3-2 Radioactivity and the nucleus (10.2 pg )

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.