ATOMS AND MOLECULES
Laws of chemical combination
Various chemical reactions take place according to the certain laws, known as the Laws
of chemical combination. These are as follows:
(i) Law of conservation of mass:
It was proposed by Lavoisier and verified by Landolt.
For example:
In a reaction of decomposition of calcium carbonate:
CalciumCarbonate100g→CalciumOxide56g+Carbondioxide44g
The mass of reactants before the reaction is equal to the mass of products after the
chemical reaction.
The mass of reactants before the reaction is equal to the mass of products after the
chemical reaction.
(ii) Law of constant or definite proportion:
It was proposed by J.L. Proust.
For Example: CO2 can be formed by either of the following processes:
(i) By heating CaCO3
CaCO3→ΔCaO+CO2
(ii) By heating 2NaHCO3→ΔNa2CO3+H2O+CO2
2NaHCO3→ΔNa2CO3+H2O+CO2
CO2 is collected separately as a product of each reaction and the analysis of CO2of each
collection reveals that it has the combination ratio of Carbon and Oxygen as 12: 32 or 3:8
by mass.
(iii) Law of multiple proportions:
It was proposed by Dalton and verified by Berzelius.
For Example:
Carbon and Oxygen when combine, can from two oxides,CO (Carbon monoxide)
& CO2 (Carbon dioxide).
In CO, ratio of masses of carbon and oxygen is 12:16 or 3:4
In CO2, ratio of masses of carbon and oxygen is 12:32 or 3:8
Thus, we can see that the masses of Oxygen which combines with a constant mass of
Carbon (12 g) bear a ratio of 16: 32 or 1: 2.
Dalton's atomic theory
John Dalton, a British Chemist and scientist gave this atomic theory in 1808. He
gave the theory on the basis of Laws of Chemical Combination.
Main postulates of Dalton's atomic theory:
Limitations of Dalton's atomic theory :-
According to Dalton, atoms were thought to be indivisible. But it is now known that
atoms can be further divided (into electrons, protons and neutrons).
Daltons atomic theory said that all the atoms of an element have exactly the same
mass. But this fact was disregarded after discovery of isotopes.
Daltons atomic theory said that atoms of different elements have different masses. But
this fact was disregarded after discovery of isobars.
According to Dalton, atoms of different elements combine in simple whole number
ratios to form compounds. This is not observed in complex organic compounds like
Sugar(12 : 22 : 11).
The theory fails to explain the existence of allotropes; it does not account for
differences in properties of charcoal, graphite, diamond.
Atoms and their symbols
Atom - An atom is defined as the smallest particle of an element which may or may not
be capable of free existence. However, it is the smallest particle that takes part in a
chemical reaction.
Characteristics of Atoms
Atom is the smallest particle of matter.
All elements are made of tiny particles called atoms.
Atoms are very small in size and cannot be seen through naked eyes.
Atom does not exist in free-state in nature (except that of noble gases). But, an atom
takes part in a chemical reaction.
The properties of matter depend upon the characteristics of atoms.
Atoms are the building blocks of an element similar to the bricks that combine
together to make a building.
The size of an atom is indicated by its radius.
In ancient times, atom was considered indivisible.
Symbols of Elements
A symbol is an abbreviation for the full name of an element.
(i) Daltons Symbols:
Dalton was the first scientist to suggest specific symbols in terms of figures for
different elements known at that time.
These symbols are difficult to draw and are inconvenient to use hence they are not
used.
(ii) Berzelius Symbols:
Berzelius, a Swedish chemist, suggested a more scientific method by using an
abbreviation for representing the full name of an element by using one or two letters from
the name of the element.
Symbol of many elements are taken from their English names, while symbols of some
elements are taken from their Greek or Latin names.
Molecules
Molecule is the smallest particle of an element or a compound which can exist
independently in nature & shows all the properties of that substance.
A molecule may be formed either by the combination of atoms of an element, or by
the combination of atoms of two or more different elements. On this basis of molecules
may be classified in following two categories:-
Atomicity:-
The number of atoms present in one molecule of an element or a compound is known as
its atomicity.
Ions
Atoms of several elements exist in the form of ions. Atoms or molecules with negative or
positive charge over them are called ions.
(i) Cations= Ions having positive charge over them are called cations.
A cation is formed by the loss of one or more electrons by an atom.
e.g. Sodium ion(Na+),Potassium ion (K+),Magnesium ion (Mg2+)etc.
(ii) Anions + Ions having negative charge over them are called anions.
An anion is formed by the gain of one or more electrons by an atom.
e.g. Chloride ion (Cl−),Fluoride ion (F−),Oxide ion (O2−)etc.
(iii) Monoatomic ions
Ions formed by one atom are called monoatomic ions.
e.g. Aluminium ion (Al3+), Magnesium ion (Mg2+),Oxide ion (O2−) Iodide ion (I−)etc.
(iv) Polyatomic ions
Ions formed by two or more atoms are called polyatomic ions.
e.g. Ammonium ion (NH+4), Hydroxide ion (OH−),etc.
Valency
Valency is the combining capacity of an element. It can be used to find out how atoms
of an element will combine with the atom (s) of another element to form a chemical
compound.
It is equal to the magnitude of positive or negative charge present on the ion.
It is also defined as the number of electrons an atom loses or gains or shares to
achieve stable nearest noble gas configuration.
