Q3 of 17 Page 24

The mass number of an atom is 31. The M shell of this atom contains 5 electrons.

How many neutrons does this atom have?

We know that mass number = number of protons + number of neutrons.

∴ Number of protons + number of neutrons = 31


⇒ 15 + number of neutrons = 31


⇒ Number of neutrons = 31 – 15


⇒ Number of neutrons = 16


Hence, the atom is having 16 neutrons.


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The mass number of an atom is 31. The M shell of this atom contains 5 electrons.

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The mass number of an atom is 31. The M shell of this atom contains 5 electrons.

What is the atomic number of this atom?

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The mass number of an atom is 31. The M shell of this atom contains 5 electrons.

Draw the Bohr model of the atom.

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Bohr models of atoms A, B, C, D are given (Symbols are not real).

a) Write the atomic number, mass number and electronic configuration of the atoms,


b) Among these, which are isotxopes? Why?