This topic accounts for approximately 8% of your exam marks.
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Stable8%
Properties of alpha, beta and gamma radiation and nuclear equations tested consistently.
What an atom is
An atom is the basic building block of all matter. Atoms are tiny (about 1 × 10⁻¹⁰ m in radius), so around 100 million atoms could sit side by side across a thumbnail
Every atom has two parts:
a tiny dense at the centre
a cloud of orbiting the nucleus
The nucleus is about 10 000 times smaller than the whole atom but contains almost all of the mass. If the atom were scaled up so the nucleus was a marble in the middle of a football stadium, the electrons would be orbiting the seats in the back row
The three sub-atomic particles
Particle
Location
Relative charge
Relative mass
Proton
In the nucleus
+1
1
Neutron
In the nucleus
0
1
Electron
Orbiting the nucleus
Together protons and neutrons make up the nucleus and are called
In a neutral atom, the number of electrons equals the number of protons, so the positive nuclear charge is exactly cancelled by the negative electron charge
If an atom loses or gains electrons, the charges no longer balance and the atom becomes an ion: positive if electrons have been lost, negative if extra electrons have been gained
Atomic and mass numbers
(Z) = the number of in the nucleus. This single number defines which element an atom is:
Hydrogen: Z = 1
Carbon: Z = 6
Sodium: Z = 11
Uranium: Z = 92
(A) = the total number of nucleons (protons + neutrons) in the nucleus
The number of in an atom can therefore be found by subtraction:
number of neutrons = mass number − atomic number = A − Z
Nuclear notation
A nucleus is described by writing its element symbol together with its mass number and atomic number:
ᴬ_Z X
where:
A = mass number (top, total nucleons)
Z = atomic number (bottom, protons)
X = the element's chemical symbol
Some examples:
¹₁H is hydrogen (1 proton, 0 neutrons, 1 electron)
²³₁₁Na is sodium (11 protons, 12 neutrons, 11 electrons)
²³⁸₉₂U is uranium-238 (92 protons, 146 neutrons, 92 electrons)