Download OCR A Level Physics B Delivery Guide Learner Resource 1: Atomic

Learner Resource 1: Atomic nomenclature
xA
z
The number x is often called the mass number and gives an indication of the relative
mass of the atom. It can also be called the nucleon number because it is equal to the
number of neutrons and protons in the nucleus of the atom. The word nucleon covers
both neutrons and protons.
The number z is the atomic number or proton number and is equal to the
number of protons in the nucleus. It is also equal to the number of electrons in
the neutral atom.
The letter A represents the atomic symbol and this is unique for each element and it is
determined by the number of protons in the nucleus (and therefore the number of electrons in the
neutral atom).
We can find the number of neutrons by subtracting the proton number (number of protons) from
the nucleon number (number of protons + neutrons).
n=z–x
So,
56
26
e.g.
Fe
This has 26 protons and 56 nucleons (neutrons and
protons) so it contains 56 – 26 = 30 neutrons.
Fill in the number of nucleons, protons and neutrons in the following atoms.
45
20
Ca nucleons =
protons =
neutrons =
Ti
protons =
neutrons =
protons =
neutrons =
47
22
17
8
O
nucleons =
nucleons =
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Isotopes
Isotopes contain the same number of protons (so they have the same atomic number) but a
different number of neutrons (so they have a different mass / nucleon number).
e.g. The following are two common isotopes of Chlorine.
35
17
Cl
This has 17 protons, 17 electrons and 35-17 = 18 neutrons
37
17
Cl
This has 17 protons, 17 electrons and 37-17 = 20 neutrons
These two isotopes occur in the ratio of 3 to 1 and so the RMM (relative molecular mass) of Chlorine
is 35.5.
Questions
Work out the number of protons, electrons and neutrons for the following isotopes.
234
92
U
235
92
U
238
92
U
Explain which of these isotopes do you think will be the least stable?
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