}_{2}{\mathrm{O}}_{4}^{2-}$ Chlorate ${\mathrm{ClO}}_{3}^{-}$ Oxide ${\mathrm{O}}^{2-}$ Cyanide ${\mathrm{CN}}^{-}$ Peroxide ${\mathrm{O}}_{2}^{2-}$ Chromate ${\mathrm{CrO}}_{4}^{2-}$ Sulphide ${\mathrm{S}}^{2-}$ Permanganate ${\mathrm{MnO}}_{4}^{-}$ Sulphite ${\mathrm{SO}}_{3}^{2-}$ Thiosulphate ${\mathrm{S}}_{2}{\mathrm{O}}_{3}^{2-}$ Manganate ${\mathrm{MnO}}_{4}^{2-}$ Phosphide ${\mathrm{P}}^{3-}$ Hydrogen phosphate ${\mathrm{HPO}}_{4}^{3-}$

what is the stm
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
Rafiq
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
Damian
How we are making nano material?
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What is meant by 'nano scale'?
What is STMs full form?
LITNING
scanning tunneling microscope
Sahil
how nano science is used for hydrophobicity
Santosh
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
Rafiq
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Mahi
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
Rafiq
what is Nano technology ?
write examples of Nano molecule?
Bob
The nanotechnology is as new science, to scale nanometric
brayan
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Damian
Is there any normative that regulates the use of silver nanoparticles?
what king of growth are you checking .?
Renato
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
why we need to study biomolecules, molecular biology in nanotechnology?
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
why?
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
anyone know any internet site where one can find nanotechnology papers?
research.net
kanaga
sciencedirect big data base
Ernesto
Introduction about quantum dots in nanotechnology
what does nano mean?
nano basically means 10^(-9). nanometer is a unit to measure length.
Bharti
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
absolutely yes
Daniel
how to know photocatalytic properties of tio2 nanoparticles...what to do now
it is a goid question and i want to know the answer as well
Maciej
Abigail
for teaching engĺish at school how nano technology help us
Anassong
How can I make nanorobot?
Lily
Do somebody tell me a best nano engineering book for beginners?
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NANO
how can I make nanorobot?
Lily
what is fullerene does it is used to make bukky balls
are you nano engineer ?
s.
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
Tarell
what is the actual application of fullerenes nowadays?
Damian
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
Tarell
The fundamental frequency of a sonometer wire streached by a load of relative density 's'are n¹ and n² when the load is in air and completly immersed in water respectively then the lation n²/na is
Properties of longitudinal waves