St

ru

ct

ur

e

of

at

o

m

      When scientists started exploring matter, they discovered that the smallest particle of an element that

maintains its chemical identity through all chemical and physical changes is called and 'atom'.

A Search for the Particles of Atom of Electrons

      Although Dalton's theory of the existence of atoms was published in 1808, there was no clear idea of

what an atom might look like until the early part of the twentieth century.

The Structure of the Atom

      Scientific investigation on the structure of atom and its subatomic particles is an area where still

absorbs scientists.

Atomic Number, Mass Number or Nucleon Number

      The nuclei of atoms are made up of protons and neutrons. These two components of the nucleus are

referred to as nucleons.

Arrangement of Electrons in an Atom (Bohr-Bury Scheme)

      Electrons revolve around the nucleus in different energy levels or shells and each shell is associated

with definite energy.

Valence Shell and Valence Electron

      The outermost shell of an atom is called its Valence shell. It is the decisive shell during a chemical

reaction.

Isotopes

       It is interesting to note that atoms of a given atomic number can have different number of neutrons.

Summary

Atom

is the

smallest

 

particle

of an

element,

which

maintain

s its

identity

througho

ut

chemical

and

physical

changes

.

 

Atoms

are

made up

of three

fundame

ntal

particles

:

electron

s,

protons

and

neutrons

.

 

Disch

arge

tube

experim

ents by

William

Crookes

and J.J.

Thomso

n led to

the

discover

y of

electron.

 

Goldst

ein's

experim

ent with

a

perforat

ed

cathode

in the

discharg

e tube

led to

the

discover

y of

anode

rays and

the

existenc

e of

protons.

 

The

existenc

e of the

nucleus

in the

atom

was

establish

ed by

Rutherfo

rd by his

alpha-

particle

scatterin

g

experim

ent.

 

James

Chadwic

k

discover

ed the

neutron.

 

Neutro

ns are

neutral

particles

, protons

are

positivel

y

charged

and

electron

s are

negative

ly

charged

particles

.

 

The

nucleus

contains

protons

and

neutrons

collectiv

ely

called

nucleon

s. It is

positivel

y

charged.

The

mass of

an atom

is

concentr

ated in

the

nucleus.

 

The

mass of

electron

s is

negligibl

e

compare

d to the

mass of

the

protons.

 

Ordina

ry

hydroge

n is the

only

element

that has

no

neutron

in it.

 

Accor

ding to

Bohr

electron

s

revolve

around

the

nucleus

in fixed

orbits

called K,

L, M, N

etc.

 

The

energy

of the

shells

increase

s as the

distance

form the

nucleus

increase

s.

 

The

number

of

electron

s in

each

orbit can

be

calculate

d by the

formula

2n2

 

The

number

of

protons

is the

atomic

number

of an

element.

It is

denoted

as Z and

is the

identity

of an

element.

 

Mass

number

is

defined

as the

sum of

the

number

of

protons

and

neutrons

in the

nucleus

of the

atom. It

is

denoted

as A.

 

Valen

cy is the

combini

ng

capacity

of an

element.

 

Metals

have 1,

2 or 3

electron

s in their

valence

shells.

 

Non-

metals

have 4,

5, 6 or 7

electron

s in their

valence

shells.

 

All

noble

gases

have 8

electron

s in their

valence

shells

(except

helium

which

has

two).

 

Atoms

of the

same

element

having

same

atomic

number

but

different

mass

numbers

are

called

isotopes

.

 

Isotop

es show

similar

chemical

propertie

s but

different

physical

propertie

s due to

the

differenc

e in the

number

of

neutrons

.

 

     

Structure of the Atom

 

 

 

Question

(1): What is

an electron?

State its

relative mass

and charge.

Answer:  The electron is a negatively charged particle found in the atoms of all the elements. Its

relative mass is 1/1840 a.m.u and relative charge is 1.

Question (2): Define the following terms: a) Nucleons b) Atomic number c) Mass number d)

Nucleus

Answer:  a) Nucleons: The particles found inside the nucleus i.e., protons and neutrons are called

nucleons.

b) Atomic number: The number of protons present in the nucleus of an atom is called the atomic

number. It is denoted as Z. The number of protons is equal to the number of electrons.

c) Mass number: Mass number is defined as the sum of the number of protons and neutrons in the

nucleus of the atom. It is denoted by the alphabet 'A'.

d) Nucleus: The solid core of an atom made up of protons and neutrons, is called the nucleus

Question (3): What are cathode rays? How are they formed?

