|
8 |
JOURNEY INSIDE THE ATOM |
REDISCOVERING THE ROOTS OF ATOMIC THEORY
-
All matter (living and
non-living) are composed of tiny particles called atoms.
-
Acharya Kanada (India)
proposed that repeated division of matter ultimately leads to indivisible
particles called parmanus. His ideas are recorded in the Vaisesika
Sutras.
-
A parmanu is
infinitely small and cannot be perceived by the senses. Parmanus combine
to form dyads (two parmanus), triads (three parmanus) and so on,
making up all matter. However, this theory does not explain the proportions in
which parmanus combine to form different substances.
-
Greek philosophers Leucippus
& Democritus proposed a similar idea and called them atomos (=
indivisible).
-
The concept of 'atom'
originated as an imaginary idea not from experiments.
-
In 1808, John Dalton
proposed atomic theory based on experiments. He stated that all matter
is made of indivisible particles called atoms. i.e., they are
fundamental building blocks of matter.
-
Dalton’s theory was the
first scientific description of the structure of matter and laid the foundation
for the modern understanding of atomic structure.
A SHORT HISTORICAL JOURNEY THROUGH ATOMIC MODELS
-
Until the late 19th
century, atoms were thought to be the smallest indivisible units of matter.
-
The discovery of radioactivity
showed that atoms emit radiation (energy and particles), proving they are made
of smaller particles.
- J. J. Thomson (1897) studied the conduction of electric
current through gases at a very low pressure. He used a glass tube with two
electrodes and applied a high voltage. He observed rays moving from the cathode
(negative electrode) to the anode (positive electrode). These were called cathode
rays. By studying these in electric and magnetic fields, he concluded that
they are streams of negatively charged particles, with much smaller mass than
atoms. These were later called electrons. It showed that atoms are
composed of smaller subatomic components.
A line diagram of cathode ray tube
-
The nature of cathode rays
was independent of the material of the cathode and the gas filled in the
cathode ray tube. It showed that electrons are a fundamental component present
in all atoms.
-
The charge of an electron
(-1.602 × 10⁻¹⁹ C) is taken as -1 as a matter of convention and convenience.
Meet
a Scientist:
·
J. J. Thomson discovered the electron, the first identified
subatomic particle. He received the Nobel Prize in Physics (1906) for
his studies on the electrical conductivity of gases, which led to this
discovery. As head of the Cavendish Laboratory in Cambridge, he guided
many scientists, including Ernest Rutherford.
Thomson's model of an atom
-
Since atoms are neutral, J.
J. Thomson proposed that an atom is a sphere of positive charge with electrons
distributed throughout it.
-
This is called plum
pudding model (pudding= positively charged matter, plums= electrons) or watermelon
model (pulp= positive charge, seeds = electrons).
Pause and Ponder
1.
Suppose you made up your own 'atom', as Thomson
described, using clay for the positive charge and small beads for the electrons
spread through it. What will happen if:
(i)
the positive charge on the clay is lesser than the
total negative charge of the beads?
(ii)
by mistake, the clay itself carries a bit of negative
charge? Would your model still represent a neutral atom?
Answer:
(i)
The model becomes negatively charged and is not
a neutral atom.
(ii)
No. The extra negative charge makes the model non-neutral.
2.
Could an orange or a lemon, which also contain seeds
inside soft pulp, be a good comparison? In what ways does it match Thomson's
idea and where does it fall short?
Answer: It can be used as a structural
comparison like the watermelon or plum pudding. Pulp represents positive
charge; seeds represent electrons.
Falls
short: Seeds are
not evenly spread, and the pulp is not actually positively charged.
3.
Why did Thomson conclude that electrons are present
in all atoms?
Answer: Thomson concluded electrons are
in all atoms because the behaviour of cathode rays remained completely independent
of the cathode material and the gas used in the tube. This proved they are
a fundamental component of all elements.
