3-2 Radioactivity and the nucleus

(10.2 pg 280-283)

• At first the atom was thought to be a solid ball but then when electrons were discovered it was thought to be like plum pudding with the negative electrons embedded in a positive atom (Fig.1 p.280).

• Ernest Rutherford conducted a very famous experiment, where he shot a positively charged particles at a piece of gold foil (‘the gold foil experiment’ – Fig.2 p.280).

• He found that while most particles made it through the foil, some were deflected, and others bounced back towards the source of emission.

• He concluded that most of the atom is made of ‘empty space’, and that the core of the atom carried a positive charge (he called the core nucleus and the positive charges the protons).

• He also suggested that electrons orbit the nucleus like planets orbit the Sun (Fig.3 p.281).

• In 1932 James Chadwick discovered that the nucleus also contains a neutral component called the neutron.

• As you know, it has almost the same mass as the proton (which is 1800 times heavier than the electron), but is neutral.

• It was later discovered that all atoms of the same element have the same number of protons, but can have different amounts of neutrons.

• Atoms of the same element with a different number of neutrons are called isotopes.

• Isotopes have different mass numbers!

• To show the isotope of an atom we use their symbol, mass number (number of protons + neutrons) and atomic number (number of protons). Please see Fig.5 p.282.