Chemistry (NTSE/Olympiad)  

4. Structure Of Atoms

Mass Number

The sum of the number of protons and neutrons in an atom of the element.
Mass number (A)
= No. of protons (p) + No. of neutrons (n)
Example. Helium atom has 2 protons, 2 neutrons and 2 electron. Its mass number is equal to 2 + 2 = 4.
Example. Oxygen has 8 protons, 8 neutrons and 8 electrons. its mass number is 16.
In some cases, particularly heavier elements, the number of neutrons is more than the number of protons.
Example. Mercury has atomic number equal to 80. So, it has 80 protons and 80 electrons. But the mass number of mercury is 200. Therefore, the number of neutrons in mercury is 200 – 80 = 120.
Generally, an atom is represented by its symbol for the element. Atomic number is written on the lower side of the symbol and the mass number is written on the upper side.

A = Mass number
Z = Atomic number
X = Symbol of element
Example. Indicates that lithium has atomic number equal to 3 and mass number equal to 7.

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