Download 10.2

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.