Testing Thomson's model: The gold
foil experiment
-
In 1911, Geiger &
Marsden, under Ernest Rutherford, tested Thomson’s atomic model
using the gold foil experiment (ฮฑ-ray scattering experiment).
-
They directed a narrow beam
of alpha (ฮฑ) particles (positively charged particles emitted by
radioactive elements) at an extremely thin gold foil.
-
According to Thomson’s
model, the positive charge in the atom was spread out evenly. So, the particles
should pass straight through the gold foil or be slightly deflected. Instead,
most passed through undeflected, some were sharply deflected, and a few bounced
back. This deflection from the straight path
is called scattering.
Schematic view of the gold foil experiment
-
Thomson’s model failed to
explain gold foil experiment.
A. Rutherford's model of an atom
-
From the gold foil
experiment, Rutherford concluded that the positive charge is concentrated in a
tiny nucleus. He proposed that:
o Most of the atom is empty space, as most ฮฑ-particles
passed undeflected.
o The nucleus is dense, contains all the positive charge and most
of the mass of an atom.
o Electrons revolve around the nucleus like planets around the
Sun, so it is called the planetary model of the atom.
- Rutherford found that the nucleus is about 10⁵ (one lakh)
times smaller than the atom. He calculated that the atom's diameter is
≈10⁻¹⁰ m and the nucleus is ≈10⁻¹⁵ m. If an atom were the size of a cricket
ground (100 m), the nucleus would be only a black pepper grain at its centre.
Ready
to Go Beyond
·
A 0.1 mm (10⁻⁴ m) thick sheet of paper is
about 10⁶ (one million) atoms thick, assuming an atomic diameter of 10⁻¹⁰
m.
Think as a Scientist
Observe the gold foil
experiment. Predict the observations you would expect if the gold foil in the
experiment were made thicker. Also, draw a simple diagram to show the
observations you expect.
Hint: Compare thin foil vs thick foil.
How does the thickness affect the chances of hitting a nucleus?
Answer: For a thicker gold foil, the
ฮฑ-particles pass through more layers of atoms, so:
o Fewer
particles go straight through.
o More
particles are deflected.
o A few more
are scattered backward.
Pause and Ponder
4.
What do you think would happen if ฮฑ-particles were
replaced with negatively charged particles in Rutherford's gold foil
experiment?
Answer: If negatively charged particles such
as electrons were used instead of ฮฑ-particles, they would be strongly attracted
to the positively charged nucleus and, being very light, easily deflected by
atomic electrons. Hence, they would not clearly reveal the structure of the
nucleus.
5.
Rutherford found that a few ฮฑ-particles bounced back
sharply. How does this single surprising result completely rule out Thomson's
'plum pudding model' of the atom?
Answer: According to Thomson's plum
pudding model, ฮฑ-particles should have passed through the atom with only slight
deflections. The sharp deflection and bouncing back of a few ฮฑ-particles showed
that the positive charge and most of the mass are concentrated in a tiny, dense
nucleus, disproving Thomson's model.
6.
If you could ask Rutherford one question about his
work, what would it be?
Answer: "What was your reaction
when you first saw an ฮฑ-particle bounce back, and how did you come up with the
15-inch shell and tissue paper analogy?"
B. Limitations of
Rutherford's model
-
Rutherford’s model could
not explain stability of atoms.
- An electron moving in a circular path is continuously
accelerating. Hence, it should lose energy while revolving around the nucleus. On
losing energy, an electron should spiral into the positively charged nucleus,
causing the atom to collapse. Since atoms are stable, this does not happen,
showing that Rutherford's model was incomplete.
Meet a Scientist
·
Ernest Rutherford, born in New Zealand, worked with J. J. Thomson
at Cambridge and became known as the Father of Nuclear Physics.
·
He discovered the atomic nucleus and explained how
some elements naturally break down. For this he won the 1908 Nobel Prize in
Chemistry.
·
In 1911, he proposed the nuclear model of the atom.
C. Discovery of the proton
-
Rutherford showed
that the nucleus carries some positively charged particles. He named them protons.
