b) radioactivity

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Radioactivity

7.2)

Describe the structure of an atom in terms of protons, neutrons and electrons

and use symbols such as 146C to describe particular

nuclei.

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 or gain electrons. When they do, they form

charged particles called ions:

If an atom loses

one or more electrons, it becomes a positively

charged ion

If and atom gains one or more electrons, it becomes a negatively charged ion

7.3)

Understand the terms atomic (proton) number, mass (nucleon) number and isotope.

The atomic number of an element tells us how

many protons each of its atoms contains.

The atomic mass is the total number of

protons and neutrons in the nucleus of an atom

Isotopes

are

atoms of an element with different numbers of neutrons but the same protons.

Therefore they have the same atomic number but different mass number.

7.4)

Understand that alpha and beta particles and gamma rays are ionising radiations

emitted from unstable nuclei in a random process.

Isotopes affect the stability of a nucleus. Too many

or too few neutrons will make the nucleus unstable. An unstable nucleus will

eventually decay.

When unstable nuclei decay they give out ionising

radiation. Ionising radiation causes atoms to gain or loose electric charge,

forming ions. Unstable nuclei decay at random (we cannot predict when decay

will happen).

Alpha radiation

Alpha radiation consists of fast-moving particles

that are thrown out of unstable nuclei when they decay (alpha particles).

Alpha particles are helium nuclei (helium atoms without

their orbiting electrons)

Alpha particles have a short range (the range of

ionising radiation is the distance it can travel through matter) – alpha

particles can only travel a few centimetres in air and cannot penetrate more

than a few millimetres of paper.

They have a limited range because they interact with atoms along their paths, causing ions to form.

The relative charge of an alpha particle is +2

Beta radiation

Beta particles are very fast moving electrons that

are ejected by a decaying nucleus.

The result of radioactive decay is to change the

balance of protons and neutrons in the nucleus to make it more stable – beta

decay involves a neutron in the nucleus splitting into a proton and an electron

– the proton remains in the nucleus and the electron is ejected at high speed

as the beta particle.

They have a greater range than alpha particles – they

interact with matter in their paths less frequently (they are smaller and carry

less charge) – they can travel long distances through air, pass through paper

easily and are only absorbed by denser materials like a millimetre or two of

aluminium

The

relative charge of a beta particle is -1

Gamma rays

Gamma rays are electromagnetic waves with very short

wavelengths

They therefore have no mass and no charge

They are weakly ionising and only interact

occasionally with atoms in their paths

They are extremely penetrating and can pass through

all but the very densest materials with ease – it takes several centimetres

thickness of lead or a metre of so of concrete to stop gamma radiation

Gamma

radiation is emitted in ‘packets’ of energy called photons

As ionising radiation passes through matter, its

energy is absorbed. This means that radiation can only penetrate matter up to a

certain thickness. This depends on the type of radiation and the density of the

material that it is passing through.