Download ATOMS AND MOLECULES

ATOMS AND MOLECULES
Atoms and molecules are the building blocks of elements and
compounds. To know about the early developments of the
idea of atoms we must have information about the laws of
chemical combinations.
Laws Of Chemical Combinations
Antoine L. Lavoisier laid the foundation of chemical sciences
by establishing two important laws of chemical combination.
Joseph L. Proust also helped in the establishment of these
laws. The Laws are the following: (i) Law of conservation of mass (estd. In 1774)
(ii) Law of constant proportion (estd. In 1779)
Law Of Conservation Of Mass
This law states that:
M atter (m ass) is neither created nor destroyed in a
chem ical reaction. The total am ount of m atter of the
reactants is converted into products. The m ass and
hence m atter is alw ays conserved. That is:
The mass of the substance
before the reaction
=
The mass of the substance
after the reaction
Law Of Constant Proportion
This law states that:
I n a pure chem ical substance, the elem ents are alw ays
present in a definite proportion by m ass.
Example
(i) In a substance called water the ratio of mass of hydrogen
and mass of oxygen is always 1 : 8. Thus if 9 gram of
water is decomposed, 1 gram of hydrogen and 8 gram of
oxygen is always obtained.
(ii) In ammonia nitrogen and hydrogen are always present in
the ratio of 14:3 by mass, whatever is the method of
source from which it is obtained.
Illustration Of This Law By The Production Of Water
If 3 grams of hydrogen is burned with 8 grams of oxygen then
9 grams of water is formed and 2 grams of hydrogen will
remain un-reacted.
6.
The relative number and kind of atoms are constant in a
given compound.
Drawbacks Of Dalton’s Atomic Theory Of Matter
1.
2.
3.
One of the major drawbacks of Dalton’s atomic theory of
matter is that atoms were thought to be indivisible .But
now it is known that atom can be divided further into
electrons, protons and neutrons.
Dalton’s atomic theory said that all the atoms of an
element have exactly the same mass.
Dalton’s atomic theory said that atoms of different
elements have different masses.
Modern Atomic Theory Of Matter
Modern atomic theory states that: 1. An atom can be divided into three subatomic particles
called electrons, protons and neutrons.
2. For some elements all their atoms do not have the same
mass. The atoms having the different masses for the
same element are called isotopes.
3. Atoms of different elements can have the same masses.
These types of atoms are called isobars.
Atom
An atom is the smallest particle of an element, which can take
part in chemical reactions. Atoms are the building blocks of all
matter around us.
Atomic Size
An atom is very small. Its radius is measured in nanometers.
1m = 109 nm
The atoms are too small in size to be imagined that more than
millions of atoms when stacked together would make a layer
barely as thick as this sheet of paper.
Symbols Proposed By Dalton
Dalton was the first scientist to use the symbols of the
elements in a very specific sense i.e., one atom of that
element. Some of the symbols used by him are: -
Dalton’s Atomic Theory Of Matter
John Dalton (1766-1844) stated basic theory about nature of
matter as: 1. All matter is composed of extremely small particles called
atoms.
2. Atoms are indivisible in nature, which cannot be created
or destroyed in a chemical reaction.
3. All atoms of a given element are identical, both in mass
and in properties. Atoms of different elements have
different masses and properties.
4. Atoms cannot be created, destroyed or transformed into
atoms of other elements.
5. When atoms of different elements combine with each
other compounds are formed.
Modern Symbols Of The Atoms Of different
Elements
In the modern concept of symbols now-a-days IUPAC
(International Union of Pure and Applied Chemistry) approves
names of elements .Many of the symbols are the first one or
two letters of the element’s name in English. Some symbols
have been taken from the names of the elements in Latin
Greek or German. The first letter is always written in capital
letter and the second in small letter.