-
Protons are much heavier
than electrons and have an equal but opposite charge.
-
An atom is electrically
neutral when the number of protons equals the number of electrons. E.g.,
helium has 2 protons & 2 electrons, sodium has 11 protons &
11 electrons. The equal positive and negative charges make all atoms electrically
neutral.
Pause and Ponder
7.
Assertion (A): Rutherford concluded that most of the mass of an
atom is concentrated in a small region at the centre called the nucleus.
Reason
(R): According to
Thomson's model, electrons are embedded in a uniformly distributed positive
charge sphere.
Choose
the correct option:
(i)
Both A and R are true, and R is the correct
explanation of A.
(ii)
Both A and R are true, but R is not the correct
explanation of A.
(iii)
A is true, but R is false.
(iv)
A is false, but R is true.
Answer: (ii) Both A and R are true, but R
is not the correct explanation of A.
Bohr's model of the atom
Niels Bohr (1913, Copenhagen University, Denmark) proposed a new
model of the atom that explains why atoms are stable and do not collapse into
the nucleus.
According to Bohr:
o Electrons do not move randomly around the nucleus but follow
fixed circular paths called stationary states, orbits, or shells.
In each shell, an electron has a definite amount of energy, so they are also
called energy levels.
o Shells are represented by the letters K, L, M, N, ... or by the
numbers n=1, 2, 3, 4, ...
Energy levels in an atom
o Electrons revolve only in the shells. While moving in a fixed
shell, an electron does not lose energy.
o The first energy level K (n=1) is closest to the nucleus and has
the least energy.
o The energy of shells increases with distance from the nucleus.
Thus, an electron in the L-shell (n = 2) has more energy than one in the
K-shell (n = 1).
o An electron can move to another shell by absorbing or releasing
a fixed amount of energy equal to the difference between the energies of the
two levels.
o Each shell can hold only a certain number of electrons.
Bohr's model explained many experimental observations about atoms,
earning him the 1922 Nobel Prize in Physics.
Bohr's model also has limitations. It led to the proposal of the quantum
mechanical model of atom.
Threads
of Curiosity
Why are
Bohr's shells called K, L, M, N… and not A, B, C, D?
·
The names K, L, M, N... came from Charles
Barkla's X-ray studies.
·
He named the first observed X-ray line K,
leaving A–J for any earlier series that might be discovered (though none were).
·
Bohr later adopted the same notation for atomic shells.
WHAT COMPONENTS CONTRIBUTE TO THE MASS OF AN ATOM?
-
Rutherford's model showed
that most of the mass of an atom is concentrated in its nucleus. The mass of
the electrons is negligible.
-
However, scientists found
that a helium atom (2 protons) has about four times the mass of a
hydrogen atom (1 proton), not double. This suggested that the nucleus
contains another particle that adds mass but has no charge.
Discovery of the Neutron
-
James Chadwick (1932)
discovered neutron (n), a subatomic particle with a mass nearly
equal to that of a proton but no electrical charge (neutral).
- Neutrons are found in the nucleus of all atoms except hydrogen. Thus,
an atom's mass comes mainly from its protons and neutrons in the
nucleus.
Symbols and relative charges of subatomic particles
|
S.No. |
Subatomic
particle |
Symbol |
Relative
charge |
|
1. |
Electron |
e⁻ |
-1 |
|
2. |
Proton |
p⁺ |
+1 |
|
3. |
Neutron |
n⁰ |
0 |
Threads of
Curiosity
·
Lighter atoms usually have equal numbers of protons and
neutrons (e.g., carbon: 6 & 6, oxygen: 8 & 8).
·
Heavier atoms have more neutrons than protons (e.g., iron: 26p,
30n; uranium: 92p, 146n).
·
Protons repel each other. Neutrons being neutral
reduce this repulsion by intervening and increasing the distance between
protons, and also by strengthening the force, called the nuclear force,
that binds all particles together. Hence, heavier atoms need more
neutrons for stability.
What if an atom had no empty space? How would
this have affected the size of various objects?
