Electronic Structure: The Distribution of Electrons in an Atom

Learning Outcomes

After studying this lesson, you shall be able to:

1. Structure of an Atom
2. Distribution of Electrons in an Atom

The Structure of an Atom

1. An atom consists of a tiny dense nucleus surrounded by electrons.
2. The nucleus contains positively charged protons and neutral neutrons.
3. A neutral atom has an equal number of protons and electrons.
4. Atoms can gain electrons and there by become negatively charged, or they can lose electrons and become positively charged.
5. However, there won՚t be any change in the number of protons.

6. Most of the mass of an atom is in its nucleus and most of the volume of an atom is
occupied by its electrons.

7. The atomic number of an atom equals to the number of protons and the number of
electrons.

8. The mass number of an atom is the sum of its protons and neutrons.

The number of neutrons of an atom can be varied, so an atom can have different mass
number

The Distribution of Electrons in an Atom

1. The electrons are the greatest importance in organic chemistry.
2. A neutral atom of each element contains an equal number of protons and electrons.
3. According to quantum mechanics, the electrons in an atom can be thought of as occupying a set of shells.
4. The first shell is the closest to the nucleus and subsequent shells lies farther from the nucleus.
5. Each shell contains subshells known as atomic orbitals that have a characteristic shape and energy.
6. They occupy a characteristic volume of the space.
7. The atomic orbital that is close to the nucleus, is lower in its energy.

1. The first shell consists of only atomic orbital.
2. The second shell consists of s and three degenerate p atomic orbitals and the third shell contains, in addition, five degenerate atomic orbitals.
3. The fourth and higher shells contain, in addition, seven degenerate f atomic orbitals.
4. Degenerate orbitals are orbitals that have the same energy.

1. Each atomic orbital can have maximum of two electrons.
2. There are only two electrons in the first shell as it has only s atomic orbitals.
3. The second shell can have a total of eight electrons for one s and three p atomic orbitals.
4. The third shell has nine atomic orbitals, ones, three p, and five d atomic orbitals, so eighteen electrons can occupy these nine atomic orbitals.

MCQs

1. Mass of an atom depends on the number of __ .

a) Electron

b) Proton

c) Neutron

d) Nucleus

Answer: C

2. Each atomic orbital can have maximum of __ electrons.

a) 1

b) 2

c) 18

d) 8

Answer: B