INSIDE THE ATOM
What are elements made from? The answer is very small particles called atoms. Each
element is made from its own type of atom. Although there are many different types of
atoms they all have some important features in common:
 Atoms have a small centre called a nucleus. The nucleus has a positive charge. This
is because it contains positively charged particles called protons. Neutral particles
called neutrons may also be present.
 Atoms have particles called electrons which move around the nucleus. Electrons
have a negative charge.
 Atoms are electrically neutral. This is because the positive charge of the nucleus is
„cancelled out“ by the negative charge of the electrons.
The periodic table is arranged so that it tells us not only about the properties of the
elements but also about the number of protons and alectrons in their atoms. We will now
look at three important features of the periodic table.
1
Each element has a special number which is called the atomic number. It
increases by one unit as you move from one element to the next.
2
The elements are arranged in groups and the elements in each group all have
similar properties. For example, all the elements in Group 0 are very unreactive
gases.
3
Why is it that all the elements in Group 1 are reactive metals? One reason is
that they all have the same number of electrons on the outside of their atoms.
The electrons an an atom are arranged in layers. The way an atom reacts depends on
the number of electrons in its outer layer. Since all the elements in Group 1 have just
one outer electron, they all react in similar ways. They have similar chemical
properties.
The number of outer electrons can be shown in two ways. First, there is a series of
numbers called the electron arrangement. The electron arrangement for sodium is
2,8,1. The number of outer electrons is the last number in the electron arrangement.
This means that sodium has one outer electron.
The second way in which the number of outer electrons is given is by the group
number. Every element in Group 1 has one outer electron, every element in Group 2 has
two outer electrons and so on.
Questions:
How many outer electrons are there in the following?
lithium
oxygen
neon
Group 0 (the noble gases) is called Group 8 in some older periodic tables. Why do you
think this is?
1
Match the terms in the left column with the definitions in the right column.
1 nucleus
2 neutrons
3 protons
4 electrons
5 atomic number
6 mass number
a) positively charged particles found in the nucleus.
b) negatively charged particles which move round the nucleus.
c) the centre of an atom. It has a positive charge.
d) neutral particles found in the nucleus.
e) the sum of the numbers of protons and neutrons in an atom.
f) the number of protons in an atom.
A BIT OF HISTORY
Rutherford in his Cambridge laboratory
How could you find out what was inside a container
lorry if all the doors were sealed? One way would be
to smash it open- it would be drastic, but it would
work! This was the kind of problem which faced
scientists when they started to try to find out what
was inside atoms. One ingenious solution was to use
large amounts of energy to separate the particles
which were inside. Most of this work was carried out
at Cambridge University.
One of the first experiments, in 1897, used
electricity to separate the electrons from a sample
of chlorine gas. The electrons were attracted to a
positive electrode- this is how it was discovered that
electrons are negatively charged. Later it was shown
that atoms also contain positive particles- these
were called protons.
However, no one knew how the electrons and protons were arranged in an atom. In
1911, Ernest Rutherford, using his special talents for good scientific guesswork and
high-powered mathematics, concluded that the atom must have the protons in a central
nucleus with the electrons arranged round the outside. Despite this successful
prediction, something was missing: the calculations for the mass of atoms did not add
up- the protons and electrons alone did not make up the total mass of the atom.
Then, in 1932, a third particle was discovered- the neutron. It had the same mass as a
proton but had no electrical charge – it was neutral.
From these and other discoveries, scientists were able to conclude that atoms consist
of a central nucleus containing the protons and neutrons, with the electrons arranged
outside the nucleus.
Find and underline the phrasal verbs in the text.
2