Answer: If atoms had no empty space:
·
Atoms would be much smaller and tightly packed.
·
All objects would become extremely tiny. E.g.,
a human, a book, or a building would shrink to a very small size because most
of an atom is empty space.
Ready
to Go Beyond
·
The discovery of the neutron led to the atomic
age.
·
Since neutrons have no charge, they can penetrate atomic
nuclei easily, enabling the creation of artificial radioactive elements,
nuclear fission (splitting of uranium
atoms etc.), nuclear power & nuclear weapons.
Meet a Scientist
·
James Chadwick was a student of Rutherford at the Cavendish
Laboratory at the University of Cambridge.
·
He earned the 1935 Nobel Prize in Physics for the
discovery of neutron.
India's Scientific Contributions
·
The Bhabha Atomic Research Centre (BARC), Mumbai, uses advanced
neutron-scattering experiments using reactors, such as Dhruva.
·
This helped to study superconductors, battery
electrodes & drug molecules, helping improve medicines, energy storage, and industrial alloys in India.
SYMBOLS OF ELEMENTS
-
By 1869, 69 elements
were known. Today, 118 elements are known, including both natural
and artificially made elements.
-
John Dalton (1803)
introduced the first pictorial symbols to represent the elements.
Symbols of some elements given by Dalton
-
Berzelius (1813) proposed
using Latin names to create alphabetic chemical symbols.
-
Today, the International
Union of Pure and Applied Chemistry (IUPAC), approves the names and symbols
of elements.
Some of the norms of
writing symbols:
o Many symbols are the first letter or the first two letters of
the name of the element.
o First letter is capital letter (uppercase) while the second
letter is small letter (lowercase). E.g., hydrogen (H), aluminium (Al), cobalt (Co),
etc.
o Symbols of some elements are formed from the first letter of the
name and a letter other than the second letter in the name. E.g., chlorine, Cl;
zinc, Zn, etc.
o Symbols of some elements come from their Latin, Greek, or German
names rather than English. E.g.,
§
Symbol for iron is Fe
(Latin ferrum)
§
Symbol for mercury is Hg
(Greek hydrargyros)
§
Symbol for tungsten is W
(German wolfram)
Symbols are internationally recognised, allowing scientists worldwide to
communicate clearly regardless of language.
Names of some
common elements and their symbols
|
Element |
Symbol |
Element |
Symbol |
Element |
Symbol |
Element |
Symbol |
|
Aluminium |
Al |
Chlorine |
Cl |
Iron (Ferrum) |
Fe |
Silicon |
Si |
|
Argon |
Ar |
Cobalt |
Co |
Lead (Plumbum) |
Pb |
Silver
(Argentum) |
Ag |
|
Barium |
Ba |
Copper (Cuprum) |
Cu |
Magnesium |
Mg |
Sodium
(Natrium) |
Na |
|
Boron |
B |
Fluorine |
F |
Neon |
Ne |
Sulfur |
S |
|
Bromine |
Br |
Gold (Aurum) |
Au |
Nitrogen |
N |
Uranium |
U |
|
Calcium |
Ca |
Hydrogen |
H |
Oxygen |
O |
Zinc |
Zn |
|
Carbon |
C |
Iodine |
I |
Potassium
(Kalium) |
K |
|
|
Pause and Ponder
8.
Imagine you are a scientist who has discovered a new
element. Name this element after yourself and justify that the symbol you have
chosen follows the IUPAC rules.
Answer:
Element name is Akbarium
Symbol: Ak
Justification: The symbol Ak uses the first
letter as uppercase and the second letter as lowercase, following IUPAC
rules for chemical symbols.
9.
What problems could arise if every scientist used
different symbols for the same element?
Answer: It causes confusion, miscommunication,
and errors in scientific research and education.
ATOMIC NUMBER
- Atoms of different elements differ in the number of protons and
electrons.
- The number of protons in the nucleus of an atom of an element is
called atomic number. Its symbol is Z.