Examples
Element
Hydrogen
Aluminium
Copper (Cuprous)
Sodium (Natrium)
Silver (Argentum)
Iron (Ferrum)
Potassium (Kalium)
Tin (Stannum)
Lead (Plumbum)
Symbol
H
Al
Cu
Na
Ag
Fe
K
Sn
Pb
Importance Of Using A Symbol
(i)
A symbol is used to represent one mole of atom of the
elements.
(ii) A symbol signifies atomic mass, relative atomic mass or
the molar mass of the atoms of the elements.
(iii) Symbols are used to indicate the type of elements in a
molecule.
Atomic Mass
Mass of an atom of an element is called atomic mass. Atomic
mass of an element is the sum of number of neutrons and
number of protons in its atom. For example, the atomic
number of oxygen is 8. In atom of oxygen number of protons
is 8 and number of neutrons is 8. So, atomic mass of oxygen
is 16u.
Unit Of Atomic Mass
amu is the unit of atomic mass. One atomic mass unit is a
mass unit equal to exactly 1/12th the mass of one atom of
carbon-12 .
1 a.m.u. = 1.66 × 10 -24 g
Relative Atomic Mass
It is the mass of an atom of a substance in comparison to
1/12th of the mass of 1 carbon atom.
Example
The relative atomic mass of oxygen is 16u. It means that an
oxygen atom is 16/12 = 4/3 times as heavy as an atom of
carbon-12. This also means that the mass of one atom of
oxygen is 16 times greater than 1/12 the mass of one atom of
carbon-12.
Gram Atomic Mass
When the mass of an atom taken in grams is numerically equal
to its atomic mass it is called gram atomic mass.
How Do Atoms Exist
Atoms of most elements are not able to exist independently
(except the atoms of noble gases). In some cases two or more
atoms form molecules and ions. The state of matter which we
see, feel, and touch is a collection of large number of
molecules or ions.
Molecule
A molecule is the smallest particle of a substance which has
the properties of that substance and can exist in free state. A
molecule is made up of two or more atoms of same or
different elements chemically bonded together.
Types Of Molecules
1. Molecule Of an Element
2. Molecule Of a Compound
Molecule Of An Element
It can contain two or more similar kinds of atoms, e.g.
molecule of hydrogen which contains two atoms of hydrogen
and written as H 2 , molecule of oxygen which contains 2 atoms
of oxygen and written as O 2 .
Atomicity
The total number of atoms bonded in a molecule of an
element is called its atomicity.
Classification Of Molecules Of Elements Based On
Atomicity
Monoatomic
Diatomic
Triatomic
Tetra atomic
Polyatomic
He, Ne, Ar, Xe, Kr, Rd(Non- Metals)
Na, Al, Fe, Cu, K, Mg (Metals)
H 2 , O 2 , N 2 , Cl 2 , Br 2 , F 2
O3
P4
S 8 , B 12 , C 60
Molecule Of Compound
It contains two or more different types of atoms, e.g. H 2 O
contains hydrogen atoms and oxygen atoms.
Classification Of Molecules Of Compounds Based On
Atomicity
Monoatomic
Diatomic
Triatomic
Tetra atomic
Polyatomic
He, Ar, Ne, Xe, Kr, Rd
HCl, NaCl, CO, KCl, NaF, KF
H 2 O, CO 2
NH 3 , P 4 , H 2 O 2 , Na 3 P, Li 3 N, K 3 P, K 3 N
S 8 , C 2 H 5 OH, CH 3 OH, CH 4
Molecular Formula
It tells us that how many atoms of each kind are present in its
one molecule. For example, hydrogen peroxide has molecular
formula H 2 O 2 . That means it contains two atoms of hydrogen
and two atoms of oxygen.
Molecular Mass
Molecular mass of a substance indicates the number of times
one molecule of the substance is heavier than 1/12 (one
twelfth) of a C-12 atom.
Calculation Of Molecular Mass
The molecular mass is equal to the sum of the atomic masses
of all the atoms present in one molecule of the substance.
