Capitolo Parte B - Il mondo dell'atomo

Le particelle dell’atomo

Summing-up

A ■ The electrical nature of matter

The electrical nature of matter can be directly experienced, and it can be demonstrated that:

the electric charge can be of two types: positive (+) and negative (−);
charges of the same sign repel and charges of opposite sign attract;
a body is electrically neutral when it has as many positive as negative electric charges;
the rubbing causes the transfer from one body to another of negative electrical charges, termed electrons.

Research in the 20th century identified the electron as a fundamental constituent of the atom.

B ■ The fundamental particles

The three fundamental particles in order of mass are neutron, proton and electron.
Atoms consists of a dense central nucleus comprised of neutrons and protons, the nucleons, surrounded by a cloud of negatively charged electrons.

C ■ Rutherford’s experiment

Rutherford, Geiger and Marsden used α-particles (helium nuclei) to investigate the atomic structure of gold atoms.
Rutherford’s atomic model:
- the positive charge and the mass of an atom are concentrated in a nucleus, the diameter of the nucleus being 100 000 times smaller than that of the atom;
- the much lighter electrons occupy the empty space around the nucleus;
- electrons move around the nucleus like the planets around the Sun;
- the number of electrons is equal to that of the protons, the atom being neutral.

D ■ Atomic number identifies elementst

The atomic number (Z) indicates the number of protons in the atom.
The mass number (A) is the sum of protons (Z) and neutrons (nº) in the atomic nucleus.
Isotopes are atoms of the same element that have the same chemical properties but different masses as they contain different numbers of neutrons.
The relative atomic mass, stated in the periodic table, is the average mass or weighted average.

E ■ Nuclear transformations

Radioactive decay transforms the nucleus of an element into the nucleus of a different element, the process of emission of radiation is called radioactivity.
The emission of radiation, termed radioactivity, has three forms: α-rays consist of helium nuclei, with 2 + charge and mass number 4; β-rays are beams of fast electrons; and γ-rays are electromagnetic radiation, like light and X-rays, but of higher energy.
All nuclei with Z ≥ 84 are unstable and therefore radioactive.
The law of radioactive decay: the half-life of a radioactive isotope is the time taken for its amount to reduce by half.

F ■ Nuclear Energy, Nuclear Fission and Nuclear Fusion

In science, nuclear energy is the energy required to separate nucleons.
The relationship between energy and mass is given by E = m·c ² i.e. E and m are directly proportional.
The energy involved in a nuclear transformation corresponds to the difference in energy associated with the nuclei of the products and the nuclei of the reactants.
Nuclear fission occurs when a heavy nucleus splits into two smaller nuclei of similar mass.
In nuclear fusion two light nuclei combine to produce a heavier one.

Summing-up. Atomic Particles.