Download Elements, their Symbol, Atomic Number and Molar Mass

QUESTIONS
xv
Atoms and Molecules
3
CHAPTER
Tips and Tricks
l
A compound is composed of two or more elements
chemically combined in definite proportions.
l
Ion is a charged particle and can be positively or
negatively charged.
l
During chemical changes, the sum of masses of
all the reactants remains equal to the masses of
products. This is known as Law of conservation of
mass.
l
The combining capacity of an element is known
as its valency.
l
The chemical formula of a molecular compound
is determined by the valency of the combining
elements.
l
In ionic compounds, the charge on positive ions
and negative ions is taken into account so that the
overall chemical formula gives a neutral
compound.
l
To bring uniformity, relative masses of all atoms
are given in atomic mass unit (u) taking mass of
one atom of a particular isotope of carbon (C-12)
as 1/12 u.
l
The number of atoms present in 12 g of carbon –12
is 6.023 × 1023 which is called Avogadro number
or Avogadro constant.
l
A mole denotes 6.023 × 1023 atoms, molecules or
ions.
l
Mass of 1 mole of a substance expressed in grams
is called its molar mass.
l
l
In a compound, the elements are always present
in definite proportion of mass. This is known as
law of Definite Proportions.
According to Dalton’s atomic theory, all matter
whether element or compound is composed of
small particles called atoms or molecules.
l
Each element has a name and a unique chemical
symbol.
l
An atom is the smallest particle of an element and
retains its identity throughout all physical or
chemical changes.
l
A molecule is a group of two or more atoms that
are chemically bonded together and forms the
smallest part of an element or a compound. This
can exist independently and shows all the
properties of that substance.
2
3
ATOMS AND MOLECULES
Elements, their Symbol, Atomic Number and Molar Mass
Element
Actinium
Aluminium
Americium
Antimony
Argon
Arsenic
Astatine
Barium
Berkelium
Beryllium
Bismuth
Bohrium
Boron
Bromine
Cadmium
Caesium
Calcium
Californium
Carbon
Cerium
Chlorine
Chromium
Cobalt
Copper
Curium
Dubnium
Dysprosium
Einsteinium
Erbium
Europium
Fermium
Fluorine
Francium
Gadolinium
Gallium
Germanium
Gold
Hafnium
Hassium
Helium
Holmium
Symbol
Atomic
number
Ac
Al
Am
Sb
Ar
As
At
Ba
Bk
Be
Bi
Bh
B
Br
Cd
Cs
Ca
Cf
C
Ce
Cl
Cr
Co
Cu
Cm
Db
Dy
Es
Er
Eu
Fm
F
Fr
Gd
Ga
Ge
Au
Hf
Hs
He
Ho
89
13
95
51
18
33
85
56
97
4
83
107
5
35
48
55
20
98
6
58
17
24
27
29
96
105
66
99
68
63
100
9
87
64
31
32
79
72
108
2
67
Molar mass/
(g mol–1)
227.03
26.98
(243)
121.75
39.95
74.92
210
137.34
(247)
9.01
208.98
(264)
10.81
79.91
112.40
132.91
40.08
251.08
12.01
140.12
35.45
52.00
58.93
63.54
247.07
(263)
162.50
(252)
167.26
151.96
(257.10)
19.00
(223)
157.25
69.72
72.61
196.97
178.49
(269)
4.00
164.93
Element
Hydrogen
Indium
Iodine
Iridium
Iron
Krypton
Lanthanum
Lawrencium
Lead
Lithium
Lutetium
Magnesium
Manganese
Meitnerium
Mendelevium
Mercury
Molybdenum
Neodymium
Neon
Neptunium
Nickel
Niobium
Nitrogen
Nobelium
Osmium
Oxygen
Palladium
Phosphorus
Platinum
Plutonium
Polonium
Potassium
Praseodymium
Promethium
Protactinium
Radium
Radon
Rhenium
Rhodium
Rubidium
Ruthenium
Symbol
Atomic
number
Molar mass/
(g mol–1)
H
In
I
Ir
Fe
Kr
La
Lr
Pb
Li
Lu
Mg
Mn
Mt
Md
Hg
Mo
Nd
Ne
Np
Ni
Nb
N
No
Os
O
Pd
P
Pt
Pu
Po
K
Pr
Pm
Pa
Ra
Rn
Re
Rh
Rb
Ru
1
49
53
77
26
36
57
103
82
3
71
12
25
109
101
80
42
60
10
93
28
41
7
102
76
8
46
15
78
94
84
19
59
61
91
88
86
75
45
37
44
1.0079
114.82
126.90
192.2
55.85
83.80
138.91
(262.1)
207.19
6.94
174.96
24.31
54.94
(268)
258.10
200.59
95.94
144.24
20.18
(237.05)
58.71
92.91
14.0067
(259)
190.2
16.00
106.4
30.97
195.09
(244)
210
39.10
140.91
(145)
231.04
(226)
(222)
186.2
102.91
85.47
