In 1803, A British school teacher John Dalton provided
the basic theory about the nature of matter which provides explanation for the
law of conservation of mass and the law of definite proportions.
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 may be stated as follows:
(i)
All matter is made of very tiny particles called atoms.
(ii) Atoms are indivisible particles, which
cannot be created or destroyed in a chemical reaction.
(iii) Atoms of a given element are identical
in mass and chemical properties.
(iv)
Atoms of different elements have different masses and chemical
properties.
(v)
Atoms combine in the ratio of small whole numbers to form compounds.
(vi) The relative number and kinds of atoms
are constant in a given compound.
Drawbacks
of Dalton’s Atomic Theory:
ð
The atom is further subdivided into protons,
neutrons and electrons.
ð The
atoms of same elements are not similar in all respect. They may vary in mass
and density. These are known as isotopes. For example: chlorine has two
isotopes having mass numbers 35 a.m.u and 37 a.m.u.
ð Atoms
of different elements are not different in all respects. Atoms of different
elements that have the same atomic mass are called isobar.
ð According
to Dalton atoms of different elements combine in simple whole number ratio to
form compounds. This is not seen in complex organic compounds like sugar C12H22O11.
ð
The theory fails to explain the existence of
allotropes like Diamond and Graphite which having different properties even
these are made up of same kind ot atom namely Carbon.
The introduction of matter wave concept by de Broglie,
the principle of uncertainty by Heisenberg etc., paved the way for modern
atomic theory [Modification in
Dalton’s atomic theory]
Modifications in Dalton’s atomic theory - Modern atomic
theory are as follows.
ð Atom
is considered to be a divisible particle.
ð Atoms
of the same element may not be similar in all respects. eg: Isotopes (17Cl35,17Cl37 )
ð Atoms of different elements may be similar in
some respects eg. Isobars (18Ar
40 , 20Ca 40 )
ð Atom is the smallest particle which takes part
in chemical reactions.
ð The
ratio of atoms in a molecule may be fi xed and integral but may not be simple e.g.,
C12H22O11 is not a simple ratio (Sucrose)
ð Atoms
of one element can be changed into atoms of other element by transmutation.
ð The
mass of an atom can be converted into energy. This is in accordance with
Einstein’s equation E = mc2
Atom: It is the smallest
particle of an element which may or may not have independent existence. The
atoms of certain elements such as hydrogen, oxygen, nitrogen, etc .do not have
independent existence whereas atoms of helium, neon, argon, etc. do have
independent existence. Thus we can say that all elements are composed of atoms.
Q. How do we know the presence of atoms if they do not
exist independently for most of the elements?
Answer: Atom join in
different way to form matter (neutral molecules or ion) that we are able to
touch, feel and see.
How big are atoms?
Atoms are extremely small. They
are so small, that you cannot see them with most microscopes. Now, Scan tunneling Microscope (STM) is the modern
instrument that made it possible to take photograph of atom. The size of an
isolated atom can 't be measured because we can 't determine the location of
the electrons that surround the nucleus. We can estimate the size of an atom,
however, by assuming that the radius of an atom is half the distance between
adjacent atoms in a solid. This technique is best suited to elements that are
metals, which form solids composed of extended planes of atoms of that element.
The results of these measurements are therefore often known as metallic
radii.
Q. What is the unit of measurement of atomic radius?
Ans: Picometers (pm) or
Angstroms (Å)
Q.The size of sodium atom is bigger than that of hydrogen
atom. Why?
Answer: Size of atom is the
distance between the nucleus and outermost shell ( valence shell ) of an atom.
The atomic number of sodium is greater than that of hydrogen. So, it needs more
number of shells to fill electrons and hence will have more number of shells
than hydrogen. Hence, atomic size of sodium is bigger than that of sodium.
Naming of an element
Dalton was the first scientist
to use the symbols for elements in a very specific sense.
Q. Why are Dalton’s symbols not used in chemistry?
Answer: Dalton was the first
scientist to use the symbol for the name of the elements a specific sense but
it was difficult to memorize and in uses so Dalton's symbol are not used in
chemistry
Berzelius suggested that the
symbols of elements be made from one or two letters of the name of the element.
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. The first letter of a symbol is always written as a capital
letter (uppercase) and the second letter as a small letter (lowercase)
For example: (i) hydrogen, H (ii) aluminum, Al and not AL (iii)
cobalt, Co and not CO.
Symbols of some elements are
formed from the first letter of the name and a letter, appearing later in the
name. Examples are: (i) chlorine, Cl, (ii) zinc, Zn etc.
Other symbols have been taken
from the names of elements in Latin, German or Greek. For example, the symbol
of iron is Fe from its Latin name ferrum, sodium is Na from natrium, and
potassium is K from kalium. Therefore, each element has a name and a unique
chemical symbol.
