Atoms and Molecules
Q. What is an atom?
Ans. Atoms are the building blocks or structural units of matter.
Q. What is a molecule?
Ans. A molecule is the smallest part of matter which shows all the
properties of the matter. Two or more atoms chemically combine to form molecules.
Q. Who used the term atom for the first time?
Ans. The Greek philosopher Democritus coined the term
“atom” for the first time from the Greek word atomos,
which means “indivisible”.
The Indian sage and philosopher Acharya Kanad (Maharishi Kanad)
is credited with first proposing the idea of atoms
(Paramanu) in India.
Q. How big are atoms?
Ans. An atom is extremely small and cannot be seen with the naked eye.
The size of an atomic radius is measured in nanometres (nm).
1 nm = 10−9 m
The smallest atom is Helium and the largest atom is Francium.
Q. How do atoms exist?
Ans. The Indian philosopher Pakudha Katyayama stated that
atoms normally exist in a combined form, which gives rise to various forms
of matter.
Most atoms of elements are found in the form of molecules or ions.
Q. Who gave the atomic theory?
Ans. The atomic theory was given by the English scientist
John Dalton in 1808.
Q. Write Dalton’s atomic theory.
Ans. According to Dalton’s atomic theory, all matter, whether an
element, a compound or a mixture, is composed of small particles called atoms.
The postulates of this theory are as follows:
- All matter is made of very tiny particles called atoms, which participate in chemical reactions.
- Atoms are indivisible particles and cannot be created or destroyed in a chemical reaction.
- Atoms of a given element are identical in mass and chemical properties.
- Atoms of different elements have different masses and chemical properties.
- Atoms combine in the ratio of small whole numbers to form compounds.
- The relative number and kinds of atoms are constant in a given compound.
Q. What are the drawbacks of Dalton’s atomic theory?
Ans. Dalton’s atomic theory has some drawbacks:
-
According to Dalton, atoms are indivisible, but later it was discovered
that atoms are made up of subatomic particles.
-
Dalton stated that atoms of a given element are identical in mass,
but the discovery of isotopes proved that atoms of the same element
may have different masses, e.g., hydrogen has isotopes of 1u, 2u and 3u.
-
Dalton stated that atoms of different elements have different masses,
but the discovery of isobars showed that atoms of different elements
may have the same mass, e.g., Calcium and Potassium both have mass 40u.
Q. Who gave the laws of chemical combination?
Ans. The laws of chemical combination were given by
Antoine L. Lavoisier.
Q. What are the laws of chemical combination?
Ans. There are two laws of chemical combination:
-
Law of Conservation of Mass, given by
Antoine L. Lavoisier.
-
Law of Constant Proportions, given by
Joseph L. Proust.
Q. State the Law of conservation of mass.
Ans. The law of conservation of mass states that
“Mass can neither be created nor destroyed in a chemical reaction.”
Q. State the Law of constant proportions.
Ans. The law of constant proportions states that
“In a chemical substance, the elements are always present
in definite proportions by mass.”
Q. What does IUPAC stand for?
Ans. IUPAC stands for
International Union of Pure and Applied Chemistry.
It is an independent worldwide scientific organization.
Q. What are the functions of IUPAC?
Ans. IUPAC approves the names of elements, their symbols
and their mass units.
Q. How do all elements get their names?
Ans. Elements have been named after a number of things, such as:
-
The name of the place where they were found for the first time.
Example: The name copper was taken from Cyprus.
-
Specific colours.
Example: Gold was taken from an English word meaning
yellow.
Q. Who started the symbolisation of an element?
Ans. Dalton was the first scientist to use symbols for elements
in a specific sense, but they were difficult to use and memorise.
Later, Berzelius suggested that symbols of elements should be
made from one or two letters of the name of the element.
Therefore, Berzelius is credited with the symbolisation of elements.
Q. What are the rules used by IUPAC to assign a symbol to an element?
Ans. The rules used by IUPAC to assign a symbol to an element are :-
-
Many symbols are the first one or two letters of the element’s
name in English.