101.07
4
Element
Rutherfordium
Samarium
Scandium
Seaborgium
Selenium
Silicon
Silver
Sodium
Strontium
Sulphur
Tantalum
Technetium
Tellurium
Terbium
CCE SCIENCE–IX
Symbol
Atomic
number
Molar mass/
(g mol–1)
Element
Symbol
Rf
Sm
Sc
Sg
Se
Si
Ag
Na
Sr
S
Ta
Tc
Te
Tb
104
62
21
106
34
14
47
11
38
16
73
43
52
65
(261)
150.35
44.96
(266)
78.96
28.08
107.87
22.99
87.62
32.06
180.95
(98.91)
127.60
158.92
Thallium
Tl
81
204.37
Thorium
Th
90
232.04
Thulium
Tm
69
168.93
Tin
Sn
50
118.69
Titanium
Ti
22
47.88
Tungsten
W
74
183.85
Uranium
U
92
238.03
Vanadium
V
23
50.94
Xenon
Xe
54
131.30
Ytterbium
Yb
70
173.04
Yttrium
Y
39
88.91
Zinc
Zn
30
65.37
Zirconium
Zr
40
91.22
The value given in parenthesis is the molar mass of the isotope of largest known half-life.
Atomic
number
Molar mass/
(g mol–1)
5
ATOMS AND MOLECULES
Formative Assessment
ORAL QUESTIONS (Conversation Type)
1. (a) Are the laws of combinations based on
conservation of energy or conservation of
mass?
(b) What is the advantage of this law in
chemical reactions?
(c) Which statement of Dalton’s atomic theory
is in conformity with the above law?
(d) On what accounts regarding nature of
atoms, Dalton’s atomic theory fails?
(e) Does the fact that ‘atom can be subdivided’
affect normal chemical reactions?
2. (a) What is an atom?
(b) Do elements exist as atoms?
(c) How are atoms of elements represented in
chemistry?
(d) What is the purpose of writing atoms by
symbols?
(e) What is the symbol of mercury and silver?
3. (a) How is the mass of an atom measured?
(b) What is one atomic mass unit?
(c) Why is such a scale chosen?
(d) How do you account the fractional masses
of many elements?
4. (a) What is valency?
(b) How is valency of an element determined?
(c) What is the use of valency?
(d) Give an example.
(e) What is molecular mass?
ORAL QUESTIONS
1. Mass of reactants is equal to mass of products.
Which postulate of Dalton‘s atomic theory
demonstrates this?
2. What is the Latin name and symbol of sodium?
3. What is the Latin name and symbol of gold?
4. What is the atomicity of phosphorous?
5. What is the atomicity of sulphur?
6. Give the name and symbol of a triatomic
molecule.
7. Hydrogen and oxygen always combine in the
ratio of 1 : 8 to form water. Which law of
combination does it demonstrate?
8. Which parameter gives the combining capacity
of an element?
9. In what form atoms of a solid exist?
10. What is the smallest particle of a compound
capable of stable existence?
11. What is a charged particle called?
12. What unit represents weight of
1
th mass of
12
C-12 atom?
13. What represents the chemical composition of a
compound?
14. What mass represents 1 mole?
15. What number represents 1 mole?
16. What is Avogadro number constant?
17. How many molecules will be present in 1 gram
molecular mass of a substance?
18. How are C12 and C14 related?
19. How are C14 and N14 related?
20. What is the term used for mass in grams of
1 mole of a substance?
TRUE OR FALSE
1. A compound may consist of two or more
elements in varying composition.
2. Law of conservation of mass states that energy
can neither be created nor destroyed.
3. The occurrence of isotopes leads to drawback
of Dalton’s atomic theory of matter.
4. Lead is represented as Ld.
5. A molecule of a compound is indicative of its
properties.
6. Copper carbonate is a heteroatomic molecule.
7. Nitrate is a polyatomic anion.
8. The molecular formula of aluminium nitrate is
AlN.
9. The atomic mass of Mg is 24 grams.
10. One mole of a gas occupies 22.4 litres at any
temperature and pressure.
6
CCE SCIENCE–IX
QUIZ
B. ⇒ Across
1. Symbolic representation of a compound (7)
1. According to me a compound is composed of
the same elements combined together in the
same proportion by mass.
