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50 views15 pagesAtomic Structure
Atomic structure class 9 icse
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/ 15
Atomic Structure
Use Cordova Smart Class Software on the smart board in class to make learning enjoyable
We know that atoms and molecules are the
fundamental building blocks of matter. We see
different kinds of matter around us. They all
are made up of different kinds of atoms and
molecules. Earlier, it was thought that atoms
are indivisible. But, now we know that atoms
are divisible and have smaller particles in
them. i sub-atomic
articles (electron, proton and neutron).
MAHARISHI KANAD’S VIEW OF ATOM
Inancient time, Indian and Greek philosophers
were curious to know about the nature of
matter. Maharishi Kanad was a great Indian
philosopher. According to him, when we
keep on dividing matter, we shall get smaller
and smaller particles. Ultimately, we shall get
the smallest particle that cannot be further
divided. He named this smallest indivisible,
particle ‘parmanu’ (param means ultimate and
anu means particle).
During the same time, poe philosophers,
Democritus and Leuappus, also_gave the
_same idea. However, they called the smallest
“indivisible particles ofm matter as atoms. The
word atom came from a Greek word ‘atomos’
which_means_indivisible., Almost after_two
millennia, John Dalton gave his atomic theory.
DALTON’S ATOMIC THEORY
In 1808, John Dalton, an English
scientist, gave_a theory about
the nature of matter. It is called
Dalton’s_atomic theory. The
main features of Dalton’s atomic
theory are as follows:
John Dalton
1/Matter is made up of extremely small and
indivisible particles called atoms.
f Atoms can neither be created nor
destroyed.
All the atoms of a particular element are
identical. They have the same size, mass,
/ density and chemical properties.
4. Atoms of different elements are different
in size, mass, density and chemical
properties.
5. Atoms of an element combine with the
atoms of other elements to form molecules
of compounds.
6. Atoms are the smallest unit of an element
that take part in a chemical reaction
SUB-ATOMIC PARTICLES OF AN ATOM
We know that atoms are made up of smaller
particles— electrons, protons and neutrons.
These particles are called sub-atomic particles
or fundamental particles.
Chemistry - 8 zag baat we know that subatomic particles
exist? Qn tral, But it
an ain a positive or a negative charge. This
indicates that an atom contains tiny particles
that carry either a positive or a negative
charge. Under ordi ee
char; each other and make the
atom neutral
DISCOVERY s
The discovery of electrons came as a result of
the study of conduction of electricity through
gases. This was done by William Crookes, @
British scientist, in 1878,
Experiment Conducted By William Crookes
In_1878, William Crookes studied the
conduction of electricity through gases at
a very I ure. For this purpose, he
took a discharge tube. A discharge tube is a
lindrical glass tube, about 60 cm long, sealed
at both the-ends.and fited wi wo circular ith two circular
metal plates called electrodes. A source of high
voltage is connected to both the electrodes.
‘One_of these elecirades_ connected tthe
positive terminal of the source is anode and
the ott ected to the negative termi
Of the source is cathode. This discharge tube
Fig. 4-1 Discharge tube
3 Chemistry -8
od abigh volta se, he noted th,
When het ronducting elect ity_and sot
25 8 ;
ET eee
invis ee rays originated from q
anode (Si they were called cathog,
whode, eherefores t
cath
ie 42))
Taintsgreenish glow
athode rays
cathode
)
Yaz scum pam
i ction of cathode rays
Fig Aa Ptisharge tbe
J} Thomson’s Experiment
J) Thomson wasa British scientist. He studied
‘the characteristics _of cathode _rays
Giscovered electron in 1897, He performe
different experiments. with discharge tube.
(He applies
the cathode rays in the discharge tube. Hi
observed that cathode rays were deflect
towards the positive plate (Fig. 43).
negatvelycharged plate
positively-charged plate
Fig, 43 Deflection of cathode
rays by an electric field
(when he applied magnetic field in the pa
of cathode rays, they were deflected in th
| direction in which moving negative charg
deflects) On the basis of his observations Thomson_concluded_that_cathode. rays are
made_up_of _negatively-charged particles
called electrons, All-atoms contain electrons
ap their integral part.
‘The properties of an electron are independent
ofthe nature of gas filled in the discharge tube.
Properties Of Electrons
¥ Anelectron has a definite mass. It carries a
definite charge.
A Blectrons ate negatively-charged sub:
atomic particles. Charge on one electron is
1.6 x 10°” coulomb.
§. The mass of an electron is 9.1 x 10% g
1
on
Thus, its mass is negligible. Tt is
times the mass of an atom of hydrogen
(1.67 x 10 g).