-
The first letter of a symbol is always written as a
capital letter and the second letter as a
small letter.
-
Some symbols have been taken from the names of elements in
Latin, German or Greek.
Q. Table of the symbols of elements.
Ans.
| Element |
Symbol |
Element |
Symbol |
Element |
Symbol |
| Aluminium | Al |
Copper | Cu |
Nitrogen | N |
| Argon | Ar |
Fluorine | F |
Oxygen | O |
| Barium | Ba |
Gold | Au |
Potassium | K |
| Boron | B |
Hydrogen | H |
Silicon | Si |
| Bromine | Br |
Iodine | I |
Silver | Ag |
| Calcium | Ca |
Iron | Fe |
Sodium | Na |
| Carbon | C |
Lead | Pb |
Sulphur | S |
| Chlorine | Cl |
Magnesium | Mg |
Uranium | U |
| Cobalt | Co |
Neon | Ne |
Zinc | Zn |
Q. Give some examples of elements whose symbols have been made from Latin names.
Ans. Some examples are:
-
Gold has the symbol Au, derived from its Latin name
Aurum.
-
Silver has the symbol Ag, derived from its Latin name
Argentium.
-
Iron has the symbol Fe, derived from its Latin name
Ferrum.
-
Sodium has the symbol Na, derived from its Latin name
Natrium.
-
Potassium has the symbol K, derived from its Latin name
Kalium.
Q. Name the first scientist who talked about the atomic mass.
Ans. John Dalton was the first scientist who talked about
atomic mass in his atomic theory.
Q. How were the atomic masses calculated?
Ans. Since determining the mass of an individual atom was a
relatively difficult task, relative atomic masses were determined
using the laws of chemical combinations and the compounds formed.
In 1961, 1/12th part of the mass of an atom of
carbon-12 isotope was selected by IUPAC as the standard.
Before this, 1/16th part of the mass of an atom of
oxygen was used as the standard.
Q. What is relative atomic mass?
Ans. The relative atomic mass of an atom of an element is
defined as the average mass of the atom as compared to
1/12th the mass of one atom of carbon-12.
The relative atomic masses of all elements have been determined
with respect to an atom of carbon-12.
Q. Define the atomic mass unit.
Ans. One atomic mass unit (amu) is defined as a mass equal to
exactly one-twelfth (1/12th) of the mass of one atom of
carbon-12.
The SI unit of atomic mass is called unified mass, which is denoted
by the symbol “u”.
Q. Define 1 unified mass.
Ans. 1 unified mass (1 u) is equal to
one-twelfth the mass of one atom of carbon-12.
Q. Write atomic masses of some elements.
Ans.
| Element |
Atomic Mass (u) |
| Hydrogen | 1 |
| Carbon | 12 |
| Nitrogen | 14 |
| Oxygen | 16 |
| Sodium | 23 |
| Magnesium | 24 |
| Sulphur | 32 |
| Chlorine | 35.5 |
| Calcium | 40 |
Q. What is a molecule?
Ans. A molecule is in general a group of two or more atoms that are
chemically bonded together. It is the smallest particle of an element
or a compound that is capable of independent existence and shows all
the properties of that substance.
Q. How many types of molecules are there?
Ans. There are two types of molecules:
1. Molecules of elements: The molecules of an element are constituted
by the same type of atoms.
Example: H2, Cl2, N2, O2,
O3, P4, S8, Al, He, Ne, Ar etc.
2. Molecules of compounds: Atoms of different elements join together
in definite proportions to form molecules of compounds.
Example: CO2, H2O, NH3, NH4 etc.
Q. What is atomicity?
Ans. The number of atoms constituting a molecule is known as its
atomicity.
Examples:
Atomicity of H2 = 2,
O3 = 3,
P4 = 4,
S8 = 8,
H2O = 3,
NH3 = 4,
NH4 = 5.
Q. What does monatomic mean?
Ans. The molecules of an element which are constituted by
only one atom are called monatomic.
