Class 9th – STRUCTURE OF ATOM (Atomic & Mass Number)

We have already discussed the model of an atom and also were the subatomic particles are present inside an atom.

Atomic Number

As we know that the protons are present inside the nucleus , so the atomic no. of an element is defined as the number of protons present in an atom.

Therefore, atomic number of an atom = number of protons in an atom

The symbol for atomic number is Z

Atoms of same element have same number of protons inside the nucleus, thus atom of same element have same atomic number. and atoms of different elements have different atomic number.

The neutral atom have same number of protons and electrons. So in that case the number of electrons is equal to the number of protons.

Thus for neutral atom

Atomic number = number of protons = number of electrons

Example

When we talk about the following atom

Mass number

The total number of Protons and Neutrons present in an atom is called its mass number.

Mass number =  Number of protons + Number of Neutrons

Mass number is denoted by the symbol A

Mass number = Atomic number + Number of Neutrons

As we know that Atomic number = Number  of Protons

Representation of atomic and mass number

Let symbol of an element be Y, so it can be written as

[Table Pending]

RELATED QUESTIONS

Q1.  WRITE THE ELECTRONIC CONFIGURATION OF ATOMIC NUMBER 16

ANSWER:  K SHELL 2

                  L SHELL 8

                 M SHELL 6

Q2. CALCULATE THE SUB-ATOMIC PARTICLES OF AN ATOM WHOSE MASS NUMBER IS 22 AND ATOMIC NUMBER IS 10

ANSWER:  MASS NUMBER = NUMBER OF PROTONS + NUMBER OF NEUTRONS

                  MASS NUMBER = ATOMIC MASS + NUMBER OF NEUTRONS

                                22           =       10 + NUMBER OF NEUTRONS

                          22-10           =     NUMBER OF NEUTRONS

THEREFORE, NUMBER OF NEUTRONS = 12

                     NUMBER OF PROTONS =  10

                    NUMBER OF ELECTRONS = 10