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Atoms and isotopes
An atom is made from a nucleus surrounded by electrons. The nucleus contains protons and neutrons.
Isotopes are atoms that have the same number of protons, but different numbers of neutrons.
The nuclei of some isotopes are unstable. They emit radiation and break down to form smaller
In the nuclear model, atoms contain three subatomic particles called protons, neutrons and electrons.
Whilst the protons and neutrons are found in the centre of the atom, the electrons are arranged in
energy levels, or shells, around the nucleus.
Each subatomic particle has different particles (due to their relative masses and charges):
Protons have a relative mass of 1 and a charge of +1 (positive)
Neutrons have a relative mass of 1 and no charge (0 = neutral)
Electrons have a relative mass of almost zero (1/1370) and a charge of 1 (negative)
The number of electrons in an atom is always the same as the number of protons so atoms are
electrically neutral overall. Atoms can lose and gain electrons and when they do, they form charged
particles called ions:
If an atom loses one or more electrons, it becomes positively charged
If an atom gains one or more electrons, it becomes negatively charged
The number of protons in the nucleus of an atom is called its atomic number:
The atoms of a particular element all have the same number of protons
The atoms of different elements have different numbers of protons
The total number of protons and neutrons in an atom is called its mass number.
The mass number is put on the top left hand corner of the chemical symbol, whilst its atomic
number is placed in the bottom left corner.
When unstable isotopes split up or 'decay' and release radiation, they form different atoms with a
different number of protons.
When an atom gives off alpha or beta radiation, its nucleus changes. It becomes the nucleus of a
different element because the number of protons in the nucleus determines which element the atom
When there is alpha decay, the number of particles decreases by two when there is a change in
the number of protons and neutrons.
When there is beta decay, if there is a change in the number of protons, the number of particles
increases by 1, whilst if there is a change in the number of neutrons, the number of particles
decreases by 1.
Most of the background radiation on Earth is natural. Some sources include cosmic rays (from outer
space), animals, rocks (giving off radon gas), soil and plants.
Artificial sources include nuclear weapons and power stations and xrays. These make up to 15% of
the average background radiation dose.
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Nuclear fission and fusion
Nuclear reactors use a type of reaction called nuclear fission, which involves splitting atoms.
Uranium or plutonium isotopes are normally used because their relatively large nuclei that are easy
to split, especially when hit by neutrons.
When the nucleus is hit by a neutron:
It splits into two smaller nuclei, which are radioactive
Two or three more neutrons are released
Some energy is released.
If the additional neutrons released hit other uranium nuclei, causing them to split, causing a
chain reaction.…read more