Download Summary 1b.1 - Uddingston Grammar School

National 4/5
Summary Notes
Sub –Topic 1 b.1 : Elements, Compounds,
Mixtures, Atoms and the Periodic Table
Elements
 Although there are so many different substances, they are actually
made up of a few very basic substances.
 These basic substances are called elements.
 An element is a substance made of only one kind of atom.
 Its atoms are different from the atoms of other elements.
 Some elements exist as molecules – 2 atoms bonded together.
 Molecules consisting of 2 atoms are often described as diatomic.
 Elements existing naturally as diatomic molecules are hydrogen,
nitrogen, oxygen, fluorine, chlorine, bromine, iodine.
 A diagram of an element could look like this
or
Uses of Elements
 Different elements have different properties. This means that different
elements have different uses.
 Gold is used for jewellery because it does not corrode.
 Helium is used in balloons because it is lighter than air.
Chemical Symbols
 Each element has a name of its own.
 Each element has its own symbol.
 It can be one letter (capital) e.g carbon is C
 It can be two letters (first capital, second lower case) e.g. magnesium is Mg
 It can be two letters based on its latin name e.g. iron is Fe
 Symbols for the elements are found on the Periodic table and in the data
booklet.
Compounds
 When atoms of different elements combine together compounds are
formed.
 A compound is a substance formed when two or more elements have
chemically joined together.
 A diagram of a compound could look like this :- see below
Mixtures
Mixtures
 When we put substances together they may or may not react together.
 If they react together, they form a compound.
 A mixture is a mix of substances which have not reacted together.
 A diagram of a mixture could look like this
or




It is not easy to separate the elements of a compound from each other
because they have combined or joined with each other.
The atoms are firmly attached to each other.
It is much easier to separate the substances in a mixture because they
have not combined with each other.
Usually it is possible to separate the substances in a mixture by quite
simple means.
Separating Mixtures
 Mixtures can usually be separated, sometimes quite easily.
 Filtration is used to separate a solid from a liquid.

Evaporation is used to separate a mixture of a dissolved solid in a liquid.
Dissolving
 A solid which will dissolve in water is said to be soluble in water.
 A solid which will not dissolve in water is said to be insoluble in water.



A solid substance dissolves in a liquid to form a solution.
The solid which dissolves is called the solute.
The liquid which does the dissolving is called the solvent.
solute + solvent  solution

E.g.
sugar + water
 sugar solution

When a substance dissolves in a solvent to form a solution, no new
substance is formed – solutions are mixtures.
Concentration Of Solutions
 If there is not very much solute in the solution, we say that we have a
dilute solution.
 If there is a lot of solute dissolved in the solution, we say that we have a
concentrated solution.
 If there is so much solute dissolved in the solution that no more can
dissolve we say that we have a saturated solution.
 If we add more solute to a saturated solution it will not dissolve. It will
simply lie at the bottom of the beaker. Even stirring will not make any
more dissolve. The solution can take no more of that solute.
Elements
 Everything in the world is made up of about 100 simple substances called
elements.
 Elements cannot be made into anything simpler.
 All the elements are found in the Periodic Table.
 They are listed in a special way and this allows us to find out about
different elements.
 Each horizontal row of elements makes up a period.
 Each vertical column of elements makes up a group.





Elements in the same group of the Periodic Table have similar properties,
that is, their chemical behaviour is similar.
The alkali metals are group 1 – these are very reactive metals
The Halogens are group 7 – these are quite reactive non-metals.
The Noble Gases are group 0 – these are very unreactive gases.
The Transition Metals are the block in the middle – they are not in a
group.
group 0
group 1
group 7
Different Types Of Elements
1.
Metals and Non-metals
Elements can be classified as metal or non-metal.
Metals
usually shiny
good conductors of
electricity and heat
Non-metals
usually dull
NOT good conductors of
electricity and heat
On the Periodic Table there is a black zig zag line on the chart. This
line separates metal elements from non-metal elements. The metals
are on the left of the line, the non-metals are on the right.
Non metals
Metals
2.
Solids, Liquids & Gases
Elements can be classified as solids, liquids and gases.
3.
Natural and Man-made Elements
Most elements occur naturally in the world. A few do not – they did
not exist until they were made by scientists. These elements are manmade or synthetic. All the elements with Atomic Number greater than
92 (Uranium) are man-made.
More About Atoms
All elements have just one kind of atom in them.
Atoms are built of three kinds of even smaller particles – protons, electrons
and neutrons.
Name
proton
electron
neutron
Symbol
p
e
n
Charge
+1
-1
0
Mass (amu)
1
1/2000
1




