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CHAPTER 3 : ATOMIC STRUCTURE

Dalton and his contemporaries considered the atom as the smallest indivisible particle of matter. However, the concept of indivisibility was proved to be incorrect by many experiments and observed facts.

3.1 Constituents of Atom

The atom is further made of smaller particles named as subatomic or ultimate particles. The atom has been split by physicists mainly into three subatomic particles namely protons, neutrons and electrons (Table 1).

Table 1 Fundamental Units of Atom

Particle

Relative wt.

Electric charge

Atomic location

Symbol

Proton

1.0073

1

nucleus

+ or p

Neutron

1.0087

0

nucleus

n

Electron

0.0005

-1

orbital

- or e-

A proton carries a positive charge. It is placed in the nucleus of an atom and has mass nearly equal to 1 a.m.u.

A neutron is chargeless. It is placed in the nucleus of an atom and has mass also nearly equal to 1 a.m.u.

An electron is negatively charged. It is placed in the orbital and has negligible mass.


Letís examine the nuclear and electron structure of some of the elements (Table 2)

Table 2 Nuclear and Electron structure of some elements

Atomic number

Element

Atomic weight

Protons

Neutrons

Electrons in neutral atoms

1

Hydrogen

1

1

0

1

2

Helium

4

2

2

2

3

Lithium

7

3

4

3

4

Berylium

9

4

5

4

5

Boron

11

5

6

5

6

Carbon

12

6

6

6

7

Nitrogen

14

7

7

7

8

Oxygen

16

8

8

8

9

Fluorine

19

9

10

9

10

Neon

20

10

10

10

Protons and Neutrons are present in the nucleus of an atom whereas Electrons are placed in 4 different orbitals (s, p, d, f)) surrounding the nucleus. Atomic weight or mass number is obtained by adding the number of protons and neutrons (collectively called nucleons) in each atom. The total number of protons in the nucleus of an atom of an element is known ae the atomic number of that element. The atomic number also coincides with the number of planetary electrons in a neutral atom.

Rutherfordís atomic model is represented in the following figure:

Index

3.1 The Atom
3.2 Isotopes
3.3 Atomic Weight
3.4 Radioactivity
3.5 Icons
3.6 Orbital
Chapter 4





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