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Chemistry




Unit 4 - How Atoms are Held Together

This section introduces the idea of covalent bonding, leading to
molecules and chemical formulae.

Covalent Bonds and Molecules

Compounds are made up of atoms of more than one element. In
Chemistry, compounds can be identified as either covalent
compounds or ionic…

Page 2

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Chemistry




Carbon Monoxide is made up of diatomic molecule.
Non-metal atoms bond together to form molecules.

Structures

Models are always used initially to determine the full structural
formula.
When using the Molymods the spheres are held together by
connections. Each connection represents a single covalent bond
which is represented as…

Page 3

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Chemistry




Physical Properties: a) Very hard. b) High Melting Point. c) Does not
conduct electricity.
Structure: It exists as one giant network, where each carbon is
found tetrahedrally joined to 4 others by covalent bonds. These
bonds exist throughout the whole 3D structure.

- (Carbon) this consists of weak covalent…

Page 4

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Chemistry




Some chemical names give an indication in the formula;
e.g. carbon dioxide - this tells us that the molecule contains two
oxygen atoms.
Examples of prefixes:

Mono (e.g. carbon monoxide) means 1.
Di (e.g. sulphur dioxide) means 2.
Tri (e.g. sulphur trioxide) means 3.
Tetra (e.g. carbon tetrachloride) means…

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Chemistry




The final formula should contain small numbers to the bottom
right of the symbols.
Leave out the number 1 from any formula.
Cancel ell even numbers (except where you have used a name
with a clue i.e. a meaningful name) e.g. X2Y2 = XY.

More Formulae

Metals and non-metals…

Page 6

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Chemistry




sharing of a pair of electrons.
Molecule - a group of atoms held together by covalent bonds.
Diatomic Molecule - molecules containing two atoms.
Chemical Formula - gives the number and type of atoms in the
molecule.

Unit 4 - How Atoms are Held Together Summary

Atoms join together…

Page 7

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Chemistry




water molecules are bent. Similarly, the compound of nitrogen and
hydrogen, called ammonia, is pyramidal in shape.

How Atoms are Held Together - Facts to Know by the end of the
Unit

Atoms are held together by bonds.
The type of bonding between a metal and a non-metal is…

Page 8

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Chemistry




the molecule is indicated by a subscript after the symbol of the
element e.g. CH4 indicates 1 carbon atom and 4 hydrogen atoms
(the subscript 1 is not written).
The names of some covalent compounds contain a prefix. The
chemical formulae of such compounds can be determined from
their…

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