2. 2
Matter and Structure of Atoms
• Subatomic particles
• Atomic Number and Mass Number
• Isotopes and Atomic Mass
• Electronic structure of Atoms
• Electron Configurations
Contents
3. 3
Atom
The word atom is derived from the Greek word
‘atom’ which means indivisible (inseparable).
Atoms are the building blocks of everything we
see around us; yet, we cannot see atom or
even a billion of atoms with the naked eyes
Atom is the smallest unit of an element that
retains (keeps) its chemical properties.
Surrounding orbiting
electrons
Positively charged
nucleus
4. 4
Measuring the size of atoms
The size of atoms is measured in nanometers (nm).
One nanometer is a meter divided by 109.
Name of Object Length in Meter
Girl 1 meter
Cassette 1/10m (1decimeter)
Fingernail 1/100m (1centimeter)
A pin head 1/1000m (1millimeter)
A diameter of blood capillary 1/10,000 meter
Blood cells 1/100,000 meter
Bacteria 1/1,000,000m (micrometer)
Influenza virus 1/10,000,000 meter
A molecule of soap 1/100,000,000 meter
5. 5
1/1,000,000,000 meter =1 nanometer(1nm)
All atoms are less than 1 nm in diameter
The size of atoms in nanometer
Atoms Size in nm
Hydrogen atom 0.074 nm
Carbon atom 0.15 nm
Copper atom 0.26 nm
Magnesium 0.32 nm
Hydrogen atoms are so small that
about 14,000,000,000 (14 billions ) hydrogen
atoms in a line would measure
a meter
6. 6
Molecules
A smallest unit of two or more atoms held
together by covalent bond and behaves
as an electrically neutral single particle.
H2 Cl2 H2O CO2 CH4
HCl
Cl2
H
O
H
H2O
7. 7
Types of Molecules
1-Monoatomic Molecules
Molecules composed of single atom which can live alone.
Elements of group VIII ( noble gases) can live as mono-
atomic molecules as:
He , Ne , Ar , Kr and Xe .
2-Diatomic molecules
Molecules made up of two atoms ( same or different ).
Elements, H, N,O, F, Cl, Br, I , exist as diatomic
molecules as: H2, N2,O2,F2,Cl2,Br2, I2 & HCl
3-Polyatomic molecules
Molecules composed of more than two atoms.
H2O, CO2, CH4 , C2H5OH.
9. 9
Structure of Atom
The atom is comprised of a positively charged
nucleus composed of protons and neutrons.
Small nucleus is surrounded by orbiting
electrons.
Protons and neutrons have similar masses
and electrons are much lighter (over 1,000
times lighter).
Protons (p+) and electrons (e-) have equal
and opposite charges while neutrons (n0)
have no charge.
The light negatively charged electrons
move around in an orbit in the space
around the nucleus.
11. 11
1. Matter is composed of tiny indivisible
particles - atoms.
2. All atoms of an element are similar to one
another but different from atoms of other
elements.
3. Atoms of various elements in a compound are
in a fixed numerical ratio.
4. In chemical reactions, atoms are combined,
separated, or rearranged but never created
or destroyed during a chemical reaction.
I- Dalton’s Atomic Theory
13. 13
Bombardment of -Particles on thin gold foil
resulted
Some -Particles to pass straight, few
deflected and some bounced back.
Conclusion:
1. Most part of an atom is empty space , occupied
by fast moving electrons.
2. The dense core of atom from where particles
deflected is Nucleus.
3. Most of the mass of atom is in nucleus where
proton & neutrons reside.
Structure of Atom
14. 14
electron, proton and neutron
The chemistry of an element depend on
subatomic particles
Name Symbol Charge Approximate
Mass (amu)
Mass in Grams
electron e- -1 0.0005 9.11 x 10-28
proton p+ +1 1.0 1.67 x 10-24
neutron n 0 1.0 1.67 x 10-24
Subatomic particles
15. 15
The space filling, the nature of electron wave is
electron cloud.