4. The symbol of an electron is ,e°. The
superscript 0 represents mass of an
electron. The subscript -1 represents unit
negative electrical charge.
DISCOVERY OF PROTONS
When the existence of electrons was proved,
scientists thought that there must be some
positively-charged particles in addition
to the electrons in an atom because atoms
are electrically neutral in nature. In 1886,
E Goldstein discovered_these_positively-
charged particles. He experimentally proved
the existence of protons in an atom. He
produced anode rays in a discharge tube.
He made the cathode perforated so that the
anode rays pass through it (Fig. 4.4).
He passed electricity at high voltage (10,000
volts) through a gas at a very low pressure
(0.001 mm Hig). He observed ared glow behind
the cathode, These anode rays move exactly in
the opposite direction of cathode rays.
The
red glow
produced by
anode rays
prorated
li cathode
sreenish glow
produced by,
cathode rays
high
voltage}
source
thet:
Fig, 44 Schematic diagram of discharge tu!
perforated cathode and production of anode rays
They consist of po: ively-charged particles
called protons.
be with
erties Of Protons
Corser are positively-charged particles.
The charge ofa proton is equal to the charge
of an electron but opposite in nature, i.e.,
l.6 x 10°? coulomb.
. The mass of a proton is almost equal to
the mass of an atom of hydrogen, i.
1.67 x 104g.
The electrons revolve rapidly around the
nucleus (in extranuclear region) in fixed
circular orbits. These circular orbits are
called energy levels or shells (or orbits).
The energy levels or shells are represented
by the numbers 1, 2, 3, 4 and so on. They
gemen!
the
0 EXP!
ore Shemical properties of Matter. He ya,
a
ded the Nobel Prize in 1922 for his “(
awar e
on the structure of atom.
the shell farthest from the nucleus has
maximum energy:
6, The number of protons and electrons in,
‘ fe the same. Thus, they balance
atom art
total charge of the atom and the atom,
electrically neutral.
No two atoms contain the same number
protons (or electrons). Thus, the properti
are also represented by the letters K, L, M, @& aioe element are different from the oth
ts
Nand so on (Fig. 49). 6 aac
Oyo.
w
energy
3 x —energy
J} [levels
> Fyn
Fig, 4.9 Electrons in various energy levels or shells
5, Each shell is associated with a fixed
amount of energy. The shell nearest to the
nucleus has the minimum energy and
|
Pat Chemistry - 8 xo CW
N fontwi ducal
WV
‘Wie know that electrons revolve in fixed shells
orbits around the nucleus at a very high spec
Each orbit is associated with a fixed amount
energy.
Electron neither lose nor gain energy whi
revolving in an orbit. As a result, it preven
the electrons from falling into the nucleus am
makes the atom structurally stable.
ATOMIC NUMBER (Z)
The number of protons present in
fn wi hota ll g
Qin OOF ro nucleus of an atom of an element is calledil
atomic number. It is denoted by the letter
For example, an atom of sodium has
protons in it, so, the atomic number of sodit
is 1 orZ=11 . ee
In a neutral atom, the number of protons is
equal to the number of electrons in it
So, we can also say:
eS (2)=Number of protons
= Number of electrons
‘No two elements can have the same atomicnumber
(same number of protons), therefore, atomic
number can be used to identify an element.
MASS NUMBER (A)
The total number of protons and neutrons
present in the nucleus of an atom of an
element is called its mass number. It is
denoted by letter A.
‘Mass number of an element (A) =
Number of protons + Number of neutrons
An element is represented as:
Xi or | Xx
where, X = Symbol of the element,
A = Mass number
Z = Atomic number
For example, an element sodium (Na) having
mass number 23 and atomic number 11 is
represented as Na or ,,Na®. The number
of neutrons in an atom can be calculated by
subtracting the atomic number from its mass
number.
| Number of neutrons =
bees Mass number —
In case of 4Na, the number of neutrons
= Mass number (A) - Atomic number (Z)
=23-11=12
Atomic Mass
Atomic number
Mass of an atom is actually very less because
of the small size of atoms. It is difficult to
determine the mass of an atom individually.
So, the atomic mass of an atom is compared
with the mass of an atom of carbon (C- -12).
Unit of atomic mass is amu (atomic mass
unit),(One amu is exactly one twelfth of the
mass of an atom of carbon-12.
Atomic mass is the number of See an atom
of a given element is heavier than 1 of the
mass of one atom of carbon-12.