Answer:  Cathode rays are a stream of negatively charged particles. These particles called

electrons are shot from the metal cathode of a discharge tube when an electric current is passed

through a gas at a very low pressure. A discharge tube is a long glass tube having two metal

electrodes. When the pressure of air in the discharge tube is reduced to 0.001 mm of mercury and

a high voltage is applied to the electrode, the emission of light by air stops. But it is noticed that

the wall of the discharge tube at the end opposite to the cathode begins to glow with greenish

light. Since these rays are formed at the cathode they are known as cathode rays.

Question (4): a) Which atom contains only two fundamental particles? b) Which particle has

constant charge to mass ratio for all matter?

Answer:  a) The hydrogen atom contains only two fundamental particles.

b) Cathode ray particles i.e., electrons, have constant charge to mass ratio for all matter.

Question (5): What are anode rays? State three properties of anode rays.

Answer:  Anode rays are stream of positively charged particles shot out from the anode of a

discharge tube when a current is passed through a gas.

Anode rays travel in straight lines. They cast shadows of the objects placed in their path.

Anode rays can produce mechanical effects. This is evident by the fact they can rotate a light

paddle.

Anode rays are positively charged as they are deflected towards the negative plate in an electric

field.

Question (6): What happens to e/m ratio of positive rays and why?

Answer:  The mass and charge of positively charged particles depends upon the gas, which is

taken in the discharge tube. Different gases contain particles having different masses and different

charges and consequently give different types of positive rays. In other words the charge to mass

ratio e/m is not constant.

Question (7): Who discovered the nucleus within an atom? And how?

Answer:  Ernest Rutherford discovered the nucleus within an atom in his alpha-ray scattering

experiment. The arrangement of the alpha-particle scattering experiment is as follows:

Rutherford produced a narrow beam of particles from a radioactive source (e.g., radium or

polonium), which was allowed to strike an extremely thin gold foil. Rutherford proposed that if the

spherical model proposed earlier which made for a uniform distribution of positive and negative

particles was correct then the alpha particle striking the gold atoms would be uniformly deflected.

However the observation were:

Most of the alpha particles passed straight through the gold foil without suffering any deflection

from their original path

A few of them were deflected through small angles, while a very few deflected to a large extent

A very small percentage (1 in 100000) was deflected through 90o(turned back)

Question (8): Describe Rutherford's model of an atom.

Answer:  The atom of an element consists of a small positively charged nucleus which is

situated at the centre of the atom and which carries almost the entire mass of the atom.

The electrons are distributed in the empty space of the atom and are revolving around the

nucleus at high speed.

The number of electrons in an orbit is equal to the number of positive charges (protons) in the

nucleus. Hence the atom is electrically neutral.

The volume of the nucleus is negligibly small as compared to the volume of the atom.

Most of the space in the atom is empty.

The arrangement is just like a solar system.

Question (9): What is the drawback of Rutherford's nuclear model of the atom?

Answer:  Drawback of Rutherford's nuclear model: It could not explain the stability of the atom

because according to the electromagnetic theory if the charged particle undergoes accelerated

motion it must lose energy continuously and the electrons should collapse in the nucleus.

Question (10): Name the fundamental particles whose relative charge is a) +1 b) - 1 C) 0

Answer:  +1 proton

- 1 electron

0 neutron

Question (11): Compare the characteristics of protons, neutrons and electrons.

Answer:  Comparison Between Proton, Neutron and Electron

Question (12): Discuss Bohr's model of an atom.

Answer:  Neils Bohr, a Danish Scientist, put forth the structure of an atom that provides an

explanation for the major drawback of Rutherford's model. The main postulates are:

i) An atom is made up of three particles: electrons, protons and neutrons. Electrons have negative

charge; protons have positive charge whereas neutrons are neutral. Due to the presence of equal

number of negative electrons and positive protons the atom on the whole is electrically neutral.

ii) The protons and neutrons are located in a small nucleus at the centre of the atom. Due to the

presence of protons, nucleus is positively charged.

iii) The electrons revolve rapidly round the nucleus in fixed circular paths called energy levels or

shells. The energy levels or shells are represented either by numbers 1, 2, 3, 4, 5 and 6 or by

letters K, L, M, N, O and P. The energy levels are counted from centre outwards.

iv) Each energy level is associated with a fixed amount of energy, the shell nearest to the nucleus

has minimum energy and the shell farthest from the nucleus has the maximum energy.

v) There is no change in energy of electrons as long as they keep revolving in the same energy

level, and the atom remains stable. When an electron jumps from a lower level to a higher level it

absorbs energy and when an electron comes down from a higher energy level to a lower energy

level it loses energy.