- This number determines the identity of an element and its
chemical behaviour.
- Since an atom is neutral, the number of protons equals the
number of electrons. E.g.,
o Hydrogen has 1 proton and 1 electron, so Z= 1
o Helium has 2 protons and 2 electrons, so Z= 2
- Lithium has 3 protons, 3 electrons and 4 neutrons. Its atomic number (Z) is = 3
Neutrons and protons in the nucleus
MASS NUMBER
- It is the total number of protons and neutrons present in
the nucleus of an atom. It is denoted by A.
Mass number = Number
of protons + Number of neutrons
- E.g., helium has two protons, but its mass is about four times
that of a proton.
-
The protons and neutrons
present in the nucleus are called nucleons.
- Since a neutron has nearly the same mass as a proton,
the mass of a helium atom is mainly due to 2 protons and 2
neutrons.
Mass number of different elements
|
Element |
Protons (p⁺) |
Neutrons (n⁰) |
Mass number
(A) |
|
Hydrogen |
1 |
0 |
1 |
|
Helium |
2 |
2 |
4 |
|
Lithium |
3 |
4 |
7 |
- The electron has negligible mass, so it is ignored
in calculations.
-
The standard notation of an
atom shows its symbol, atomic number (Z), and mass number (A).
- E.g., the notation for carbon (Z= 6, A = is 12) is ¹²₆C
Pause and Ponder
10.
An atom with an atomic number of 26 has 56 nucleons.
Find out its number of electrons, protons and neutrons.
Answer:
Electrons
= 26
Protons
= 26
Neutrons
= 56 − 26 = 30
11.
The nucleus of an atom contains 20 protons. If its
mass number is 41, find the number of neutrons in it.
Answer: Neutrons = 41 − 20 = 21
12.
An atom has 18 neutrons and an atomic number of 17.
What is its mass number?
Answer: Mass number = 17 + 18 = 35
13.
An atom ²³A has 11 electrons. Find the number of
neutrons in it.
Answer:
Protons
= 11 (electrons = protons in a neutral atom)
Neutrons = 23 − 11 = 12
HOW ARE ELECTRONS DISTRIBUTED IN DIFFERENT ENERGY
LEVELS?
Bohr and Bury suggested the following rules:
o Maximum number of electrons present in a shell is given by the
formula 2n², (n = number of the shell).
Hence, K-shell (n=1) 2 × 1² = 2 electrons.
L-shell (n=2) 2 × 2² = 8 electrons.
M-shell (n=3) 2 × 3² = 18 electrons.
o The outermost shell can hold a maximum of 8 electrons
(the first shell can hold only 2).
o Electrons fill the shells stepwise, from inner to outer (i.e.,
in the order K, L, M, N, ...). A shell is filled only after the inner shell is
complete.
-
Hydrogen has only one
electron, so it must be in K-shell.
-
Helium has 2 electrons that
are arranged in the K shell.
Building up atoms
-
The distribution of
electrons among various shells is known as the electronic configuration
of the atom.
