Atomic Number and Atomic Mass

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The nuclei of atoms is made up of protons and neutrons. These two components of the nucleus are referred to as nucleons. The electrons occupy the space outside the nucleus. Since an atom is electrically neutral, the number of protons in the nucleus is exactly equal to the number of electrons. This number is the atomic number given by the symbol Z.

Atomic number represents the number of protons in an atom. As atoms are electrically neutral, an atom contains as many electrons as it has protons.

Atomic mass, mass number or nucleon number

The total number of protons and neutrons present in one atom of an element is known as its mass number.

Mass number = number of protons + number of neutrons

It can also be said that:

Mass number = Atomic number + number of neutrons

Atomic number and mass number can also be represented on the symbol of an element as follows:

Let's take an example:

  • Represent the atom of sodium whose atomic mass is 23 and atomic number is 11. Calculate the number of protons, electrons and neutrons.

23Na11

Atomic number Z =11

Atomic mass A = 23

No. of protons = Z = 11

No. of electrons = 11

No. of neutrons = A - Z

23 - 11 = 12

Study the table carefully

   Lithium - 7  Silicon - 28  Copper - 65  Dysprosium - 164  Uranium - 238
 Proton Number Z  3  14  29  66  92
 Nucleon number A  7  28  65  164  238
 Number of neutrons N (A-Z)  4  14  36  98  146
 Symbol 
7
 Li
3
 
28
  Si
14
 
65
  Cu
29
 
164
   Dy
 66
 
238
   U
 92

Attempt the following:

  • The atomic number of an element is 12. How many protons and electrons are there in the atom?

Solution:

Atomic number = 12

Number of protons = Number of electrons = Atomic number = 12
  • The nucleus of an atom of an element contains 11 protons and 12 neutrons. Determine the atomic number and mass number of the element.

Solution:

Atomic number = number of protons = 11

Mass number = number of protons + number of neutrons

= 11 + 12 = 23

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