Question (13): Calculate the atomic number of the element that has 12 neutrons and a mass

number of 23.

Answer:  Number of neutrons = 12

Mass number = 23 = number of protons + number of neutrons

23 = number of protons + 12

Number of protons = 23 - 12 = 11

Question (14): From the symbol State i) Mass number ii) Atomic number iii) Electronic

configuration

Answer:  i) Atomic mass / mass number - 31

ii) Atomic number - 15

iii) Electronic configuration - 2, 8, 5 K L M

Question (15): What is an electronic configuration of an element?

Answer:  The arrangement of electrons in the various shells of an atom of the element is known

as electronic configuration of the element.

Question (16): State Bohr-Bury scheme of electronic configuration.

Answer:  According to Bohr-Bury scheme:

The maximum number of electrons that can be accommodated in any energy level of the atom

is given by 2n2 (n is the number of that energy level). For e.g., 3rd energy level n = 3 Maximum

number of electrons = 2n2

2 x (3)2

2 x 9 = 18

The outermost shell of an atom cannot accommodate more than 8 electrons even if it has a

capacity to accommodate more electrons.

Question (17): What is the maximum number of electrons that can be accommodated in L shell?

Answer:  The maximum number of electrons that can be accommodated in the L shell is 8

electrons.

(2n2 : n = 2 Therefore, 2 x 2 x 2 = 8 electrons)

Question (18): Write the electronic configuration of elements whose atomic numbers are between

1 and 10.

Answer:  Hydrogen (1) : 1

Helium (2) : 2

Lithium (3) : 2, 1

Beryllium (4) : 2, 2

Boron (5) : 2, 3

Carbon (6) : 2, 4

Nitrogen (7) : 2, 5

Oxygen (8) : 2, 6

Fluorine (9) : 2, 7

Neon (10) : 2, 8

Question (19): What is the electronic configuration of a positively charged sodium ion Na+?

Answer:  The electronic configuration of a positively charged (sodium ion) Na+:

Question (20): Write short notes on (a) Isotopes (b) Valence electrons.

Answer:  a) Isotopes are atoms of the same element having the same atomic number but

different atomic mass. Isotopes differ in the number of neutrons in the nuclei.

Isotopes have same chemical properties but differing physical characteristics.

b) Valence electrons - The outermost shell of an atom that accommodate electrons in orbit is

called the valence shell. Electrons present in the valence shell are called valence electrons.

Example: Na (11) - 2, 8, 1. The number of valence electrons is 1.

Question (21): Which of the following elements is a metal and which is a non-metal?

Answer:  X - 11 - 2, 8, 1 Y - 9 - 2, 7 X - Metal Y - Non-metal

Question (22): State the number of valence electrons in a) alkali metals b) alkaline earth metals

c) halogens.

Answer:  Alkali metals = 1 (e.g. sodium)

Alkaline earth metals = 2 (e.g. calcium)

Halogens = 1 (e.g. chlorine)

Question (23): Predict the valencies of helium, phosphorus, sulphur and neon.

Answer:  Helium - He : 2

K shell has 2 electrons, which is completely filled. Hence its valency is 0.

Phosphorus - P - 15 : 2, 8, 5

Valency = 8 - 5 = 3

Sulphur - S - 16 - 2: 8, 6

Valency - 8 - 6 = 2

Neon - Ne - 10 : 2, 8

Valency is 0

Question (24): How can valency be calculated? Give examples of elements whose valencies are

1, 2, 3 and 4 respectively.

Answer:  There are different ways in which valency can be calculated:

i) The valency of an element can be determined by looking at the group number of the element.

Elements having 1, 2 or 3 electrons in the valence shell are metals.

ii) For non-metals valency is calculated by using the relationship Group number - 8; in case of a

non-metal 4 to 7 electrons exist in their valence shell.

iii) The same relation i.e. (Group number - 8) stands good for rare gases also.

iv) The valency of an electro positive ion is equal to the positive charge on it.

