ATOMS, ELEMENTS AND COMPOUNDS
Everything is made of atoms and there are just over 100 different atoms
known. These are all shown on the periodic table where they arranged in
groups with similar properties. An atom is the smallest part of a substance
that can exist on it’s own. 7 million atoms joined together in a straight line
would be about 1mm long. Each atom has its own unique symbol and can
exist as a solid, liquid or gas depending on the temperature.
All atoms have a nucleus (the big bit in the middle). The nucleus contains
protons (which have a positive charge) and neutrons (which have no charge).
For any atom, the number of protons is the same as the number of electrons
(which have a negative charge) and so atoms are electrically neutral overall. If
an atom loses or gains electrons it becomes charged and is called an ion.
This is a picture of a sodium atom. It has 11 protons, 11 electrons and 12
neutrons.
An element is a substance which is made of only one type
of atom
Compounds are two or more different elements joined
together by chemical bonds.
TOPIC 10.1.1: FUNDAMENTAL IDEAS
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Mass number
The number of protons plus the number of neutrons is called the mass
number.
Fluorine has 9 protons and 10 neutrons. The mass number is 9 + 10 = 19.
The mass number is the total number of particles in the nucleus.
Electron shells
Electrons in atoms occur in shells (shown as circles around the nucleus).
The shells can also be called energy levels. The maximum number of
electrons in each shell, going from the middle to the outside, is 2, 8, 8, 18.
Below is a picture of a potassium atom. Its electron structure is 2, 8, 8, 1.
An atom which has the maximum number of electrons in its outer shell
will be stable. It has achieved the electronic structure of a nobel gas and this
means that it will not react with other atoms.
If the outer shell of an atom has less than its maximum number of electrons
(see potassium above) then it will not be stable. It will react with other atoms
to get a full outer shell.
The number of electrons in the outer shell of an atom is the same as the
group number on the periodic table., so all elements in the same group will
have similar chemical properties. Lithium, sodium and potassium all have one
electron in their outer shell and all react in a similar way.
TOPIC 10.1.1: FUNDAMENTAL IDEAS
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Chemical reactions
All chemical reactions involve atoms trying to get a full outer shell of electrons.
The total number of atoms is the same before and after the reaction.
When an atom reacts with another atom to form a chemical bond, it will either
1. Lose electrons to form a stable positive ion.
2. Gain electrons to form a stable negative ion.
or 3. Share electrons to form a stable molecule.
Metals lose electrons to form positive ions whilst non-metals gain electrons to
form negative ions. These oppositely charged ions then attract each other and
a compound is formed. The strong forces of attraction between these ions are
called IONIC BONDS.
Magnesium is in group 2 of the periodic table.
A magnesium atom will lose 2 electrons to form a stable 2+ ion.
Chlorine is in group 7 of the periodic table.
A chlorine atom will gain 1 electron to form a stable 1- ion.
Two chlorine atoms will each gain one electron from the magnesium atom.
The atoms become ions.
The attraction between the oppositely charged ions forms the ionic bond
between magnesium and chlorine. The formula for magnesium chloride is
MgCl2 since two chlorine atoms were needed to accept one electron each
from the magnesium which was losing two electrons.
magnesium + chlorine
Mg(s) + Cl2(g)
TOPIC 10.1.1: FUNDAMENTAL IDEAS
magnesium chloride.
MgCl2(s)
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When non-metals react with non-metals they share electrons and form a
covalent bond. Covalent means sharing. If you do not know whether an
element is a metal or a non-metal, see the periodic table.
A chlorine atom has 7 electrons in its outer shell. Chlorine is in group 7 of the
periodic table. Two chlorine atoms will each share one electron to get a full
outer shell and form a stable Cl2 molecule.
Each chlorine atom now has 8 electrons in its outer shell by sharing the two
electrons where the shells touch. This is a single covalent bond and can also
be represented as Cl2 or
Cl-Cl
where – represents a covalent bond.
TOPIC 10.1.1: FUNDAMENTAL IDEAS
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Chemical formulae
The chemical formula of a compound tells us the ratio of each type of atom (or
ion) in the compound. (See MgCl2 or Cl2 above). These can be worked out
quickly and simply for ionic compounds using the table of ions on the back of
your periodic table which you will always have in an exam.
E.g. aluminium oxide is Al2O3 since the ions involved are Al3+ and O2-. In
order that the charges balance (and the formula be neutral overall) you need
2 x Al3+ (6 positives) and 3 x O2- (6 negative).
Balancing equations
During a chemical reaction mass can not be lost or gained so the atoms or
ions simply get re-arranged. No atoms are created or destroyed. Think of a
reaction in terms of LEGO. You are simply disconnecting the lego blocks to
re-build them in a different way. You have the same number of each type of
atom before and after a chemical reaction.
Eg When hydrogen gas and oxygen gas react together they produce water.
H2 + O2 =
H2O
does not balance
When balancing equation you can not change the formula of a substance by
adding subscript numbers. You can put numbers in front of the formula in
order to have more than one of the species.
In this example we have not used one of the oxygen atoms that we had at the
start. To use the second oxygen we would need to make more water
H2 + O2 = 2H2O
still does not balance
Now we have more hydrogen atoms on the right than we had at the start. But
we can make this balance by putting a 2 in front of the H2 ie we need two H2
for every one O2.
2H2 + O2 = 2H2O
TOPIC 10.1.1: FUNDAMENTAL IDEAS
is balanced
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