Download 3-2 Radioactivity and the nucleus

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 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.
PG 279 # 6, 8, 11
PG 283 # 1, 8, 9, 11-13