Chemical Formula
Chemical formula is a way of representing one molecule of a compound showing the
elements and the numbers of atoms of each element present in it.
Rules for writing formula of compound-
Formula of a compound is given using the symbols of constituent elements.
Symbol of the more metallic element is written first in formula.
Number of atoms of each of the constituent element present in the molecule is
indicated by a subscript.
When either of the ions or both the ions are polyatomic and their valency is more than
1, we enclose the polyatomic ions in brackets. No brackets are necessary if the valency of
polyatomic ion is 1.
While writing formula of compound, if the valencies of respective elements numbers
have a highest common factor (H.C.F), then former should be divided by H.C.F to get the
simple ratio between the combining elements.
Formula of a binary compound is written by criss-crossing the valencies of elements
present in a molecule of the compound.
A chemical compound is always electrically neutral, so the positive and negative
valencies or charges of the ions in the compound must add up to zero.
Atomic and molecular mass
Atomic mass
Mass of one atom of an element is called its atomic mass. Since, atoms are very
small the actual mass of an atom is very small.
For example, the actual mass of one atom of Hydrogen is equal to 1.673×10−24g.
Atomic mass is the relative atomic mass of an atom with respect to 1/12th of the mass
of an atom of Carbon (C - 12 isotope) taken as 12 amu.
Atomic mass =Average mass of an atom1/12×Mass of an atom of C−12
amu is the abbreviation of Atomic mass unit, but now it is denoted just by 'u'.
One atomic mass unit: It is equal to 112th of the mass of an atom of Carbon-12
isotope. One atomic mass unit = 1/12 of the mass of one atom of C-12.
It is found that, the actual atomic mass of a Carbon-12 atom =1.9926×10−23g.
Therefore, 1u=1.9926×10−2312=1.6605×10−24g
Thus, by multiplying the relative atomic mass with 1.6605×10−24g, we can get the
absolute or actual mass of an atom.
Molecular mass
Molecular mass of a molecule (of an element or a compound), may be defined as a
number which indicates how many times heavier is a molecule of that element or
compound as compared with 1/12 of the mass of an atom of Carbon-12.
It is expressed in a.m.u.
Molecular mass = massofonemoleculeofthesubstance1/12×MassofoneatomofC−12
Molecular mass of a substance is an additive property and can be calculated by adding
the atomic masses of all the atoms present in one molecule.
For example, molecular mass of CO2is =12+16×2=44u
Formula unit mass:
Ionic compounds do not exist in molecular form. Therefore, instead of molecular
mass, the term formula unit mass is used.
The formula unit mass of a substance is the sum of the atomic masses of all the atoms (or
ions) present in one formula unit of the compound.
e.g. formula unit mass of NaCl = Atomic mass of Na + Atomic mass of Cl
= 23 + 35.5
=58.5u
Mole Concept
The concept of "mole" was introduced to calculate the number of atoms (or) molecules
by knowing the mass of a given substance. It is the link between the mass of atoms (or
molecules) and the number of atoms (or molecule).
Definition: Mole is defined as the amount of substance that contains as many
specified elementary particles (atoms, molecules or ions) as the number of atoms present
in 12 g of Carbon-12 isotope.
One mole is also defined as the amount of substance which contains Avogadro
number (6.023×1023) of particles.
Avogadro number: Number of atoms or molecules or ions present in one mole of a
substance is called Avogadro number. Its value is 6.022×1023.
Therefore, one mole of any substance =6.022×1023 particles( may be atoms,
molecules, ions)
For example:
Oxygen atom is O and Oxygen molecules is O2
1 mole of Oxygen atoms (O) =6.022×1023 Oxygen atoms
1 mole of Oxygen molecules (O2) =6.022×1023 Oxygen molecules
Gram atomic mass (Molar Mass):
Mass of one mole atoms of an element is called its molar mass or gram atomic mass.
Gram atomic mass is equal to the atomic mass of an element expressed in grams.
For example:
The atomic mass of oxygen (O) is 16 u so, gram atomic mass of Oxygen will be 16
grams.
Mass of one atom of an element in grams = Gramatomicmass6.02×1023
Gram molecular mass (Molar Mass)
Mass of one mole molecules of any substance (element or compound) is called its
Gram molecular mass or molar mass.
Gram molecular mass is equal to the mass of one molecule of a substance (element or
compound) in grams.
For example :
The molecular mass of oxygen (O2)is 32 u so, gram atomic mass of oxygen will be 32
grams.
Mass of one molecule of an element or compound in grams =Gramatomicmass6.02×1023
Important formulas:
Mass of the Substance
Number of Moles =
Molar Mass
No. of particles of Substance
Number of Moles =
Avogadro’s number
Mass of the substance No. of particles of Substance
=
Molar Mass Avogadro’s number
Extra Edge
Gram Molar Volume:-
Volume occupied by one mole of any gas at STP is called Gram molar
volume.
The value of gram molar volume is 22.4 litres.
Volume of 1 mole of any gas at STP= 22.4litres
Brief Overview:
Percentage composition of a compound
Determining mass percentage of an element from molecular formula:
The molecular formula of a compound may be defined as the formula which specifies the
number of atoms of various elements in one molecule of that compound.
With the help of molecular formula of a compound we can calculate its percentage
composition by mass.