3. A quantity giving 6.02 × 1023 molecules of a
compound (4)
2. I had announced that atoms of a given element
are identical in mass and chemical properties.
3. I am the building block of an element.
4. I represent sodium as Na.
5. I am the smallest unit that gives the relative
mass of an atom.
6. I am a monoatomic molecule.
7. I am a giant molecule containing sixty carbon
atoms.
8. I decide how many atoms of two elements will
combine to form a molecule?
9. I am a number that gives one gram atom of the
element.
10. I give mass in grams of 1 mole of a substance.
PUZZLES
A. ⇒ Across
1. Smallest particle of a compound (8)
3. Two atoms with same atomic mass but different
atomic numbers (7)
⇓ Down
2. Scientist who gave atomic theory of matter (6)
4. An element having atomicity of 4 (11)
⇓ Down
2. A representation of an atom of an element. (6)
4. An element with a symbol of Cr (8)
SUGGESTED PROJECT/ACTIVITY
1. Carry out simple precipitative reaction and see
that law of conservation of mass is followed.
Mix weighed amount of
(a) barium chloride and sodium sulphate
(b) lead nitrate and sodium chromate.
and see that the amount of products formed
remains the same.
7
ATOMS AND MOLECULES
Assignments
Name: ............................
Class: ......... Section: ......... Roll No.: ........ Grade: .......... Teacher’s sign: .......................
CLASS ASSIGNMENT
1. Which postulate of Dalton’s atomic theory is a
result of law of conservation of mass?
.....................................................................................
.....................................................................................
2. In what way occurrence of isobars contradicts
Dalton’s atomic theory?
.....................................................................................
6. Give the value of one atomic mass unit.
.....................................................................................
.....................................................................................
.....................................................................................
7. What is the symbol of gold?
.....................................................................................
.....................................................................................
8. Give names of three triatomic molecules.
.....................................................................................
.....................................................................................
.....................................................................................
.....................................................................................
3. Three samples of ammonia obtained in three
different ways contain nitrogen and hydrogen
in the ratio 3 : 14. Which law of combination
does it conform to? Explain.
.....................................................................................
.....................................................................................
.....................................................................................
.....................................................................................
4. What is an atom? What would be the size of
the smallest atom?
.....................................................................................
.....................................................................................
.....................................................................................
.....................................................................................
5. What do you mean by symbol of elements?
.....................................................................................
.....................................................................................
.....................................................................................
.....................................................................................
9. Are one atom of carbon and 2 atoms of oxygen
equal to one molecule of carbon dioxide?
Explain.
.....................................................................................
.....................................................................................
.....................................................................................
.....................................................................................
.....................................................................................
10. Write down the formulae of
(i)
(ii)
(iii)
(iv)
hydrogen sulphide
aluminium sulphate
calcium bicarbonate
barium sulphate.
.....................................................................................
.....................................................................................
.....................................................................................
.....................................................................................
8
CCE SCIENCE–IX
Name: ............................. Class: ........ Section: ........
Roll No.: ........ Grade: ......... Teacher’s sign: ...........................
HOME ASSIGNMENT
1. How is Avogadro constant defined?
6. Define and explain atomic mass of an element.
.....................................................................................
.....................................................................................
.....................................................................................
.....................................................................................
.....................................................................................
.....................................................................................
2. Give an example to show that chemical
reactions obey law of conservation of mass.
.....................................................................................
.....................................................................................
7. How does an atom differ from a molecule?
.....................................................................................
.....................................................................................
.....................................................................................
.....................................................................................
.....................................................................................
.....................................................................................
.....................................................................................
.....................................................................................
.....................................................................................
8. Discuss the utility of mole concept.
3. Define and explain law of multiple proportions.
.....................................................................................
.....................................................................................
.....................................................................................
.....................................................................................
.....................................................................................
.....................................................................................
.....................................................................................
.....................................................................................
.....................................................................................
4. State drawbacks of Dalton’s atomic theory of
matter.
9. Calculate the number of atoms in 1 g of copper.
(Atomic mass of copper is 63.50.)
.....................................................................................
.....................................................................................
.....................................................................................
.....................................................................................
.....................................................................................
.....................................................................................
.....................................................................................
.....................................................................................
5. How do atoms exist?
.....................................................................................
.....................................................................................
.....................................................................................
10. How many moles of oxygen atoms are present
in one mole of the following compounds?
NaNO3, H2SO4, Cl2O7
.....................................................................................
.....................................................................................
.....................................................................................
.....................................................................................
.....................................................................................
.....................................................................................