Example
One molecule of H 2 O contains two atoms of hydrogen and one
atom of oxygen so,
Mass of H atom = 1
Mass of 2H atoms = 2 × 1 = 2
Mass of O atom = 16
Molecular mass of H 2 O = Mass of 2 H atoms + Mass of O
atom
= 2 + 16 = 18
∴ Molecular mass of water is 18 u.
Gram Molecular Mass
When the mass of a molecule taken in grams is numerically
equal to its molecular mass, it is called gram molecular mass.
Therefore the gram molecular mass of water is 18g.
(v) The number of anions required is written on the right
bottom of the symbol of anion but without showing the
charge on the anion.
An ion is an atom having a charge over it.
Example
Suppose we have to write the formula of hydrogen sulphate
(Sulphuric Acid). Now hydrogen sulphate is made up of two
types of ions: Hydrogen ion H+ and Sulphate ion SO 4 2- . We
know that hydrogen ion H+ has 1 unit of positive charge where
as sulphate ion SO 4 2- has two units of negative charge. This
means that 2 H+ ions are needed to balance the two negative
charges (or valencies) of a sulphate ion SO 4 2- . So the
hydrogen sulphate compound is made up of two H+ ions and
one SO 4 2- ion, that is, 2H+ SO 4 2-. Now when we write the
formula of hydrogen sulphate then 2H+ is written as H 2 and
SO 4 2- is written as SO 4 (because the charges are not shown in
the formula) so the formula of hydrogen sulphate becomes H 2
SO 4 .
Ion
Type Of Ions
Cation
An ion carrying positive charge is called cation.
Anion
An ion carrying negative charge is called anion.
Types Of Ions Depending upon The Number Of Atoms
1. Simple Ions
Ions formed from single atom are called simple ions.
Example
Na+, Mg2+, Al3+, O2-, Cl-.
2. Compound Ions
Ions formed from groups of joined atoms are called compound
ions.
Example
NH 4 +, OH- , SO 4 2-, CO 3 2-, NO 3 - etc.
Gram Ionic Mass
When the mass of an ion taken in grams is numerically equal
to its ionic mass, it is called gram ionic mass. Therefore the
gram ionic mass of NaCl is 58.5g.
Valency of Ion
The valency of an ion is equal to the charge on the ion. If an
ion has 2 units of charge its valency is 2, if the charge is 3
units, the valency is 3. A cation has a positive valency and an
anion has a negative valency. Prefixes mono, di, tri, tetra are
used according to the presence of unit charge on the ion.
Example
Monovalent Cations
Monovalent Anions
Divalent cations
Divalent Anions
Trivalent Cations
Trivalent Anions
H+, Li+, K+, Na+, NH 4 +, Ag+, Cu+
F-, Cl-, Br-, I-, As-, NO 2 -, NO 3 -,
HSO 4 -, HCO 3 - , MnO 4 - , CN-, OHMg 2+, Ca2+, Zn2+, Fe2+, Hg2+, Cu2+.
O2-, S2-, So 3 2-, SO 4 2-,CO 3 2Al3+, Cr3+ , Fe3+
N3-, P3-, PO 4 3-
Steps To Write The Formulae
For Ionic compounds
1. First Method
(i) Write down the name of the compound whose formulae
are required.
(ii) Write down the symbols of its ions. Note that as
convention the cation is written on left and anion is
written on right side.
(iii) The number of cations and anions is adjusted in such a
way that the total number of positive valencies of cation
becomes equal to the total number of negative valencies
of anions.
(iv) The number of cations required is written on the right side
bottom of the symbol of cation but without showing
charge on the cation.
2. Second Method
(i) First write down the symbols of hydrogen and sulphate
ion.
(ii) Below the symbol of hydrogen ion, write the valency or
charge of hydrogen ion which is 1+.
(iii) Below the symbol of sulphate ion, write the valency or
charge of sulphate ion which is 2-.
Example
SO 4 2Ions - H+
21+
(iv) Now cross – over the valencies or charges of hydrogen
ion sulphate ion.
