Atomic Structure Book Back Questions 9th Science Lesson 11

9th Science Lesson 11

11] Atomic Structure

Book Back Questions with Answer and Do You Know Box Content

Do You Know?

In 1920 Rutherford predicted the presence of another particle in the nucleus as neutral. James Chadwick, the inventor of neutron was student of Rutherford.

Besides the fundamental particles like protons, electrons and neutrons some more particles are discovered in the nucleus of an atom. They include mesons, neutrino, antineutrino, positrons etc.

Atomic number is designated as Z why? Z stands for Zahl, which means NUMBER in German. Z can be called Atomzahl or atomic number. A is the symbol recommended in the ACS style guide instead of M (massenzahl in German).

Problem 1: Calculate the atomic number of an element whose mass number is 39 and number of neutrons is 20. Also find the name of the element.

Solution: Mass number = Atomic number + Number of neutrons

Atomic number = Mass number – number of neutrons = 39 – 20 = 19

Element having atomic number 19 is Potassium (K).

Atoms are so tiny their mass number cannot be expressed in amu (atomic mass unit). New unit is U. Size of an atom can be measured in nano metre (1 nm = 10^-9 m) Even though atom is an invisible tiny particle now-a-days atoms is an invisible tiny particle now-a-days atoms can be viewed through SEM that is scanning electron microscope.

Problem 2: What is the electronic configuration of aluminium?

Solution: Electronic configuration of aluminium atom: (Z=13) K shell = 2, L shell = 8 and M shell = 3 electron. So its electronic configuration is 2, 8, 3.

The forces between the protons and the neutrons in the nucleus are of special kind called Yukawa forces. This strong force is more powerful than gravity.

Problem 3: Find the valency of Magnesium and Sulphur.

Solution: Electronic configuration of magnesium is 2, 8, 2. So, valency is 2. Electronic configuration of sulphur is 2, 8, 6. So valency is 2 i.e. (8-6).

Thumb rule for isotopes and isobars. Remember t for top and b for bottom. Isotope: Top value changes – atomic mass. Isobars: Bottom value changes – atomic number.

Choose the best answers:

1. Among the following the odd pair is

(a) ¹⁸₈O, ¹⁹₉F

(b) ⁴⁰₁₈Ar, ¹⁴₇N

(c) ³⁰₁₄Si, ³¹₁₅P

(d) ⁴⁰₂₀Cr, ³⁹₁₉K

2. Change in the number of neutrons in an atom changes it to

(a) An ion

(b) an isotope

(c) an isobar

(d) another element

3. The term nucleons refer to

(a) Protons and electrons

(b) Only neutrons

(c) Electrons and neutrons

(d) Protons and neutrons

4. The number of protons, neutrons and electrons present respectively in ⁸⁰₃₅Br are

(a) 80, 80, 35

(b) 35, 55, 80

(c) 35, 35, 80

(d) 35, 45, 35

5. The correct electronic configuration of potassium is

(a) 2, 8, 9

(b) 2, 8, 1

(c) 2, 8, 8, 1

(d) 2, 8, 8, 3

State true or false. If false, correct the statement:

1. In an atom, electrons revolve around the nucleus in fixed orbits.

2. Isotopes of an element have different atomic numbers.

3. Electrons have negligible mass and charge.

4. Smaller the size of the orbit, lower is the energy of the orbit.

5. The maximum number of electron in L shell is 10.

Fill in the blanks:

1. Calcium and Argon are examples of a pair of __________.

2. Total number of electrons that can be accommodated in an orbit is given by __________.

3. ______________ isotope is used in the nuclear reactors.

4. The number of neutrons present in ₃⁷Li is _________________.

5. The valency of Argon is _____________.

Match the following:

1. Dalton – hydrogen atom model

2. Chadwick – discovery of nucleus

3. Rutherford – first atomic theory

4. Neils Bohr – plum pudding model

– discovery of neutrons

Answers:

Choose the best answers:

1. 2. an isotope 3. protons and neutrons 4. 35, 45, 35 5. 2,8,8,1

State true or false. If false, correct the statement:

1.True

2.False

Correct answer: Isotopes are atoms of same elements with atomic numbers

3.False

Correct answer: Electrons have negligible mass and have negative charge

4.True

5.False

Correct answer: The maximum number of electron in L Shell is 8

Fill in the blanks:

1. Isobars 2. 2n² 3. Radio 4. 4 5. 0

Match the following:

1.Dalton- First atomic theory

2.Chadwick- Discovery of neutrons

3.Rutherford- Discovery of nucleus

4.Neils Bohr- Hydrogen atom modela