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12 SCIPAD - BIG IDEAS - BOOK 2
NATURE OF THE ATOM
Getting To Know The Periodic Table
If you look at a Periodic Table, you will notice that each element has a number associated with
it. The number is called the atomic number and represents the number of protons found in the
nucleus of a single atom of that element. Since the number of protons is equal to the number of
electrons, the atomic number also tells you how many electrons there are surrounding the nucleus.
Another number you find on some Periodic Tables is called the mass number. This tells you the
total number of particles, protons plus neutrons, found in the nucleus of an atom. So, if you know
the mass number and the atomic number of an element, it is possible to work out the number of
neutrons in the nucleus. By subtracting the number of protons from the mass number, you can
calculate the total number of neutrons.
Li
3
7
Lithium
A lithium atom has a mass of 7. State the number of protons, neutrons and
electrons found in this atom.
Number of protons or electrons = The atomic number.
Therefore, the lithium atom has 3 protons and 3 electrons.
Number of neutrons = Mass number - Atomic number.
Therefore, the lithium atom has 7 - 3 = 4 neutrons.
1. Use a Periodic Table to complete the table below. The data for the element lithium has
been done for you.
Element Atomic
Number
Mass
Number
Number of
Protons
Number of
Electrons
Number of
Neutrons
Hydrogen 1 1
Helium 2 4
Lithium 3 7 3 3 4
Beryllium 4 9
Boron 5 11 6
Carbon 6 6
Nitrogen 7 7
Oxygen 8 8
Fluorine 9 10
Neon 20
Sodium 23
Magnesium 24
Aluminium 27
Silicon 28
Phosphorus 31
Sulfur 32
Chlorine 35
Argon 40
Potassium 39
Calcium 40
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