Unit 4 Chemistry

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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 compounds. The next three sections (4, 5 and 6)
concentrate almost totally on covalent compounds. Covalent
compounds can be distinguished from ionic compounds
experimentally (this will be done in later sections (. They can also be
distinguished on paper if we know the elements which are
chemically joined together to form the compounds.
Ionic Compound: An ionic compound is a metal element chemically
joined to a non- metal element.
Example: Sodium Chloride is a compound consisting of a metal
(sodium) and a non-metal (chlorine), thus since this is a non-metal
chemically joined to a non-metal it is an ionic compound.
Covalent Compound: A covalent compound is two or more
non-metals chemically joined to each other.
Example: Hydrogen Chloride is a compound consisting of a
non-metal (hydrogen) and another non-metal (chlorine), thus since
this is a non-metal chemically joined to a non-metal, it is a covalent
compound.
Groups of Atoms
We say atoms are bonded together when they are joined together.
A molecule is a group of atoms joined together by covalent bonds.
A diatomic molecule is a molecule containing two atoms.
Covalent bonding is found in molecules.
The chemical formula gives the number of atoms of each element in
a molecule.

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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 ( - ) two connections holding a pair of shells
together represents a double covalent bond which is represented
as
( = ).…read more

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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 bonds (further apart than
hexagon rings).
Physical Properties: Lubricant, ('lead' for pencils, has a high melting
point (graphite fibers) and conducts electricity.
Structure: The carbon atoms are arranged in parallel layers of
hexagons.…read more

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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 4.
Penta (e.g. phosphorus pentachloride) means 5.
Writing Formula in Everyday Chemistry
The number of bonds which an atom forms is equal to the number
of 'extra' electrons which an atom requires to become stable.…read more

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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 combine by ionic bonding - not covalent.
However the formula of simple ionic compounds can be written
using the valency and 'cross-over' method.…read more

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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 to form molecules.
Atoms are held together by chemical bonds.
When atoms combine, 2 half-filled electron clouds overlap to form a
covalent bond.
A two atom molecule is called a diatomic molecule.…read more

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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 called ionic
bonding.
The type of bonding between non-metals is called covalent bonding.…read more

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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 meaningful names. E.g. Carbon monoxide - CO (the 'cross-over'
method should not be used).
The formulae of some covalent compounds can be determined
using the 'cross-over' method.
E.g. Carbon Hydride - CH4.…read more

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