CHAPTER-03
Atom and molecule
Around 500 B.C, Indian philosopher Maharishi kanad, postulated the theory if we go on dividing matter(padarth), we will obtain smallest particle beyond which further division can’t be possible which is known as ‘parmanu’.
Antoine L. Lavoisier laid the foundation of chemical sciences by establishing two important laws of chemical combination.
Law of chemical combination—This law is established after the experiments by Lavoisier L. Proust
The chemical reaction between two or more substances give rise to products which is governed by certain laws called Laws of chemical reaction. These are
(1)--Law of conservation of Mass—According to this law Mass can neither be created nor destroyed in a chemical reaction.
During a chemical reaction, the total mass of reactants will be equal to the mass of the product.
Ex.A(reactant)+B(reactant)=
AB (product)
mass of A + mass of B = mass of AB
(2)--Law of constant properties—In a chemical reaction, compounds always contain the same elements present in definite properties by mass .
This law is also called law of definite proportions.
Ex. The ratio of hydrogen and oxygen in pure water is always 1:8 by weight
Daltons Atomic Theory— All matter, whether an element, a compound or a mixture is composed of small particles called atoms.
Postulates of Dalton’s atomic theory
*1- All matter is made of very tiny particles called atoms, which participate in chemical reaction.
*2- Atoms are indivisible particles, which cannot be created and destroyed in a chemical reaction.
*3- Atom of a given element are identical in mass and chemical properties.
*4- Atoms of different elements have different mass and chemical reaction.
*5- Atom combine in the ratio of small whole numbers to form compounds.
*6- The relative number and kinds of atoms are constant in a given compound.
Atoms:-
The smallest tiny particle of matter which can’t be divided further is called atom, it is the smallest building block of matter.
Ex.sodium(Na),Hydrogen(H), Oxygen(O) etc.
# Atomic radius is measured in nanometer.
1 Nanometer(nm)
= 10-9 m
1 Meter(m) =
109nm
|
Radii (in m) |
Example |
|
10-10 |
Atom of hydrogen |
|
10-9 |
Molecule of water |
|
10-8 |
Molecule of haemoglobin |
|
10-4 |
Grain of sand |
|
10-3 |
Ant |
|
10-1 |
Apple |
Modern day symbols of
elements
= Dalton was the first scientist who use the symbols for elements in the very specific sense.
= Berzilius suggested that the symbols of elements should be made from one or two letters of the name of elements.
= The name copper was taken from Cyprus, a place from where it was found for first time.
= Now IUPAC (International union of pure and applied chemistry) approves names of elements.
= The first letter of a symbol is always written in a capital letter (uppercase) and the second letter in a small letter (lowercase).
Ex. Hydrogen (H)
Aluminium (Al)
Iron (Fe) etc
= Some other symbols have been taken from the names of elements in Latin, German or Greek.
Ex. Fe from its Latin name ferrum, sodium is Na from natrium, potassium is K from kalium.
|
Elements |
Symbol |
|
Aluminium |
Al |
|
Argon |
Ar |
|
Barium |
Ba |
|
Boron |
B |
|
Bromine |
Br |
|
Calcium |
Ca |
|
Carbon |
C |
|
Chlorine |
Cl |
|
Cobalt |
Co |
|
Copper |
Cu |
|
Fluorine |
F |
|
Gold |
Au |
|
Hydrogen |
H |
|
Iodine |
I |
|
Iron |
Fe |
|
Lead |
Pb |
|
Magnesium |
Mg |
|
Neon |
Ne |
|
Nitrogen |
N |
|
Oxygen |
O |
|
Potassium |
K |
|
Silicon |
Si |
|
Sodium |
Na |
|
Uranium |
U |
|
Zinc |
Zn |
Atomic mass –
Atomic mass of an elements may be defined as the average relative mass of an
atom of the elements as compared with the mass of an atom of carbon (C-12
isotope) taken as 12 amu.
Atomic mass = Mass of 1 atom of an elements / 1/12 of the mass of an atom of C-12
Gram atomic mass- The atomic mass of an elements expressed in grams is known as gram atomic mass.
|
Elements |
Atomic mass (u) |
|
Hydrogen |
1 |
|
Helium |
4 |
|
Carbon |
12 |
|
Nitrogen |
14 |
|
Oxygen |
16 |
|
Sodium |
23 |
|
Magnesium |
24 |
|
Aluminium |
26 |
|
Sulphur |
32 |
|
Chlorine |
35.5 |
|
Potassium |
39 |
|
Calcium |
40 |
Molecule:-
A group of two or more than two atoms of
the same or different elements that are chemically bonded together is
called a molecule.
