Atomic Nuclei 4

Nuclear physics is the branch of physicswhich deals with the nucleus of the atom. Within this field,

some scientists focus their attention on lookingat the particles inside the nucleus andunderstanding

how they interact, whileothers classify and interpret the properties of nuclei. This detailedknowledge

of the nucleus makes itpossible for technological advances to be made. In this next chapter, we are

going to touch on eachof these different areas within the field of nuclearphysics.

See introductory video:VPhtu at http://www.everythingscience.co.za

4.1 Nuclear structure and stability ESBAD

You will remember fromgrade 10 that an atom is made up of different types of particles, each with

a different charge: protons (positive charge) neutrons (neutral) and electrons (negative charge). The

nucleus is the part of the atom that contains the protons and the neutrons, while the electrons are

found in energy orbitalsaround the nucleus. The protons and neutronstogether are called nucleons.

It is the nucleus that makes up most of an atom’s atomic mass, because an electron has a very small

mass when compared with a proton or a neutron.

Within the nucleus, there are different forces which act between the particles. Thestrongnuclearforce

is the force between twoor more nucleons, and this force binds protons and neutrons together inside

the nucleus. The electromagnetic force causes the repulsion between like-charged (positive) protons.

In a way then, these forces are trying to produce opposite effects in thenucleus. The strong nuclear

force acts to hold all theprotons and neutrons close together, while theelectromagnetic force acts to

push protons further apart. In atoms where thenuclei are small, the strong nuclear force overpowers

the electromagnetic force. However, as the nucleus gets bigger (in elements with a higher number of

nucleons), the electromagnetic force becomes greater than the strong nuclear force. In these nuclei,

it becomes possible forparticles and energy tobe ejected from the nucleus. These nuclei are called

unstable. The particles and energy that a nucleus releases are referred to as radiation, and the atom is

said to be radioactive. We are going to look at these concepts in moredetail in the next few sections.

4.6 The Dangers of Radiation

Radioactivity was first discovered in 1896 by aFrench scientist called Henri Becquerel while hewas

working on phosphorescent materials. He wrapped a photographic plate in black paper and placed

various phosphorescentsubstances on it. Whenhe used uranium salts he noticed that the film black-

ened even if it was keptin a dark room. He eventually concluded that some rays must be comingout

of the uranium crystals to produce this effect andthat these rays were ableto pass through the paper.

FACT

Due to the high levels

of radioactivity, Marie

Curies papers from the

1890’s are considered

too dangerous to han-

dle. Even her cookbook

is highly radioactive.

These documents are

kept in lead-lined boxes,

and those who wish to

consult them must wear

protective clothing.

His observations were taken further by the Polishscientists Marie and Pierre Curie, who increasedour

knowledge of radioactive elements. In 1903, Henri, Marie and Pierre were awarded the Nobel Prize in

Physics for their work onradioactive elements. This award made Marie thefirst woman ever to receive

a Nobel Prize. Marie and Pierre Curie went on to discover two newelements, which they named

polonium (Po) after Marie’s homecountry, and radium (Ra) after its highly radioactive characteristics.

For these discoveries, Marie was awarded a Nobel Prize in Chemistry in1911, making her one of very