(v) Write the crossed over valencies as subscripts with the ion
but without their charges. In this way we get the formula
as: H
SO 4
i.e., H 2 SO 4
1
2
Formula Of Molecular Compounds Or Covalent
Compounds Are Made Up Of Non – Metals
1. First Method
(i) Write down the name of the compound whose formula is
required.
(ii) Write down the symbols of its atoms. Note that as
convention we write the less electronegative element on
left and more electronegative element on right side.
(iii) The number of atoms is adjusted in such a way that the
total valencies of both the atoms become equal.
(iv) The number of less electronegative atoms repaired is
written on the right side bottom of the symbol of element.
(v) The number of more electronegative atoms required is
written on the right bottom of the symbol of the element
and that is how we obtain the formula.
O
C
4
2
(iii) Cross over C 2 O 4
(iv) Dividing by the common factor.
(v) C 2 O 4 divided by 2 gives CO 2 .
Second Method
Carbon dioxide is made up of two elements, carbon and
oxygen. The valency of carbon is 4 & the valency of oxygen is
2. Now if we take 2 oxygen atoms, then their total valenies will
be 2 × 2 = 4, which will become equal to the valency 4 of
carbon atom. So, one atom of C will combine with 2 atoms of
O to give the formula CO 2 of carbon dioxide.
Formula Mass Of Ions
The formula mass of a substance is the sum of the atomic
masses of all ions present in a formula unit of the compound
whether molecular or not. For example formula mass of an
ionic compound NaCl which is actually Na+Cl- will be the sum
of atomic mass of sodium ion with the atomic mass of chloride
ion which is equal to
Mass of sodium ion = 23.0 gm
Mass of chloride ion = 35.5 gm
Mass of NaCl
= 23.0 gm + 35.5 gm = 58.5 gm
Mole Concept
Mole
Mole is a unit which is equal to the amount of substance which
contains as many entities (atoms, molecules or ions) as there
are atoms in 12 gram of C-12 element.
Every element in a compound has a fixed proportion by mass.
To calculate mass percentage of an element in a compound
we do as follows: Suppose AB is a compound, one element is A and another is B.
Mass Of A In Total Mass Of AB
Mass Percentage Of A=
× 100
Total Mass Of AB
Mass Percentage Of B=
Example
Take CO 2
Molar mass of CO 2 is 44 g/mol
If we take 44 gram of CO 2 it will contain 120 gram Carbon and
32.0 gram of oxygen
Percentage Of Oxygen =
32.0
× 100 = 73.0 %
44.0
Percentage Of Carbon =
12.0
× 100 = 27.0%
44.0
Number of atoms in C-12 element is = 6.023 × 1023 atoms
Therefore,
1 mole of atoms = 6.023 × 1023 atoms
Also,
1 mole of molecules = 6.023 × 1023 molecules
Also,
1 mole of ions = 6.023 × 1023 ions
The number 6.023 × 1023 is called Avogadro’s number and
denoted by N A
Example
1 mole of oxygen atoms (O) = 6.023 × 1023 oxygen atoms
1 mole of oxygen molecules (O 2 ) = 6.023 × 1023 oxygen
molecules
1 mole of oxide ions (O2-) = 6.023 × 1023 oxide ions
Also,
1 mole atoms of an element = Gram atomic mass of
element
Example
1 mole of oxygen atom = Gram atomic mass of oxygen = 16
grams
1 mole of oxygen molecules = Gram molecular mass of
oxygen = 32 grams
1 mole of O2- ions = Gram ionic mass of oxide ions = 16 gm
Now, it is concluded that
1 mole of atoms = 6.023 × 1023 atoms = Gram atomic
mass
1 mole of molecules = 6.023 × 1023 molecules = Gram
molecular mass
1 mole of ions = 6.023 × 1023 ions = Gram ionic mass
Mass Percentage Of An Element
Mass Of B In Total Mass Of AB
× 100
Total Mass Of AB