Parte B - Il mondo dell'atomo
Le particelle dell’atomo
- 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.