All noble gases (He, Ne, Ar, etc.) and all metals
(Al, Mg, Ca, Fe, Cu, etc.) are monatomic.
Q. What does diatomic mean?
Ans. The molecules of an element or compound which are constituted by
two atoms are called diatomic.
Examples: H2, Cl2, N2, O2, CO, NaCl etc.
Q. What does triatomic mean?
Ans. The molecules of an element or compound which are constituted by
three atoms are called triatomic.
Examples: O3, H2O etc.
Q. What does tetratomic mean?
Ans. The molecules of an element or compound which are constituted by
four atoms are called tetratomic.
Examples: P4, NH3, CO3 etc.
Q. What does polyatomic mean?
Ans. All the molecules of an element or compound which are constituted by
more than two atoms are called polyatomic.
Examples: H2, O3, P4, S8,
NH3, CO3 etc.
Q. What are the differences between atoms and molecules?
| Atoms |
Molecules |
| They are the smallest particles of matter that may or may not show the properties of matter. |
They are the smallest particles of matter that must show all the properties of matter. |
| They are not stable. |
They are stable. |
| They may or may not exist independently. |
They exist independently. |
| Atoms are made up of sub-atomic particles. |
Molecules are made up of atoms. |
Examples: H, P, S, Cl etc.
(Atom of hydrogen = H)
|
Examples: H2, P4, S8 etc.
(Molecule of hydrogen = H2)
|
Q. What is an ion?
Ans. The charged particles in a molecule are called
ions.
There are two types of ions:
-
Cation: Positive charge is called cation.
Examples: Al3+, Ca2+, Na+ etc.
-
Anion: Negative charge is called anion.
Examples: O2−, H−, S2− etc.
Q. What does polyatomic ion mean?
Ans. A group of atoms carrying a charge is known as a
polyatomic ion.
Examples: SO42−, CO32−,
OH− etc.
Q. Names and symbols of some ions.
| Valency |
Name of Ion |
Symbol |
Non-metallic Ion |
Symbol |
Polyatomic Ion |
Symbol |
| 1 |
Sodium |
Na+ |
Hydrogen |
H+ |
Ammonium |
NH4+ |
| Potassium |
K+ |
Chloride |
Cl− |
Hydroxide |
OH− |
| Silver |
Ag+ |
Bromide |
Br− |
Nitrate |
NO3− |
| Copper (I) |
Cu+ |
Iodide |
I− |
Hydrogen carbonate |
HCO3− |
| 2 |
Magnesium |
Mg2+ |
Oxide |
O2− |
Carbonate |
CO32− |
| Calcium |
Ca2+ |
Sulphide |
S2− |
Sulphite |
SO32− |
| Zinc |
Zn2+ |
— |
— |
Sulphate |
SO42− |
| 3 |
Aluminium |
Al3+ |
Nitride |
N3− |
Phosphate |
PO43− |
Q. What does chemical formula mean?
Ans. The chemical formula of a compound is a symbolic representation
of its composition.
For example, the chemical formula of water is
H2O,
which represents that water is composed of hydrogen and oxygen.
Examples of some chemical formulas:
| Common Name |
Chemical Name |
Chemical Formula |
| Quick lime |
Calcium Oxide |
CaO |
| Slaked lime |
Calcium Hydroxide |
Ca(OH)2 |
| Limestone |
Calcium Carbonate |
CaCO3 |
| Blue Vitriol |
Copper Sulphate Pentahydrate |
CuSO4.5H2O |
| Green Vitriol |
Ferrous Sulphate Heptahydrate |
FeSO4.7H2O |
| Baking Soda |
Sodium Hydrogen Carbonate |
NaHCO3 |
| Sulphuric Acid |
Hydrogen Sulphate |
H2SO4 |
| Nitric Acid |
Hydrogen Nitrate |
HNO3 |
| Hydrochloric Acid |
Hydrogen Chloride |
HCl |
| Water |
Hydrogen Oxide |
H2O |
| Milk of Magnesia |
Magnesium Hydroxide |
Mg(OH)2 |
| Caustic Soda |
Sodium Hydroxide |
NaOH |
| Common Salt |
Sodium Chloride |
NaCl |
| Ammonia |
Nitrogen Trihydride |
NH3 |
| Ammonium |
Nitrogen Tetrahydride |
NH4 |
Q. What does valency mean?
Ans. The combining power of an element is known as the
valency of an atom.