Example:

v) The valency of an electro negative ion is equal to the negative charge on it.

Example of elements:

Valency - 1: sodium, potassium lithium, and chlorine

Valency - 2: magnesium, calcium, zinc

Valency - 3: aluminium, nitrogen, and chromium

Valency - 4: tin, carbon

Question (25): and are symbols of two isotopes of magnesium. Compare the

atoms of these isotopes with respect to the following. i) The composition of their nuclei. ii) Their

electronic configurations. iii) Give the reason why the two isotopes of magnesium have different

mass numbers. iv) Explain why the two atoms have the same chemical reactions.

Answer: 

         

i) Protons = 12               Protons = 12

Neutrons = 24 - 12 = 12          Neutrons = 26 - 12 = 14

ii) The electronic configuration of both atoms is the same i.e., 2, 8, 2, since they both have 12

electrons.

iii) Mass number is the number of protons + neutrons. Both the atoms have same number of

protons but different number of neutrons. Hence their mass numbers are different.

iv) When chemical reactions take place, only electrons are involved in chemical reactions and the

protons and neutrons are not involved. Since both the atoms have same number of electrons, the

chemical reactions will be the same for both atoms.

Question (1): Almost the entire mass of an atom is concentrated in the__________.

1. proton

2. electrons

3. nucleus

4. neutrons

Ans:  3

Question (2): Electron was discovered by___________.

1. Chadwick

2. Thomson

3. Goldstein

4. Bohr

Ans:  2

Question (3): An atom has a mass number of 23 and atomic number 11. The number of protons are_________.

1. 11

2. 12

3. 23

4. 44

Ans:  1

Question (4): The mass of the atom is determined by________.

1. neutrons

2. neutron and proton

3. electron

4. electron and neutron

Ans:  2

Question (5): The K, L and M shells of an atom are full. Its atomic number is_______.

1. 18

2. 20

3. 10

4. 12

Ans:  1

Question (6): Which of the air pressures is appropriate for the production of cathode rays in the discharge tube?

1. 1 cm Hg

2. 1 mm Hg

3. 0.001 cm Hg

4. 0.001 mm Hg

Ans:  4

Question (7): Cathode rays are deflected towards__________.

1. positive electrode

2. negative electrode

3. both electrodes

4. none of the electrodes

Ans:  1

Question (8): The absolute charge of an electron is___________.

1. - 1.6 x 10-19C

2. + 1.6 x 10-19C

3. 1.6 x 10-19C

4. 16 x 10-19C

Ans:  1

Question (9): The proton is heavier than an electron by___________.

1. 1850 times

2. 1840 times

3. 1000 times

4. 100 times

Ans:  2

Question (10): Carbon-12 atom has_______________.

1. 6 electrons, 6 protons, 6 neutrons

2. 6 electrons, 12 protons, 6 neutrons

3. 12 electrons, 6 protons, 6 neutrons

4. 18 electrons, 6 protons and 6 neutrons

Ans:  1

Question (11): Chadwick got the Nobel Prize for the discovery of __________.

1. protons

2. neutrons

3. electrons

4. mesons

Ans:  2

Question (12): Mass number is equal to the_________.

1. number of protons + number of electrons

2. number of protons + number of neutrons

3. number of neutrons + number of electrons

4. number of electrons

Ans:  2

Question (13): The element X has 2 valence electrons. It is a____________.

1. metal

2. non-metal

3. metalloid

4. gas

Ans:  1

Question (14): The volume of the nucleus of an atom when compared to the extra nuclear part is_________.

1. bigger

2. smaller

3. same size

4. unpredictable

Ans:  2

Question (15): In Rutherford's alpha-scattering experiment, a foil of element that was used _______________.

1. gold

2. silver

3. aluminum

4. magnesium

Ans:  1

Question (16): An element has an electronic configuration of 2, 8, 7. Its valency is

1. 1

2. 7

3. 17

4. 8

Ans:  1

Question (17): The other name of is _________

1. protium

2. tritium

3. deuterium

4. proton

Ans:  1

Question (18): During a chemical reaction, atomic number_________.

1. changes

2. remains same

3. changes and then is restored

4. changes alternately

Ans:  2

Question (19): The fixed circular paths around the nucleus are called_________.

1. orbits

2. orbitals

3. nucleons

4. mesons

Ans:  1