Schematic atomic structure of the first eighteen elements showing how the electrons are filled in the K, L and M shells
Electronic distribution of atoms of the first eighteen elements
|
Name
of the element |
Symbol |
Atomic
number |
Number
of protons |
Number
of neutrons |
Number
of electrons |
Distribution (K,
L, M, N) |
|
Hydrogen |
H |
1 |
1 |
- |
1 |
1 |
|
Helium |
He |
2 |
2 |
2 |
2 |
2 |
|
Lithium |
Li |
3 |
3 |
4 |
3 |
2, 1 |
|
Beryllium |
Be |
4 |
4 |
5 |
4 |
2, 2 |
|
Boron |
B |
5 |
5 |
6 |
5 |
2, 3 |
|
Carbon |
C |
6 |
6 |
6 |
6 |
2, 4 |
|
Nitrogen |
N |
7 |
7 |
7 |
7 |
2, 5 |
|
Oxygen |
O |
8 |
8 |
8 |
8 |
2, 6 |
|
Fluorine |
F |
9 |
9 |
10 |
9 |
2, 7 |
|
Neon |
Ne |
10 |
10 |
10 |
10 |
2, 8 |
|
Sodium |
Na |
11 |
11 |
12 |
11 |
2, 8, 1 |
|
Magnesium |
Mg |
12 |
12 |
12 |
12 |
2, 8, 2 |
|
Aluminium |
Al |
13 |
13 |
14 |
13 |
2, 8, 3 |
|
Silicon |
Si |
14 |
14 |
14 |
14 |
2, 8, 4 |
|
Phosphorus |
P |
15 |
15 |
16 |
15 |
2, 8, 5 |
|
Sulfur |
S |
16 |
16 |
16 |
16 |
2, 8, 6 |
|
Chlorine |
Cl |
17 |
17 |
18 |
17 |
2, 8, 7 |
|
Argon |
Ar |
18 |
18 |
22 |
18 |
2, 8, 8 |
Pause
and Ponder
14.
Identify the number of electrons in the outermost
shell of the following elements:
(i) ¹²₆C
(ii) ¹⁹₉F
(iii) ²⁸₁₄Si
Answer:
(i)
4
(ii)
7
(iii)
4
15.
Write the electronic configuration of the elements
having atomic numbers 12, 16 and 18.
Answer:
Atomic
number 12 = 2, 8,
2
Atomic
number 16 = 2, 8,
6
Atomic
number 18 = 2, 8,
8
16.
Solve this riddle: I am an atom with a mass number of
23 and 11 protons. I am a soft metal and react vigorously with water. Who am I
and how many neutrons do I have?
Answer:
Element: Sodium (Na)
Neutrons: 23 − 11 = 12.
COMBINING CAPACITY OF AN ATOM: VALENCY
-
The number of atoms of
hydrogen or chlorine with which one atom of an element can combine to form a
compound is called its combining capacity.
-
It is expressed in terms of
hydrogen and chlorine because both possess a combining capacity of one. E.g.,
o In H₂O, oxygen combines with two hydrogen atoms, so the
combining capacity of oxygen is 2.
o In NH₃, the combining capacity of nitrogen is 3.
o In MgCl₂, the combining capacity of magnesium is 2.
-
Combining capacity of an
atom can be decided by its electronic configuration.
-
The outermost shell
containing electrons of an atom is called its valence shell. The
electrons present in it are called valence electrons.
-
If the outermost shell of
an atom has 8 electrons, it is called an octet.
-
Elements with complete octet
(2 electrons in the case of helium) are stable and largely unreactive.
-
Elements with incomplete
valence shells are reactive. They lose, gain, or share electrons to complete
their octet and become stable.
-
The number of electrons
gained, lost, or shared to complete the octet is called the valency of
the element.
-
If the element has fewer
than 4 electrons in its valence shell, it tends to lose electrons to complete
its octet. If the number of valence electrons is more than 4, it tends to gain
electrons to complete its octet. E.g.,
o Electronic configuration of sodium is 2, 8, 1. It can get an
octet by losing 1 electron. Hence, its valency or combining capacity is 1.
o Oxygen (2, 6) has 6 valence electrons. Hence, it can gain 2
electrons to attain an octet, its valency is 2.
-
The electronic
configuration of carbon is 2, 4. Carbon has four valence electrons and cannot
easily gain or lose them. Therefore, it can share four electrons with other
atoms to complete its octet. Thus, the valency of carbon is 4.
-
Some compounds appear to
violate the valency rule.
A DEEPER LOOK INTO ATOMIC STRUCTURE
Isotopes
-
Dalton proposed that all
atoms of an element are identical and have the same mass. However, isotopes
are atoms with the same atomic number but different numbers of
neutrons, giving them different mass numbers (A = p⁺ + n⁰).
-
E.g., Hydrogen
occurs as three isotopes: ¹₁H (protium, ~99.98%), ²₁H (deuterium,
~0.015%), and ³₁H (tritium, traces). All have 1 proton and
1 electron; deuterium has 1 neutron, while tritium has 2 neutrons.