Examples:
Valency of Al = 3 (Al3+)
Valency of O = 2 (O2−)
Valency of Na = 1 (Na+)
Q. Find the atomicity of Aluminium Sulphate.
Ans. Aluminium Sulphate = Al2(SO4)3
Number of Aluminium atoms = 2
Number of Sulphur atoms = 1 × 3 = 3
Number of Oxygen atoms = 4 × 3 = 12
So, the atomicity of Aluminium Sulphate = 2 + 3 + 12
= 17
Q. Find the molecular mass of Aluminium Sulphate.
Aluminium Sulphate = Al
2(SO
4)
3
Mass of Aluminium atoms = 2 × 27 = 54 u
Mass of Sulphur atoms = 1 × 3 × 32 = 96 u
Mass of Oxygen atoms = 4 × 3 × 16 = 192 u
Molecular mass = 54 + 96 + 192
= 342 u
Q. Calculate the relative molecular mass of water (H2O).
H2O = 2 × H + 1 × O
= 2 × 1 + 1 × 16
= 2 + 16
= 18 u
Q. Calculate the molecular mass of HNO3.
HNO3 = 1 × H + 1 × N + 3 × O
= 1 × 1 + 1 × 14 + 3 × 16
= 1 + 14 + 48
= 63 u
Q. Find the mass ratio of Hydrogen and Oxygen in water.
Water = H
2O
Mass of Hydrogen = 2 × 1 = 2
Mass of Oxygen = 1 × 16 = 16
Mass ratio = 2 : 16 = 1 : 8
= 1 : 8
Q. Find the atomic ratio of Hydrogen and Oxygen in water.
Water = H
2O
Atomic ratio of Hydrogen = 2
Atomic ratio of Oxygen = 1
= 2 : 1
Q. Write down the formulae of:
1. Sodium oxide → Na2O
2. Aluminium chloride → AlCl3
3. Sodium sulphide → Na2S
4. Magnesium hydroxide → Mg(OH)2
Q. Write down the names of compounds represented by the following formulae:
(i) Al2(SO4)3 → Aluminium Sulphate
(ii) CaCl2 → Calcium Chloride
(iii) K2SO4 → Potassium Sulphate
(iv) KNO3 → Potassium Nitrate
(v) CaCO3 → Calcium Carbonate
Q. Calculate the molecular masses of the following:
H2 = 2 × H
= 2 × 1
= 2 u
O2 = 2 × O
= 2 × 16
= 32 u
Cl2 = 2 × Cl
= 2 × 35.5
= 71 u
CO2 = 1 × C + 2 × O
= 1 × 12 + 2 × 16
= 12 + 32
= 44 u
CH4 = 1 × C + 4 × H
= 1 × 12 + 4 × 1
= 12 + 4
= 16 u
C2H6 = 2 × C + 6 × H
= 2 × 12 + 6 × 1
= 24 + 6
= 30 u
NH3 = 1 × N + 3 × H
= 14 + 3 × 1
= 17 u
CH3OH = 1 × C + 4 × H + 1 × O
= 12 + 4 + 16
= 32 u
Q. Calculate the formula unit masses of ZnO, Na2O, K2CO3, given atomic masses of
Zn = 65 u, Na = 23 u, K = 39 u, C = 12 u and O = 16 u.