Amount of electron charge per unit volume is
electron density
In an atom, the protons and neutrons are found in the
nucleus; the electrons are located outside the nucleus.
Electron cloud
Nucleus
Proton
Neutron
Electron Cloud and Electron density
16. 16
The atomic symbol
X
A
Z
C
A - Atomic mass = Total number of protons & neutrons
Z -Atomic number = Number of protons or electrons
C - Charge = + or – , or 0 values
We use the following symbol to describe the atom:
A= Z + n0 where n0 is the number of neutrons.
Number of Neutrons = A- Z
n0 = A - Z
Element symbol
17. 17
A = Atomic mass
Z = Atomic number
= number of protons
= number of neutrons
= number of electrons
19. 19
Atomic Number and Mass number
Atomic Number (Z): The number of protons
present in nucleus of an atom is its atomic number.
It also equals the number of electrons moving
around the nucleus.
Mass number (A): The sum of number of protons &
neutrons or total number of nucleons in nucleus of
an atom is its mass number.
Mass number = number of protons + number of neutrons
X
A
Z X
A
Z
6C12, 8O16 , 7N14
20. 20
ISOTOPES
• Most of the elements exist as mixtures of
several kinds of atoms called isotopes.
• Isotopes are the atoms of the same
element but with different number of
neutrons but the same number of protons .
• Isotopes of carbon Isotopes of chlorine
C C C Cl Cl
12
6 17
6 6
14
35 37
17
13
21. 21
Atoms can be split into smaller parts.
An atom is composed of three types of subatomic particles:
proton, neutron, and electron.
In a neutral atom, the number of positively charged protons
in the nucleus is equal to the number of orbiting electrons.
If you add or subtract a proton from the nucleus,
you make a new element.
If you add or subtract a neutron from the nucleus,
you make a new isotope of the same element you
started with.
How You generate a New Isotope
electrons.
protons
neutron
22. 22
Identifying Protons and Neutrons in Isotopes
Question1 :State the number of protons and neutrons
in Isotopes of Neon.
20 21 22
10 10 10 Ne
Ne Ne
Question 2:What are the number of protons ,neutrons
and electrons in the following isotopes ?
30
Si Zn
26
14
70
Hint: Atomic mass = Atomic number + number of neutrons
23. 23
Hydrogen in nature occurs as mixture of
its isotopes
Mass of H= 1.007amu
Isotopes
of Hydrogen
Atomic number
( Z)
( no. of protons)
Number of
Neutrons
( n0 )
(no. of neutrons)
Mass number
(A)
( p + n)
Protium H1 1 0 1+ 0 = 1
Deuterium H2 1 1 1 +1 = 2
Tritium H3 1 2 1 +2 = 3
24. 24
The nuclei of different isotopes of the same
element have the different numbers of neutrons.
Example:
Tritium
Deuterium
Protium
Isotopes of Hydrogen
1 Proton
1 Proton
1 Proton
1 Neutron 2 Neutrons
25. 25
Isotopes of Hydrogen
element symbol
H
1
1
mass number = A
atomic number = z
(Deuterium) (Tritium)
(Protium)
H
1
1 H
2
1 H
3
1
p p p
n
n
n
Nucleus
of Tritium
Nucleus
of Protium
Nucleus
of
Deuterium
26. 26
Measuring the Mass of An Atom
Average Atomic Mass/Weight:
Most of the elements exist in nature as a mixture
of several kinds of isotopes with slightly
different masses so the average of the atomic
masses of naturally occurring isotopes of an
element is taken.
Atomic mass & atomic weight are often used
interchangeably.
We can measure masses of ions by using an
instrument called Mass spectrometer.
27. 27
MEASUREMENT OF RELATIVE ATOMIC MASSES
An oxygen atom is 16/12 or 4/3 of C-12 atom.
Magnesium is 24/12 or twice of C- atom
Atomic Mass Unit (AMU): Atoms are too small
to be seen or weighed in grams. The atomic
mass of any nuclide is determined by
comparing it with the mass of C-12 atom.