Protons are positively charged
and found in the nucleus.
Neutrons have no charge
(neutral) and are found in the
nucleus.
Electrons are negatively charged
and are found outside the
nucleus.
An atom is neutral overall since
the number of positive protons
equals the number of negative
electrons.
Electron Arrangement
 We know that electrons are found outside the nucleus.
 They are arranged around the nucleus in different levels of energy.
 The level closest to the nucleus has the lowest energy.
 Electrons always go to the lowest energy level where there is space.

The electron arrangement of any atom can be worked out if we know how
many electrons can fit into each level.
Level Number
Energy
1
2
3
lowest
higher
higher still
Maximum number of
electrons
2
8
8
any extra electrons go in here





Lithium has atomic
number 3.
It has 3 electrons.
The first 2
electrons go into
the 1st level.
The next electron
goes into the 2nd
level.
The electron
arrangement is 2,1.





Carbon has atomic
number 6.
It has 6 electrons.
The first 2
electrons go into
the 1st level.
The next 4
electrons go into
the 2nd level.
The electron
arrangement is 2,4.






Chlorine has atomic
number 17.
It has 17 electrons.
The first 2 electrons
go into the 1st level.
The next 8 electrons
go into the 2nd level.
The next 7 electrons
go into the 3rd level.
The electron
arrangement is 2,8,7.
The electron arrangements of the first 20 elements are :
Why The Group Is Important
 Elements in the same group have the same number of outer electrons.
Atomic Number
 The Atomic Number of an element tells us the number of protons in the nucleus of its
atoms.
 It is the number of protons that make an element what it is.
 Since an atom is neutral it also tells us the number of electrons.
Atomic number = number of protons = number of electron




Elements from the same Group all behave like each other. They behave
like each other because they have the same number of electrons in their
outer energy levels. It is the number of electrons in the outer shell that
makes an element behave the way it does.
The Noble gases have filled outer energy levels
They are also very unreactive.
This suggests that a filled outer energy level is the most stable
arrangement.
Atomic Number
 The Atomic Number of an element tells us the number of protons in the
nucleus of its atoms.
 It is the number of protons that make an element what it is.
 Each element has a different atomic number, therefore each element
has a different number of protons in its nucleus.
 Since an atom is neutral it also tells us the number of electrons.
Atomic number = number of protons = number of electrons.
Mass Number
The mass number of an element is the number of protons PLUS neutrons in the
nucleus of an atom.
Mass number = number of protons + number of neutrons
National 5
Notation
There is a special way of writing the symbol of an element to show the atomic
number and the mass number.
mass number
symbol
atomic number
This atom has
23
Na
11
11 protons (the same as the atomic number)
11 electrons (the same as the number of protons)
12 neutrons (mass number - atomic number)
Ions
 Atoms sometimes lose or gain electrons to try and have a full outer
energy level of electrons.
 When this happens, the number of electrons does not equal the number
of protons and the particle now has a positive or negative charge.
 These particles are called ions.
 An ion can be formed when an atom :1. Gains electrons – to form a negative ion.
e.g.
15
8
2-
Protons
Electrons
Neutrons
2. Loses electrons – to form a
e.g.
27
3+
Protons
Al
Electrons
13
Neutrons
=8
= 10
=7
positive ion
= 13
= 10
= 14

Atoms form ions in order to achieve a noble gas electron arrangement.

This means they get a full outer shell of electrons.

A full outer shell of electrons is very stable.

E.g. A sodium atom has an electron arrangement of 2,8,1.

If it loses one electron it will have a full outer shell.

It now has an electron arrangement of 2,8.

It is now an ion and would now be written as Na
electron.
+
as it has lost one

E.g. A chlorine atom has an electron arrangement of 2,8,7.

If it gains 1 electron it will have a full outer shell.

It now has an electron arrangement of 2,8,8.

It is now an ion and would be written as Cl- as it has gained one electron.
Atoms And Isotopes
 Atoms with the same number of protons but a different number of
neutrons are called isotopes.
 Isotopes have the same atomic number but different mass numbers.
Most elements occur as a mixture of isotopes.
 E.g

Relative Atomic Mass is the average mass of all the isotopes of an
element.

It is rarely a whole number since it is a calculated average mass of the
isotopes.