This document discusses the molecular structure of atoms. It begins by explaining that all substances are composed of elements, which are the basic substances, and that elements are made up of very small particles called atoms. It then describes the basic structure of an atom, including that atoms have a small, positively charged nucleus surrounded by negatively charged electrons. The number of protons determines the element and equals the number of electrons. Atoms can gain, lose, or share electrons to attain stable electron configurations, forming ions or molecules.

1. Chapter 4
MOLECULAR STRUCTURE

2. 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
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3. Fig: 2. Our Solar System
Atomic Number
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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.

4. 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
POTASSIUM K 19
CALCIUM Ca 20 20 20
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5. 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
Configuration
HYDROGEN H 1 (1)
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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.

6. 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
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.
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7. 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.
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
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8. 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
d) None of the above
ii. Which particles are moving around the nucleus?
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9. 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
j) 8
k) 18
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10. 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.
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