CHAPTER - 3
ATOMS AND MOLECULES
Matter is made up of small particles called atoms. Atom is the smallest building block of matter. Atoms are very small, they are smaller than anything we can imagine or compare with.
An Indian philosopher Maharishi Kanad postulated that if we go on dividing matter(padarth) further and further into smallest particles called parmanu as indivisible.
Democritus and Leucippus proposed ‘atom’ term.
Laws of Chemical Combination:
The process of combination of two or more elements to form new compounds is governed by certain laws called laws of chemical combination. These are:
1. Law of conservation of mass.
Proposed by Lavoisier in 1744
states that ‘Mass can neither be created nor destroyed in a chemical reaction’.
This means that during a chemical reaction the sum of the masses of the reactants and products remain unchanged.
2. Law of constant proportions. Proposed by J.Proust.
states that ‘In a chemical compound the elements are always present in a definite proportion by mass’.
Eg :- Water ( H2O) always contains two elements hydrogen and oxygen combined together in the same ratio of 2:16 or 1:8 by mass. If 9 g of water is decomposed, we get 1 g of hydrogen and 8 g of oxygen.
Dalton’s atomic theory :-
i) Matter is made up of tiny particles called atoms.
ii) Atoms are indivisible and cannot be created or destroyed
in a chemical reaction
iii) Atoms of a given element are similar in mass and
properties.
iv) Atoms of different elements have different masses and
properties.
v) Atoms combine in small whole number ratios to form
compounds.
vi) In a given compound the relative number and kind of
atoms are constant.
Atom :-
An atom is the smallest particle of an element that may or may not exist independently and retains all its chemical properties.
Atoms are very small in size and smaller than anything we can imagine or compare with.
Atomic radius is measured in nanometres (nm)
1 nanometer = 10 -9 m or 1 meter = 109 nm
Eg :- The atomic radius of an atom of hydrogen is 10-10 m.
The radius of a molecule of water is 10-9 m.
Symbols of atoms of different elements :-
Dolton was first scientist to use the symbol for elements
Berzillius suggested modern symbol of elements which be made from one or two letters of the name of the element.
Symbols of some common elements:-
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 |
Atomic mass :
Since atoms are very small in size its mass is very small and determining its mass is very difficult. So the mass of an atom is compared with the mass of a standard atom.
The atom which is considered as a standard atom for comparing the masses of other atoms is carbon – 12 atom whose atomic mass is 12 u (atomic mass unit).
One atomic mass unit (u) is the mass of 1/12th the mass of a carbon – 12 atom.
The atomic mass of an element is defined as the average mass of one atom of the element compared with 1/12th the mass of a carbon – 12 atom.
Atomic masses of some elements :-
Element | Atomic mass (u) | Element | Atomic mass (u) |
Hydrogen | 1 | Magnesium | 24 |
Carbon | 12 | Aluminium | 27 |
Nitrogen | 14 | Sulphur | 32 |
Oxygen | 16 | Chlorine | 35.5 |
Sodium | 23 | Calcium | 40 |
Molecule :-
A molecule is the smallest particle of an element or compound which exists independently and shows all the properties of that substance.
A molecule is a group of two or more elements that are held together by attractive forces.
Atoms of the same element or different elements can join together to form molecules.
Type of molecules
1.Molecule of elements :-
Molecule of an element contains atoms of the same element.
Molecules of some elements contain only one atom and molecules of some elements contain two or more atoms.
Ex- H2, O2, Cl2 etc
Atomicity of an element :- is the number of atoms present in one molecule of the element.
Atomicity of some elements: -
Type of element | Name | Atomicity |
Non metal | Argon Ar | 1 – Monoatomic |
Non metal | Helium He | 1 – Monoatomic |
Non metal | Oxygen O2 | 2 – Diatomic |
Non metal | Hydrogen H2 | 2 – Diatomic |
Non metal | Nitrogen N2 | 2 – Diatomic |
Non metal | Chlorine Cl2 | 2 – Diatomic |
Npn metal | Phosphorus P4 | 4 – Phosphorus |
Non metal | Sulphur S8 | Poly atomic |
Metal | Sodium Na | 1 – Monoatomic |
Metal | Iron Fe | 1 – Monoatomic |
Metal | Aluminium Al | 1 – Monoatomic |
Metal | Copper Cu | 1 – Monoatomic |
2. Molecule of compounds :-
Molecule of a compound contains atoms of two or more different types of elements.
Molecules of some compounds: -
Compound | Combining elements | Number of atoms of each elements |
Water – H2O | Hydrogen, Oxygen | 2 - Hydrogen, 1 - Oxygen |
Ammonia – NH3 | Nitrogen, Hydrogen | 1 - Nitrogen, 3 - Hydrogen |
Carbon dioxide CO2 | Carbon, Oxygen | I - Carbon, 2 - Oxygen |
Hydrochloric acid HCl | Hydrogen, Chlorine | 1 - Hydrogen, 1 - Chlorine |
Nitric acid HNO3 | Hydrogen, Nitrogen, Oxygen | 1 - Hydrogen, 1 - Nitrogen, 3 - Oxygen |
Sulphuric acid H2SO4 | Hydrogen, Sulphur, Oxygen | 2 - Hydrogen, 1 - Sulphur, 4 - Oxygen |
Ions :-
Compounds containing metal and non metal elements contain charged particles called ions.
An ion is a charged particle having positive or negative charge. A positively charged ion is called ‘cation’ and a negatively charged ion is called ‘anion’.
Polyatomic ions- is A group of atoms carrying a charge. ex- SO42- ,NO3-
Valency :- is the combining capacity of an element.