Note:(Mass of one C-atom= 12. amu)
ISOTOPES AND ISOBARS
The atoms of an element that have the same
The atoms of an eler f
ifferent mass numbers
ifferent mass numbers
atomic number but
are called isotopes.
. ~sotopesofanclementhave thesameatomic
number because they contain the same
number of protons and electrons. So, they
have the same electronic configuration
(arrangement of electrons in orbits) and
the same chemical properties.
* Isotopes of an element have different mass
numbers because they contain different
number ofneutrons.So, they have different
physical properties such as melting point,
boiling point and density. Isotopes of some
elements are given in Table 4.2.
Table 4.2 Isotopes of elements
§.No.| Element Isotopes
1 Hydrogen 1H, 2H, 3H
2 Carbon BC, BC, HC
[kes Chlorine SCL, 21
The atoms of different elements _having
different atomic numbers_but-same_mass.
number are called isobars.
For example, calcium (Ca) and argon (Ar)
both have the same mass number but different
atomic numbers. So, calcium and argon are
isobars.
Chemistry -8 A. Tick (7) the correct options. dern Mo
a
: : ‘ 0
1, Which of the following ia not explained byM omic pa"
(@) Electrons, protons and neutrons are 847"
i ins proton
(®) Nucleus is positively charged and it contains P
yw
ed cite!
© Electrons revolve around the nucleus in Bx
i ergy: 4
@ Shell nearest to the nucleus has maximumen”™” orbit,
pile revolving
fall into it
‘Asa result,
2, Electron neither lose nor gain energy W/
(a) itis strongly attracted towards nucleus
(b) it comes closer to the nucleus
(©) it does not fall into the nucleus
(@) it takes a spiral path
3. The mass number ofan atom is equal to the numero
on
(a) neutrons C1) eles
(© protons C1 @) protons 2
. The atomic number and mass number of Cl are at _
(a) 35,17 O © 17,35 Go 1817
5. Which of the following pairs represents isotopes? rn
@ "C1, 8cl a0 2H 00 HH ial
B. Complete the following table. .
ie
QL2LD LD
£
of
QD
d neutrons
and___20___ respectiy,
O@ 38, 18
s
1
1. | Hydrogen 1 1 oO
t F 7 1 1
7 3 1 @ @
2 Carbon 6 q@ 6 @- 2
@ 13 6 6 @
6 aw 6 © 8
3. | Chlorine 7 35 G ® @
@ 37 v7 @ 20
ARRANGEMENT OF ELECTRONS AROUND _ increasing energy around the nucleus of a
THE NUCLEUS OF AN ATOM~ELECTRONIC atom is called the electronic configuration.
CONFIGURATION The orbit that is nearest to the nucleus is called
The distribution of electrons in different the first orbit. The next orbit is called th
energy levels or shells in the order of — second orbit (Fig. 4.10). K shell is represented
HE crear -e by n=1,L shell is represented by n=
on.
Zand so
/— nucleus
Fig. 4.10 Orbits or shells in an atom
The Bohr-Bury scheme gives the maximum
number of electrons that can be accommodated
ina shell of an atom. According to Bohr-Bury
scheme:
1, Maximum number of electrons that
can be present in a shell is given by the
formula 2n? where, n is the shell number
(Fig, 4.11).
Rucleus
© & EM Neches
However, when the outermost shell of an
atom Is the first shell or K shell, it cannot
accommodate more than 2 electrons. This
is known as duet rule (duplet),
3. Blectrons do not enter ina new shell unless
the inner shells are completely filled.
Let us understand the Bohr-Bury scheme of
electronic configuration with the help of the
following examples:
(a) $He-Atomic number is 2. It means
helium atom contains 2 electrons and
these electrons occupy the first shell, i.e.,
K-shell. So, electronic configuration of
helium is 2.
C-Atomic number is 6. It'means that
carbon atom contains 6 electrons and out
ofthese 6 electrons, 2 electrons are present
in K shell and 4 electrons are present in
L shell. So, electronic configuration of
carbon is 2, 4.
Table 4.4 Atomic numbers and electronic
(b)
third energy. ‘maximum
Tevel jp Sme configurations of elements
electrons that -—— ae
oor Stee BE a
dated in various . | Atomic
shel No,| Flement / number {
Fig. 4.11 Maximum number of electrons
in various shells 1_| Helium 2
Table 4.3 Maximum number of electrons {2 [Beryllium 4
ina particular shell 3_ [Carbon 6
— poll Maximum number [4 |Oxygen 8
peed iE Valence Shell
1 K
5 = The outermost shell of an atom is called its
; m1