Chapter-3Atomic structure

Atoms consist of a massive positively charged small central part called nucleus.

The nucleus contains protons and neutrons. The electrons revolve around the nucleus in

definite circular paths. These circular paths are called shells or orbits.

Each orbit has fixed energy. Therefore , these orbits are also known as energy shells or energy levels.

These orbits are represented by the symbols K, L, M, N etc....or numbers 1,2,3,4 respectively.

As long as as an electron moves in the same orbit it would not lose or gain energy.

Electron can lose energy only when it jumps from an orbit of higher energy level to another orbit of lower energy .

An electron jump from a lower energy orbit to a higher energy orbit only when it gains energy from outside.

In a neutral atom number electrons are equal to number of protons.

Atomic orbit is the circular path in which the electron moves around the nucleus.

NucleonsThe particles The particles present in nucleus present in nucleus are called Nucleons.are called Nucleons. Protons and Protons and neutrons are neutrons are nucleons.nucleons.

Comparison of properties ofconstituents of atom

No Properties electron proton neutron

1. Symbol e p n

2. Relative charge -1 +1 0

3 Relative mass 0 1 1

4. Position Outside the nucleus

In the nucleus

In the nucleus

5. Discovered by J.J Thomson E. Goldstein

James Chadwick

Distribution of electrons in shells

The systematic distribution of electrons in various energy states (K,L,M,N-shells) of the atom of an element is called its electronic configuration.

The electrons first occupy the shell with the lowest energy . The order of filling the electrons in shells follows the sequence K,L,M....

The maximum number of electrons that can be accommodated in any shell is given by 2n2 formula where 'n' is the number of the shell.

The outermost shell of an atom cannot accommodate more than 8 electrons.

The shell closest to the nucleus has The shell closest to the nucleus has n=1 .n=1 .

Hence the maximum number of electrons in different shells are as follows;

First orbit or K shell =2x12 = 2Second orbit or L shell =2x 22 =8Third orbit or M shell = 2x32 =18 and

so on.

Valence shell

The outer most shell of an atom is called its valence shell. The valence shell is also called valence orbit.

Valence electrons

The electrons present in the outermost shell of the atom are known as valence electrons.

Valence electrons are important because 1. The valence electrons decide the

reactivity of an element. 2. The valence electrons decide the

manner in which an atom form a bond with another atom.

3. The valence electrons in an atom decide the combining capacity ( or valency ) of the element.

Valency (modern definition) Valency of an element may be

defined as the number of electrons in an atom that actually take part in bond formation.

Bond formation means combining of two atoms to form a molecule.

With the exception of helium ,whose valency is zero, we can estimate the valency of an atom by the following rules

(1) When the number of valence electrons in an atom is less than 4

then,valency = number of valence electrons.

(2) When number of valence electrons in an atom is equal to or greater than 4, then

Valency= 8 – number of valence electrons.

Chemical reactivity of an element

The chemical reactivity of an atom is explained on the following basis

1. Octet rule 2. Electronic configuration

1.Octet rule

The octet rule was proposed by G.N Lewis.

This rule states that : ‘ The atom of an element combines with another atom to have eight electrons in its outer most shell’.

2. Electronic configuration

Electronic configuration of an atom helps us to understand the chemical reactivity of the element.

When the outermost shell of an atom is completely filled (have 8 electrons) the element is unreactive.

An atom having ‘8’electrons in its outermost shell is chemically inert.

E.g.: Elements like Helium, Neon , Argon etc. which have completely filled outermost shells are known as noble gases or inert gases.

These gases do not take part in chemical reaction and do not combine with other elements. Valency of noble gases are ‘0’.

An atom having less than ‘8’ electron in its outermost shell is chemically reactive. (except helium)

Home work 1. Which one is more reactive – an

element with atomic number 10 or an element with atomic number 9 ? Why ?

Atomic Number ( z)

The number of protons present in the nucleus of an atom of an element is called its atomic number.

In a neutral atom number of protons is equal to number of electrons.

So, atomic number = number of protons = number of electrons.

Z = p =e

Mass number (A)Mass number (A)

The sum of the number of protons and the number of neutrons present in the nucleus of one atom of an element is called its mass number.

Mass number = Number of protons + Number of neutrons.

Method to write the atomic symbol using atomic number and

mass number 1. First the atomic symbol of the element is

written. 2. The atomic number (Z ) is written as subscript on

the left side of the atomic symbol. 3. The mass number (A) is written as super script

on the left side of the symbol.

For example , an element “X” is represented as :

A Mass number

X Atomic symbol

Z Atomic number Example:- 24 23 4 Mg, Na , He 12 11 2

Home work

1. Calculate the number of protons , neutrons and electrons in an atom whose atomic number is 19 and mass number is 39.

2. Workout the number of protons ,neutrons and electrons in each of the following atoms.

56Fe, 93Nb, 235U, 133Cs, 137Ba 26 41 92 55 56

Isotopes Isotopes are atoms of same element

having same atomic number but different mass number.

Eg:1. The three isotopes of hydrogen are

1. Protium (11H )

2. Deuterium or D ( 21H)

3. Tritium or T (31T )

2. Isotopes of carbon 12C, 13C, 14C 6 6 6Carbon-14 is radioactive.

Isotopes contain same number of protons and electrons but have different mass number because they have different number of neutrons..

The chemical properties of elements are goverened by the number and arrangement of the electrons. So isotopes have identical chemical properties.

Characteristics of isotopes

1. Isotopes have the same electronic configuration and the same number of valence electrons and hence same chemical properties.

2. Isotopes of an element have different masses, so the physical properties which depend upon atomic masses will be different.

Many physical properties such as melting point, boiling point , density etc depend upon atomic mass.

Fractional mass of elements The fractional atomic masses of elements

are due to the existence of their isotopes having different masses.

EG: Chlorine has two isotopes 35 17 Cl and 37 17 Cl with natural abundance of 75% and 25% respectively.

The mass of 100 atoms=35x75+ 37x 25 = 3550Mass of one atom = 3550/100 =35.535.5 is the relative atomic mass of chlorine.

Homework

1. Calculate the relative atomic mass magnesium. The isotopes of magnesium and their percentage abundance are given below:

24 Mg - 78.6% 25Mg - 10.1% 26 Mg - 11.3%

Homework 2. Calculate the relative atomic mass

Lithium The isotopes of lithium and their percentage abundance are given below:

There is 8 atoms of 6Li for every 100 atoms of 7Li

Ans: The total mass of 108 atoms=

First 20 elements

Atomic number Element 1. Hydrogen

2. Helium

3. Lithium

4. Beryllium

5. Boron

6. Carbon

7. Nitrogen

8. Oxygen

9. Fluorine 10 Neon 11 Sodium 12 Magnesium 13 aluminium 14. silicon 15. Phosphorous 16. Sulphur 17. Chlorine 18. Argon 19 Potassium 20 Calcium

Deduce the electronic configuration of first 20 elements from their positions in the periodic table.

Relative atomic mass and Relative formula mass (RAM and RFM)