Some common ions and their valencies :-
Valency | Name of ion Symbol | Name of ion Symbol | Name of ion Symbol |
1 | Sodium Na+ | Hydrogen H + | Ammonium NH 4- |
1 | Potassium K+ | Hydride H - | Hydroxide OH - |
1 | Silver Ag+ | Chloride Cl - | Nitrate NO3- |
1 | Copper (I) also known as Cuprous Cu+ | Bromide Br - | Hydrogen carbonate HCO3- |
2 | Magnesium Mg 2+ | Iodide I - |
|
2 | Calcium Ca 2+ | Oxide O 2- | Carbonate CO32- |
2 | Zinc Zn 2+ |
| Sulphite SO32- |
2 | Iron (II) also known Ferrous Fe 2+ |
| Sulphate SO42- |
2 | Copper (II) also known Cupric Cu 2+ |
|
|
3 | Aluminium Al 3+ | Nitride N 3- | Phosphate PO43- |
3 | Iron (iii) also known Ferric Fe 3+ |
|
|
Writing chemical formulae :-
i) symbol of the metal or positive ion is on the left and symbol / formula
of the non metal or negative ion is on the right.
ii) Write the valencies of the elements or ions below the elements or
ions.
iii) Cross over the valencies of the combining ions.
iv) Polyatomic ions should be enclosed in bracket before writing the
formula.
Examples :-
i) Formula of hydrogen chloride ii) Formula of hydrogen sulphide
Symbol H Cl Symbol H S
Valency 1 1 Valency 1 2
Formula HCl Formula H2S
iii) Formula of Magnesium chloride iv) Formula of Carbon
tetrachloride
Symbol Mg Cl Symbol C Cl
Valency 2 1 Valency 4 1
Formula MgCl2 Formula CCl4
v) Formula of Calcium oxide vi) Formula of Aluminium oxide
Symbol Ca O Symbol Al O
Valency 2 2 Valency 3 2
Formula Ca2O2 = CaO Formula Al2O3
vii) Formula of Sodium nitrate viii) Formula of Calcium hydroxide
Symbol/ Na NO3 Symbol/ Ca OH
Formula Formula
Valency 1 1 Valency 2 1
Formula NaNO3 Ca(OH)2
ix) Formula of Sodium carbonate x) Formula of Ammonium sulphate
Symbol/ Na CO3 Symbol/ NH4 SO4
Formula Formula
Valency 1 2 Valency 1 2
Formula Na2CO3 (NH4)2SO4
Molecular mass / Formula unit mass:-
The molecular mass of a substance is the sum of the atomic masses of all the
atoms in a molecule of the substance.
Molecular mass is expressed in atomic mass units (u).
Eg:- 1.Molecular mass of water – H2O
Atomic mass of H = I u
Atomic mass of O = 16 u
Molecular mass of H2O = 1x2+10 = 2+16 = 18 u
2. Molecular mass of Nitric acid – HNO3
Atomic mass of H = 1 u
Atomic mass of N = 14 u
Atomic mass of O = 16 u
Molecular mass of HNO3 = 1+14+16x3 = 1+14+48 = 63 u
Mole concept-
The word “mole” was introduced around the year 1896 by the German chemist
Wilhelm Ostwald, who derived the term from the Latin word moles meaning a ‘heap’ or ‘pile.
The mole concept is a convenient method of expressing the amount of a substance.
a mole is defined as the amount of a substance that contains exactly
6.02214076 X1023 ‘elementary entities’ of the given substance.
The number 6.02214076 X1023 is popularly known as the Avogadro number and is
often denoted by the symbol ‘NA’. The elementary entities that can be represented
in moles can be atoms, molecules, ions
The number of moles of a substance in a given pure sample can be represented by
the following formula:
n = w/M = N/NA
Where n is the number of moles of the substance (or elementary entity), w=given
mass, M=molar mass, N is the total number of elementary entities in the sample,
and NA is the Avogadro number.
problems
1. Calculate the number moles of magnesium in 0.478g of magnesium ?
Ans: Molecular weight of magnesium =24g
24g of magnesium = 1 mole
0.478g of magnesium = ?
n = w/M
n = 0.478 X 1/24 = 0.019 mole.
2. Find the number of moles present in 24.088X1023 particles of carbon dioxide
1 mole of carbon dioxide contains 6.022x1023
Solution:
The number of moles (n ) = Given number of particles (N) / Avogadro number (N0).
= 24.088X1023 / 6.022x1023
= 4 moles
3. Calculate the number of moles in 12g of oxygen gas
Ans: 32g of oxygen gas = 1 mole
12g of oxygen gas = ?
n = w/M
= 12 X 1 / 32 = 0.375 mole.
4. Calculate the number of moles present in 14g of carbon monoxide .
Ans: 28g of CO = 1 mole
14g of CO = ?
n = w/M
n = 14 X 1 / 28 = 1/2 = 0.5 mole.
5. Find the mass of 5 moles of aluminium atoms?
Ans: Atomic mass of aluminium atom(M) = 27g
w= nxM
mass of 5 moles of aluminium atoms = 5 X 27g = 135 g.
6. Calculate the molar mass of sulphur.
Ans: Sulphur is a poly atomic molecule. It is made up of 8 sulphur atoms.
Atomic mass sulphur atom = 32 g.
Molar mass of sulphur = 8 X 32 = 256 g.
7. Calculate the mass of 0.2 mole of water molecules.
Ans: 1 mole of water molecules = 18g
0.2 mole of water molecules = ?
w= nxM
= 0.2 X 18 / 1 = 3.6 g.