Unit 4 updated

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Unit 4 updated
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

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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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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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If a covalent compound has a name which gives a clue to its formula,
then use the meaningful name. If not use a bonding picture
approach.
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.…read more

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Leave out any 1's in the formula.
e.g. PCl3.
Remember:
The valencies go below the symbols of the elements.
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.…read more

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Energy is required to overcome forces of attraction. A lot of energy
is required to break covalent bonds.
Vocabulary
Bonding - the join between different atoms.
Covalent Bonding - a bond formed between non-metals by the
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.…read more

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This is
because the electron clouds repel each other and try to get as far
away from each other as possible.
This explains why methane molecules are tetrahedral in shape and
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.…read more

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A covalent bond is formed by the sharing of electrons in the
electron clouds of an element e.g. Hydrogen and Oxygen combine
to form water.
The chemical formula gives the numbers and type of atoms of each
element in a molecule. The numbers of atoms of each element in
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).…read more

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