Ex. Two atoms of hydrogen (H2) and one atom of oxygen (O2) react with each other and form one molecule of water.
Molecules of Elements:- The molecules of an element are constituted by the same type of atoms.
Ex. A molecule of water consists of two atoms of hydrogen and one atom of oxygen to form a diatomic molecule O2 .
Molecules of compounds:- Atom of different elements join together in definite proportion to form molecules of compounds.
Ex. A molecule of water consists of two atom of hydrogen and one atom of oxygen to form a Triatomic molecule H2O . Other examples are HCl, CO2, KCl etc.
# A molecule may have atom of same or different elements, depending upon this, molecule can be categorized into two categories.
# Molecules of some compounds
|
Compound |
Combining elements |
Ratio by mass |
|
Water (H2O ) |
Hydrogen, Oxygen |
1:8 |
|
Ammonia(NH3) |
Nitrogen, Hydrogen |
14:3 |
|
Carbon dioxide (CO2 ) |
Carbon, Oxygen |
3:8 |
(1)-Homoatomic molecules- It containing atom of same
elements.
Ex. H2, O2, O3, P4 etc.
(2)-Heteroatomic molecules or compounds- It containing atoms of different elements.
Ex. H2O, CO2, NaCl, CaCO3 etc.
Ion :- Ion is an electrically charged atom or group of atoms. It is formed by the loss or gain of one or more electrons by an atom.
Two types of Ion are—
*1* Cation = It is positively charged ion and is formed by the loss of one or more electron from atom.
Ex. Sodium atom, loses one electron to form a sodium ion .
Na ─ e- → Na+
*2* Anion = It is negatively charged ion and is formed by the gain of one or more electrons by atom.
Ex. Chlorine atom gain one atom to form chloride ion Cl- .
Cl → Cl- + e-
some ionic compounds are
|
Ionic compound |
Constituting Elements |
Ratio by mass |
|
Calcium oxide |
Calcium and Oxygen |
5:2 |
|
Magnesium Sulphide |
Magnesium and sulphur |
3:4 |
|
Sodium chloride |
Sodium and Chlorine |
23:35.5 |
|
Potassium chloride |
Potassium and chlorine |
39:35.5 |
# A group of atom carrying a charge is known as polyatomic ion. Ex. NH4+, CO3--, OH- etc.
# Metals always carry positively charged ion.
# Non metals always carry negatively charged ion.
Atomicity:- The number of atoms present in one molecule of an element is called its atomicity.
|
Types of Element |
Name |
Atomicity |
|
Non metal |
Argon |
Monoatomic |
|
Helium |
Monoatomic |
|
|
Potassium |
Monoatomic |
|
|
Hydrogen |
Diatomic |
|
|
Nitrogen |
Diatomic |
|
|
Chlorine |
Diatomic |
|
|
Oxygen |
Diatomic |
|
|
Sulphur |
Polyatomic |
|
|
Phosphorus |
Tetra- atomic |
Valency:- To
combining power (or capacity) of an elements is known as its valency.
It can be used to find out how the atoms of an element will combine with the atom(s) of another element to form a chemical compound.
# Depending upon the valency, elements are Monovalent (Ex. Na+, Cl-, H+ etc), Divalent (Ex. Mg++, O--, Fe++ etc) and Trivalent (Al+++, Fe+++, N--- etc.)
Name, symbols and valency of some ions.
|
Valency |
Name of ion (Metals) |
Symbol |
|
1 |
Sodium |
Na+ |
|
Potassium |
K+ |
|
|
Silver |
Ag+ |
|
|
Copper (I)* |
Cu+ |
|
|
2 |
Magnesium |
Mg++ |
|
Calcium |
Ca++ |
|
|
Zinc |
Zn++ |
|
|
Iron (II)* |
Fe++ |
|
|
Copper (II)* |
Cu++ |
|
|
3 |
Aluminium |
Al+++ |
|
Iron (III)* |
Fe+++ |
|
Valency |
Name of ion (Non metals) |
Symbol |
|
1 |
Hydrogen |
H+ |
|
Hydride |
H- |
|
|
Chloride |
Cl- |
|
|
Bromide |
Br- |
|
|
Iodide |
I- |
|
|
2 |
Oxide |
O-- |
|
Sulphide |
S-- |
|
|
3 |
Nirtide |
N--- |
|
Valency |
Polyatomic ion |
Symbol |
|
1 |
Ammonium |
NH3+ |
|
Hydroxide |
OH- |
|
|
Nitrate |
NO3- |
|
|
Hydrogen carbonate |
HCO3- |
|
|
2 |
Carbonate |
CO3-- |
|
Sulphite |
SO3-- |
|
|
Sulphate |
SO4-- |
|
|
3 |
Phosphate |
PO4--- |
Write chemical formula:- The rules that we have to follow while writing a chemical formula are as follow:
(1)- The valencies or charges on the ion must balance.