-
Carbon has 3 isotopes: ¹²₆C,
¹³₆C, and ¹⁴₆C. All have 6 protons and 6 electrons but
differ in the number of neutrons. ¹²₆C is the most abundant
isotope.
- Isotopes have similar chemical properties because they
have the same electronic configuration (same valence electrons), but different
physical properties such as melting and boiling points.
Ready to Go Beyond
·
Atoms are too small to be weighed in kg or g, so their
masses are measured in the unified atomic mass unit (u). Earlier, it was
expressed in atomic mass unit (amu).
Bridging Science and
Society: Applications of isotopes
·
²³⁵₉₂U, an isotope of uranium, is used as a fuel in
nuclear reactor to generate electricity in nuclear power plants.
·
⁶⁰₂₇Co, a radioactive isotope of cobalt, is used in
radiation treatment for cancer.
·
¹³¹₅₃I, an isotope of iodine, is used to treat goitre
and thyroid cancer.
·
¹⁴₆C, an isotope of carbon, is used in archaeology
and geology to determine the age of fossils and artefacts.
Average atomic mass
-
Chlorine occurs naturally
as two isotopes: ³⁵Cl (atomic mass 35 u) and ³⁷Cl (atomic mass 37
u) in a 3:1 ratio.
-
The average atomic mass by simple
arithmetic mean of isotopic masses for chlorine= (35 + 37)/2 = 36 u. But
this is inaccurate because it ignores the relative abundances of each isotope.
-
Since isotopes do not occur
in equal amounts (³⁵Cl ~75%, ³⁷Cl ~25%), the weighted average atomic mass
is calculated using their relative abundances. i.e.,
= (35 × 75/100 +
37 × 25/100)
= (105/4 + 37/4)
= 142/4 = 35.5 u
-
E.g., In 1 million chlorine
atoms, about 7.5 lakh are ³⁵Cl and 2.5 lakh are ³⁷Cl, giving a weighted average
atomic mass of 35.5 u.
Meet a Scientist
·
Homi Jehangir Bhabha, the father of the Indian nuclear
programme, established Tata Institute of Fundamental Research (TIFR) and
the Bhabha Atomic Research Centre (BARC), for peaceful uses of atomic
energy for electricity, agriculture and medical treatments.
Ready to Go Beyond
-
Electron microscopes, such as Scanning Tunnelling
Microscopes (STMs) and Transmission Electron Microscopes (TEMs)
produce atomic-level images. STMs study surfaces, while TEMs reveal the
arrangement of atoms inside thin samples.
Pause and Ponder
17.
Two different atoms have 11 protons each, but one has
12 neutrons, and the other has 13 neutrons. How do their atomic numbers and
mass numbers compare? Are they the same element or different elements?
Answer:
Atomic
numbers: Same
(11)
Mass
numbers:
Different (23 and 24)
They
are the same element (isotopes) because they have the same number of protons.
18.
If a bromine atom is available in the form of, say
two isotopes, ⁷⁹₃₅Br (49.7%) and ⁸¹₃₅Br (50.3%), calculate the average atomic
mass of the bromine atom.
Answer:
⁷⁹₃₅Br: Mass = 79 u, Abundance = 49.7%
⁸¹₃₅Br: Mass = 81 u, Abundance = 50.3%
Average
Atomic Mass = (79 × 49.7) + (81 × 50.3) / 100
=
(3926.3 + 4074.3) / 100 = 80.006 u ≈ 80 u
Isobars
-
The atoms of different
elements having the same mass number but
different atomic numbers are called isobars.
-
E.g., calcium (Z=
20), potassium (Z= 19), and argon (Z= 18) have different numbers
of protons, but their mass number is 40. i.e., total number of nucleons in
their atoms is the same.
- Bohr's model is not entirely correct. Electrons do not move in fixed orbits but exist as electron clouds, where only their probable location can be predicted.
