# Structure of the Atom| Worksheet Solutions| Class 9

Worksheet solutions for chapter: Structure of the Atom for class 9 are provided below.

(1)Write electronic configuration of element X which has atomic number = 15.Write its valency also.

Ans-

Electronic configuration = 2, 8, 5

The outer shell will complete 8 electrons after gaining 3 electrons.

Hence, valency of the element is 3. Ans-

(i) These are example of isotopes.

In isotopes atoms has same atomic number and different mass number. In our example

Atomic number for both atom = 20

Mass number for both atom = 30u  and 31u

Therefore it is an example of isotopes.

(ii) These are example of isobars.

In isobars atoms has different atomic number and same mass number. In our example

Atomic number for both atom = 20 and 18

Mass number for both atom = 35u

Therefore it is an example of isobars.

(3) Differentiate between atomic number and mass number.

Ans-

Differences between the atomic number and mass number are tabulated below-

 SN Atomic number Mass Number 1. Total number of the protons present in the nucleus Total number of nucleons 2. It is represented by “Z” It is represented by “A”. 3. All atoms of an element has same atomic number. Atoms of an element can have different mass number.

(4) An element has mass number equal to 32. What do you understand by it?

Ans- Mass number = Number of protons + Number of neutron

The above statement means that total of number of protons and number of neutron are equal to 32.

(5)If number of electron in outermost shell of an atom is 6. What will be the charge of atom?

Ans-

Number of electron in outermost shell = 6

2 electrons are needed to complete the outermost shell.

The atom will receive two electrons and get negative charge.

(6) Complete the table given below –

 SN Atomic number Number of proton Number of neutron Mass number Distribution of electrons Valency 1. 18 2 20 2,8,8 2. 4 5 3. 14 5 2,8,4

Ans-

 SN Atomic number Number of proton Number of neutron Mass number Distribution of electrons Valency 1. 18 18 2 20 2,8,8 0 2. 4 4 5 9 2,7 1 3. 14 14 5 19 2,8,4 4 Ans- They are examples of isotopes because both have same atomic number and different mass numbers.

(8) Differentiate between average atomic mass and mass number.

Ans- Difference between average atomic mass and mass number are tabulated below –

 SN Average atomic mass Mass Number 1. Only mass of the isotopes can be calculated. The mass of any atom can be calculated. 2. Fractional abundance of isotope is needed in calculation. Only number of protons and neutrons are needed for calculations. 3. Average atomic mass of bromine =(Fractional abundance of isotope 1  mass of isotope 1) + (Fractional abundance of isotope 2  mass of isotope 2) Mass number = number of protons  + number of neutrons Ans- (10) Who proposed that electrons are embedded in positive sphere-

(i) J.J.Thomson        (ii) Rutherford’s Model        (iii) Bohr’s Model

Ans- J.J.Thomson

Helping Topics

Structure of the Atom

Atoms and Molecules

NCERT Solutions Class 9