(2)- When a compound consisits of a metal and non metal, the name or symbol of the metal is written first.
Ex. Calcium oxide (CaO), sodium chloride (NaCl), Iron sulphide (FeS), cupper oxide (CuO) etc., Where oxygen, chlorine, sulphur are non metal and are written on the right, whereas calcium, sodium, iron and copper are metals, and are written on the left.
(3)- In compounds formed with polyatomic ions, the number of ions present in the compound indicated by enclosing the formula of ion in a bracket and writing the number of ions outside the bracket.
Ex. Mg(OH)2.
(4)- In case the number of polyatomic ion is one, the bracket is not required.
Ex. NaOH
Formula of simple and molecular compounds:-
Steps to construct the chemical formula of a compound:
-*- While writing the chemical formula for compounds, write the constituent elements with their valencies written down the respective elements.
-*- Then crossover the valencies of the combining atoms.
Ex.
*1* Formula of hydrogen chloride
Symbol H Cl
↖ ↗
↗ ↖
Valency 1 1
Formula of the compound would be HCl
*2* Formula of hydrogen sulphide
Symbol H S
↖ ↗
↗ ↖
Valency 1 2
Formula: H2S
*3* Formula of carbon tetrachloride
Symbol C Cl
↖ ↗
↗ ↖
Valency 4 1
Formula: CCl4
*4* Formula of magnesium chloride
Symbol Mg Cl
↖ ↗
↗ ↖
Valency 2 1
Formula: MgCl2
*5* Formula for aluminium oxide:
Symbol Al O
↖ ↗
↗ ↖
Valency 3 2
Formula: Al2O3
*6* Formula for calcium oxide:
Symbol Ca O
↖ ↗
↗ ↖
Valency 2 2
Formula: CaO
*7* Formula of sodium nitrate:
Symbol Na
NO3
↖ ↗
↗ ↖
Valency 1 1
Formula: NaNO3
*8* Formula of calcium hydroxide:
Symbol Ca OH
↖ ↗
↗ ↖
Valency 2 1
Formula: Ca(OH)2
*9* Formula for ammonium sulphate:
Symbol NH4 SO4
↖ ↗
↗ ↖
Valency 1 2
Formula: (NH4)2SO4
Molecular mass:- It is the sum of the atomic masses of all atoms in a molecule of the substance. It is expressed in atomic mass unit (u).
Ex. Molecular mass of water (H2O) -
Molecular mass of hydrogen =1u
Molecular mass of oxygen =16u
water has 2 hydrogen and 1 oxygen atom
Molecular mass of water(H2O) is
= 2x1+1x16 = 18u
Formula unit mass:- The formula unit mass of a substance is a sum of the atomic masses of all atoms in a formula unit of a compound. Formula unit mass is calculated in the same manner as we calculate the molecular mass.
We use the word formula unit for those substances whose constituent particle are ions.
Ex. Formula unit mass of NaCl = Atomic mass of sodium + atomic mass of chlorine.
(Na=23, Cl=35.5)
= 1x23 + 1x35.5 = 58.5u
Mole concept:-
A group of 6.022 x 1023 particles (atoms, molecule or ions) of a
substances is called a mole of that substance.
-*- The
mass of 1 mole particles is equal to its mass in grams.
-*- 1 mole of atoms = 6.022 x 1023 atoms
-*-1 mole
of molecules = 6.022 x 1023
molecules.
Ex. 1 mole of oxygen = 6.022 x 1023 oxygen atoms.
Avogadro’s constant or number:- The number of particles present in one mole
(6.022 x
1023 particles) is
called avogadro’s number or avogadro’s constant.
Ex. #1-Find the number of moles present in 24.088 x 1023 particles of carbon dioxide,
1
mole of CO2 contain 6.022 x 1023
solve- The
number of moles(n) = Given number of particles(N)/ Avogadro number(N0)
= 24.088 x 1023 /6.022 x 1023 = 4 moles.
#2- calculate the number of atoms in 48g of Mg
Solve- Number of atoms(n)= Given mass(m) / Molar mass (M) x Avogadro number (N0)
= 48/24 x 6.022
x 1023
= 2 x 6.022
x 1023 = 12.04 x 1023 atoms
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