ZnO = 1 × Zn + 1 × O
= 1 × 65 + 1 × 16
= 65 + 16
= 81 u Ans
Na2O = 2 × Na + 1 × O
= 2 × 23 + 1 × 16
= 46 + 16
= 62 u Ans
K2CO3 = 2 × K + 1 × C + 3 × O
= 2 × 39 + 1 × 12 + 3 × 16
= 78 + 12 + 48
= 138 u Ans
Q. Calculate the formula unit mass of CaCl2.
CaCl2 = 1 × Ca + 2 × Cl
= 1 × 40 + 2 × 35.5
= 40 + 71
= 111 u Ans
Q. Write the atomic mass of oxygen and molecular mass of oxygen.
Atom of oxygen = O
Molecule of oxygen = O2
Atomic mass of oxygen = 1 × 16
= 16 u Ans
Molecular mass of oxygen = 2 × 16
= 32 u Ans
Exercise
1. A 0.24 g sample of a compound of oxygen and boron was found by analysis to contain
0.096 g of boron and 0.144 g of oxygen.
Calculate the percentage composition of the compound by weight.
Ans.
Boron = 0.096 g
Oxygen = 0.144 g
Total = 0.24 g
Boron % = ?
Oxygen % = ?
Boron % =
Boron /
Total × 100
= 0.096 / 0.24 × 100
= 40 % Ans
Oxygen % =
Oxygen /
Total × 100
= 0.144 / 0.24 × 100
= 60 % Ans
2. When 3.0 g of carbon is burnt in 8.00 g oxygen, 11.00 g of carbon dioxide is produced.
What mass of carbon dioxide will be formed when 3.00 g of carbon is burnt in
50.00 g of oxygen? Which law of chemical combination will govern your answer?
Ans. Because 3.0 g of carbon is burnt in 8.00 g oxygen,
11.00 g of carbon dioxide is produced.
So, when 3.00 g of carbon is burnt in 50.00 g of oxygen then according to the
law of constant proportion:
3.00 g of carbon will burn in 8.00 g of oxygen to produce
11.00 g of carbon dioxide and 42.00 g of oxygen will be left.
So, 11.00 g of carbon dioxide will form.
Law of constant proportion governs my answer.
3. What are polyatomic ions? Give examples.
Ans. A group of atoms carrying a charge is known as a
polyatomic ion.
For example: SO42−,
CO32−, OH− etc.
4. Write the chemical formulae of the following.
a) Magnesium chloride
Ans. MgCl2
b) Calcium oxide
Ans. CaO
c) Copper nitrate
Ans. CuNO3
d) Aluminium chloride
Ans. AlCl3
e) Calcium carbonate
Ans. CaCO3
5. Give the names of the elements present in the following compounds.
a) Quick lime
Ans. Formula of Quick lime = CaO
So, in Quick lime, Calcium and Oxygen elements are present.
b) Hydrogen bromide
Ans. Formula of Hydrogen bromide = HBr
So, in Hydrogen bromide, Hydrogen and Bromine elements are present.
c) Baking powder
Ans. Formula of Baking powder = NaHCO3
So, in Baking powder, Sodium, Hydrogen, Carbon and Oxygen
elements are present.
d) Potassium sulphate
Ans. Formula of Potassium sulphate = K2SO4
So, in Potassium sulphate, Potassium, Sulphur and Oxygen
elements are present.
6. Calculate the molecular mass of the following substances.
a) Ethyne, C2H2
Ans. C2H2 = 2 × C + 2 × H
= 2 × 12 + 2 × 1
= 24 + 2
= 26 u Ans
b) Sulphur molecule, S8
Ans. S8 = 8 × S
= 8 × 32
= 256 u Ans
c) Phosphorus molecule, P4
(Atomic mass of phosphorus = 31)
Ans. P4 = 4 × P
= 4 × 31
= 124 u Ans
d) Hydrochloric acid, HCl
Ans. HCl = 1 × H + 1 × Cl
= 1 × 1 + 1 × 35.5
= 1 + 35.5
= 36.5 u Ans
e) Nitric acid, HNO3
Ans. HNO3 = 1 × H + 1 × N + 3 × O
= 1 × 1 + 1 × 14 + 3 × 16
= 1 + 14 + 48
= 63 u Ans
