Elements, Compounds
and Mixtures
Year 8 Chemistry
Elements and Atoms
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An element:
is an absolutely pure substance that cannot be
broken down into other substances.
An atom:
is the smallest unit that an element can be
divided into so that all units are the same.
Elements, Compounds and
Mixtures
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Video
Elements, Compounds and Mixtures
An Element:
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consists of only one kind of atom,
cannot be broken down into a simpler type of matter by
either physical or chemical means, and
can exist as either atoms (e.g. argon) or molecules (e.g.,
nitrogen).
A molecule consists of two or more atoms of the same
element, or different elements, that are chemically bound
together. Note that the two nitrogen atoms which
comprise a nitrogen molecule move as a unit.
Elements, Compounds and Mixtures
A Compound:
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consists of atoms of two or more different
elements bound together,
can be broken down into a simpler type of matter
(elements) by chemical means (but not by physical
means),
has properties that are different from its component
elements, and
always contains the same ratio of its component
atoms.
Elements, Compounds and Mixtures
A Mixture:
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consists of two or more different elements and/or
compounds physically intermingled,
can be separated into its components by physical means,
and
often retains many of the properties of its components.
Elements and Atoms
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The periodic table is a list of all of the
elements that have been discovered so far.
The Periodic Table
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The periodic table is used by chemists to
classify all of the elements, putting elements
with similar properties together.
The columns of a periodic table are called
groups
The rows of a periodic table are called periods
Video
Groups
Groups
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Group 8 elements are known as the noble
gases
Group 7 elements are known as the halogens
Group 1 elements are known as the alkali
metals.
The Periodic Table
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In each box on the periodic table a different
element is found
Each has a different combination of chemical
and physical characteristics.
The Periodic Table
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The four pieces of information that
can be found in each box are:
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The name of the element
The symbol used to represent the
element
The atomic number (the smaller number)
The relative atomic mass (the larger
number)
Atoms
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Imagine you are given a small piece of pure
gold
It has to be divided into small enough pieces to
give everyone in the school a little bit
You end up crushing the gold into the smallest
piece possible
This smallest piece is an atom
Atoms
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Atoms can only be
seen with very
powerful electron
microscopes
Compounds
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Just like lego blocks join together atoms join
together as well.
When different types of elements join together
we make compounds.
Small groups of atoms are called molecules.
Compounds
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Larger structures that are made up of many
atoms, with a repeating pattern, are called
lattices.
Compounds
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The properties of compounds are usually very
different. They can have different:
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Colours
Textures
Smells
Density
Compound Formula
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A compound or chemical formula is used to
describe how elements are combined together
For a molecular compound we show the exact
number of each element
H2O
CH4
CO2
Compound Formula
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For lattice compounds we show the ratio
between the elements
NaCl
SiO2
Fe2O3
Compound Formula
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How many of each type of element is in the
following molecules
H2SO4
H2S
CH3CH2OH
Compound Formula
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Write the chemical formula for molecules
with the following elements
1 hydrogen and 1 chlorine
12 carbon, 22 hydrogen and 11 oxygen
8 carbon and 18 hydrogen
Mixtures
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Even though a compound is made up of
different elements it is not a mixture
A mixture consists of:
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Two or more different compounds
One or more elements mixed with one or more
compounds
Two or more different elements