Chapter 4

MOLECULAR STRUCTURE

Introduction

Every substance in the universe is composed of certain basic substances; we know such basic substances as elements.

When we crush two substances into fine particles or powdered form and observe it under magnifying glass than what is visible now are compounds of very minute particles. This very minute particle is known as an atom. Atoms are so minute that they are not visible even by a microscope.

Group of similar atoms is called an element whose fundamental component is atom.

Fig: 1. Atom

The circular central portion of the atom is known as Nucleus. It is also known as center of the atom. There are two types of particles namely Proton and Neutron in the center of an atom, which is called Nucleus.

Surrounding the nucleus there is one more type of particle known as electron which moves a definite circular path also called Orbit.

Proton Electrically positively charged Positive charge

Electron Electrically negatively charged Negative charge

Neutron Electrically neutral No charge

1

Fig: 2. Our Solar System

Atomic Number

2

Compare Fig: 1. and Fig: 2. and think about following:-

•Is there any resemblance between both the figures?

•What is at the center of both the figures?

•What is rotating around the center in both the figures?

What makes the electron rotate around the nucleus in an atom?

Proton and neutron are relatively very heavy particles whereas electrons are very light in their comparison.

Due to the attraction of the positively charged proton in the nucleus, the negatively charged electrons rotate around the nucleus in a circular path.

In an atom, the number of proton and electron are equal. Since the positive charges and the negative charges are equal, an atom is neutral with respect to the electrical charge.

The number of proton or electron present in an atom of an element is called “atomic number”.

Therefore;

The Atomic Number of an Element = The Number of Protons Present in it = The Number of Electrons in it.

Name of the Element

Symbol Atomic Number Number of Protons

Number of Electrons

HYDROGEN H 1 1 1

HELIUM He 2 2 2

LITHIUM Li 3 3 3

BERYLLIUM Be 4 4

BORON B 5 5

CARBON C 6 6

NITROGEN N 7 7

OXYGEN O 8 8

FLUORINE F 9 9

NEON Ne 10

SODIUM Na 11 11

MAGNESIUM Mg 12 12

ALUMINIUM Al 13 13

SILICON Si 14 14

PHOSPHOROUS P 15 15

SULPHUR S 16 16

CHLORINE Cl 17

ARGON Ar 18

3

POTASSIUM K 19

CALCIUM Ca 20 20 20

Electronic Configuration

The distribution of electrons among various molecular orbitals is called the electronic configuration of the molecule.

The closest energy level to the nucleus is called the first energy level.

Beyond the first energy level, gradually the second, third and the fourth energy levels come in that order.

In the first energy level maximum 2, in the second energy level maximum 8, in the third energy level maximum 18 and in the fourth energy level maximum 32 electrons can be accommodated.

Electrons fill orbitals in a way to minimize the energy of the atom. Therefore, the electrons in an atom fill the principal energy levels in order of increasing energy (the electrons are getting farther from the nucleus). The order of levels filled looks like this:

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, and 7p

One way to remember this pattern, probably the easiest, is to refer to the periodic table and remember where each orbital block falls to logically deduce this pattern. Another way is to make a table like the one below and use vertical lines to determine which subshells correspond with each other.

Name of the Element Symbol Atomic Number Electro

4

To know how many electrons can be accommodated in any energy level, the formula 2n2 is very helpful, where ‘n’ stands for the serial number of the energy level.

Configuration

HYDROGEN H 1 (1)

HELIUM He 2 (2)

LITHIUM Li 3 (2, 1)

BERYLLIUM Be 4 (2, 2)

BORON B 5 (2, 3)

CARBON C 6 (2, 4)

NITROGEN N 7 (2, 5)

OXYGEN O 8 (2, 6)

FLUORINE F 9 (2, 7)

NEON Ne 10 (2, 8)

SODIUM Na 11 (2, 8, 1)

MAGNESIUM Mg 12 (2, 8, 2)

ALUMINIUM Al 13 (2, 8, 3)

SILICON Si 14 (2, 8, 4)

PHOSPHOROUS P 15 (2, 8, 5)

SULPHUR S 16 (2, 8, 6)

CHLORINE Cl 17 (2,8, 7)

ARGON Ar 18 (2, 8, 8)

Do atoms have tendency to gain, lose or share electrons? Let us find the answer:

Till the outer most energy level of the atom of any element is not completely filled

with electrons, the atom is active and has the tendency to combine with another atom.

Let us take some example to understand the above mentioned concept.

Example: 1

5

The atomic number of Sodium (Na) is 11, and therefore electron configuration is (2, 8, 1). Its outer most energy level has only one electron and thus to complete the energy level, it has tendency to lose one electron.

Due to loss of an electron, the nuclear charge does not remain neutral but it becomes positively charged. Therefore after losing 1 electron Na becomes positive ion and is written as (Na+).

Example: 2

The atomic number of Chlorine (Cl) is 17; therefore the electron configuration is (2, 8, 7). Its outermost energy level has seven electrons and to complete the energy level it has a tendency to gain 1 electron.

Due to the gain of 1 electron, the nuclear charge does not remain neutral, but it becomes negatively

charged. Therefore, after gaining 1 electron Cl becomes negative ion and is written as (Cl -).

Oppositely charged ions like negative or positive charge are attracted towards each other, combine to make a molecule of sodium chloride (salt).

The elements which have 1, 2 or 3 electrons in the outer most energy level easily lose 1, 2 or 3 electrons and make positive ions having the valence +1, +2 or +3.

Eg: - Na+, Mg2+, Al3+, etc.

The elements which have 7, 6 or 5 electrons in the outer most energy level gain easily 1, 2 or3 electrons and make it negative ions having the valence -1, -2 or -3.

Eg: - Cl-, O2-, etc.

6

Sometimes there are no possibilities of gaining or losing of electrons thus two atoms share the electrons present in their outer most energy level and combine with each other to complete their energy level.

Example: 3

Hydrogen atoms share one electron each and complete the energy level having 2 electrons and making one molecule of Hydrogen (H2).

Similarly, Oxygen atoms share two electrons each and complete the energy level having 8 electrons and making one molecule of Oxygen (O2).

Exercise

Q.1. MCQs

i. Which particles are not in the nucleus?

a) Proton

b) Neutron

c) Electron

7

d) None of the above

ii. Which particles are moving around the nucleus?

a) Proton

b) Neutron

c) Electron

d) None of the above

iii. Which particles do not possess any charge?

a) Proton

b) Neutron

c) Electron

d) None of the above

iv. How many maximum electrons can be accommodated in the first energy level?

a) 2

b) 8

c) 18

d) 32

v. How many maximum electrons can be accommodated in the second energy level?

e) 2

f) 8

g) 18

h) 32

vi. How many maximum electrons can be accommodated in the third energy level?

i) 2

8

j) 8

k) 18

l) 32

vii. How many maximum electrons can be accommodated in the fourth energy level?

m) 2

n) 8

o) 18

p) 32

viii. The circular central portion of the atom is known

a) Proton

b) Nucleus

c) Center

d) Neutron

Q.2. Give two points of difference for:

i. Molecule & Atom

ii. Atom & Ion

iii. Electron & Proton

Q.3. Explain:

i. Basic unit for an element is atom, whereas the basic unit for compound is molecule.

ii. Atom is neutral with respect to the electrical charge.

iii. Atoms have tendency to gain, lose or share electron.

iv. Electron rotating around the nucleus.

9