1 amu is exactly 1/12th of mass of
a C-12 atom
28. 28
AVERAGE ATOMIC MASS
Atomic masses of naturally occurring isotopes.
Isotopes Mass
number
%
Percentage
natural
abundance
Atomic
mass
(amu)
Average atomic
mass
of elements (amu)
Hydrogen-1
Hydrogen-2
1
2
99.985
0.015
1.007825
2.014102
1.00794
Carbon-12
Carbon-13
Carbon-14
12
13
14
98.90
1.10
Trace
12
13.003355
14.003242
12.0111
Oxygen-16
Oxygen-17
Oxygen-18
16
17
18
99.762
0.038
0.200
15.994915
16.999131
17.999160
15.9994
Uranium-234
Uranium-235
Uranium-238
234
235
238
0.005
0.720
99.275
234.040947
235.043924
238.050784
238.029
30. 30
3 Naturally occurring Isotopes of Mg
Atomic symbol Mg Mg Mg
Number of Protons 12 12 12
Mass number 24 25 26
% abundance 78.9% 10.0% 11.1%
Number of neutrons 12 13 14
12
25 26
24
12
12
31. 31
1) Number of protons in all isotopes of an element same.
Thus the chemical properties are also identical except
the rate of the reaction.
2) Number of valence electrons of all the
isotopes of same element are similar and
have the same valence.
3)The physical properties, such as mass,
density, melting point, boiling point of isotopes of same
elements are different.
4)Some isotopes are unstable and tend to breakdown or
decay, to a more stable isotopes. Such isotopes are called
radio isotopes because they produce radiations. These
isotopes are useful in medicine.
Isotopes & Their Properties
32. 32
Properties of an element determined by number
and distribution of its electrons around nucleus.
1. So Z (atomic number) distinguishes atom of an
element from atom of another element, or Z
identifies which element it is.
2. If atoms of same element differ in mass (in A)
(atomic mass), i.e. they have different number of
neutrons.
Chemically isotopes are identical except rate of
reaction, as in isotopes of H.
Isotopes & their Properties
33. 33
Exercise
Name of
Elements
Symbol Atomic
Number
Mass
Number
Number of
Protons
Number of
Neutrons
Number of
Electrons
- Al - 27 - - -
- - 12 - - 12 -
Potassium - - - - 20 -
- - - - 16 15 -
- - - 56 - - 26
Take the help of Periodic Table and complete
the following table for neutral atoms
34. 34
Exercise
M. C. Q:
1. What is the correct number of Protons and neutrons in Al?
a. 13protons; 14 neutrons b. 14 protons; 13 neutrons
c. 13protons; 27neutrons d. 27protons; 13 neutrons
2. Total number of elements placed in the periodic Table are:
a. 115 b. 109 c. 90 d. 120
3. The substances He and Ne are examples of:
a. Mono-atomic molecules b. Di-atomic molecules
c. Poly-atomic mole d. None of the above
35. 35
4.Choose the correct statement
a. Protons have negative charge b. Electrons are neutral
c. Protons have positive charge d. Neutrons are charged particles
6. “Atom” is a Greek word that means:
a. Visible b. Invisible
c. Divisible d. Indivisible
5. Total number of Protons and Neutrons in an atom expresses:
a. Atomic Number (Z) b. Atomic Mass (A)
c. Charge ( C) d. Both a & b.
30
a. Atomic Number (Z) b. Atomic Mass (A)
c. Charge d. Both a & b.
7. The element Zinc is represented by symbol Zn. The number 65 stands for:,
65
36. 36
8.Which particle has a mass less than a proton:
a) Electron b) Neutron
c) Atom d) Molecules
9.The atomic masses of three isotopes of Hydrogen (Protium, Deuterium
and Tritium) are in the order of:
a) 3,2,1 b) 3,1,2
c) 2,1,3 d) 1,2,3
10.Amount of electron charge per unit volume is electron density
a) True b) False
11.Mono atomic Molecules are :
a) Molecules composed of single atom
b) Molecules composed of two atoms
c) Molecules composed of many atoms