Molecule: A molecule is the smallest or the simplest structural unit of an element (or) a
compound which contains one (or) more atoms. It retains the characteristics of
an element. A molecule can exist freely and it is a combined form of bonded
units whereas an atom is a singular smallest form of non bonded unit.
Molecules are of two types, namely homo atomic molecules
and hetero atomic molecules.
Homo atomic molecules: These are the molecules which are made up of atoms of the same element.
For example hydrogen gas consists of two atoms of hydrogen (H2).Similarly
oxygen gas consists of two atoms of oxygen (O2).
HETERO ATOMIC MOLECULES : The hetero atomic molecules are made up of atoms of
different elements. They are also classified as diatomic, triatomic, or
polyatomic molecules depending upon the number of atoms present. H2O,
NH3, CH4, etc., are the examples for hetero atomic
molecules.
Atomicity:The
number of atoms present in one molecule of an element is called the atomicity
of an element. Depending upon the number of atoms in one molecule of an
element, molecules are classified into monoatomic, diatomic, triatomic or poly
atomic molecules containing one, two, three, or more than three atoms
respectively.
Mono atomic molecules: Helium (He) Neon (Ne) Metals
; Di atomic molecules: Hydrogen H2 Chlorine Cl2
Tri atomic molecules: Ozone (O3) ; Poly atomic
molecules: phosphorous P4 Sulphur S8
Atomicity = Molecular Mass/Atomic mass
MORE TO KNOW:
Isotopes ⇒ These are the atoms of same element with
same atomic number (Z) but different mass number (A). Example (17Cl35,17Cl37
)
Isobars ⇒ These are the Atoms of the different element
with same mass number but different atomic number. Example (18Ar40, 20Ca
40 )
Isotones ⇒ These are the atoms of different elements with same
number of neutrons Example ( 6C13,
7N14 )
AVOGADRO’S HYPOTHESIS: Amedeo Avogadro put forward hypothesis and is based
on the relation between number of molecules and volume of gases that is “volume
of a gas at a given temperature and pressure is proportional to the number of
particles”.
Avogadro’s Law: Equal volumes of all gases under the same
conditions of temperature and pressure. contain the equal number of molecules.
TEST YOUR UNDERSTANDING SKILL
(a) Find the
atomicity of chlorine if its atomic mass is 35.5 and its molecular mass is
71
(b) Find the atomicity of ozone
if its atomic mass is 16 and its molecular mass is 48
WHAT IS AN ION?
An ion is a charged particle and can be negatively or
positively charged.
A negatively charged ion is called an ‘anion’ and the
positively charged ion, a ‘cation’. For example, sodium chloride (NaCl). Its
constituent particles are positively charged sodium ions (Na+) and negatively charged
chloride ions (Cl–).
Ions may consist of a single charged atom or a group of
atoms that have a net charge on them. A group of atoms carrying a charge is
known as a polyatomic ion e.g. Calcium oxide (Ca+2 O-2)
Atomic mass and Relative Atomic mass (RAM):
Q. Each element had a characteristic atomic mass even
then we are using Relative Atomic mass. Give reason?
Answer: Since determining the
mass of an individual atom was a relatively difficult task due to extremely
smaller size, relative atomic masses were determined using the laws of chemical
combinations and the compounds formed.
Relative Atomic mass (RAM): In 1961 IUPAC selected an isotope of carbon (12C)
as a slandered for comparing atomic and molecular mass of element and compound.
Relative atomic mass of an
element is the ratio of mass of one atom of element to the 1/12th
part of mass of one atom of carbon. Relative atomic mass is a pure ratio and
has no unit. If the atomic mass of an element is expressed in grams, it is
known as gram atomic mass. e.g.
Gram atomic mass of hydrogen = 1g where
as gram atomic mass of carbon = 12g
Atomic mass is expressed in atomic mass unit (amu). One atomic
mass unit is defined as 1/12th part of the mass of one atom of carbon.
Q. The atomic mass of an
element is in fraction .What does it mean?
Ans
If the atomic mass of an element is in fraction, this mean that it exists in
the form of isotopes. The atomic mass is the average atomic mass and is
generally fractional.
Chemical Formulae: The chemical formula is a symbolic representation of a compound of its
composition.
For
writing Chemical Formulae the name or symbol of the metal is written first then
non-metals with their valencies. Then we must crossover the valencies of the
combining atoms. For example:
(a) Formula for aluminium oxide: Al3+
O-2 Þ Al2O3 (b) calcium hydroxide : Ca+2 OH-1 Þ Ca(OH)2
Valency: The combining power (or
capacity) of an element is known as its valency. Valency 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.
RELATIVE MOLECULAR MASS (RMM) : The relative molecular mass of an element or
a compound is the ratio of mass of one molecule of the element or a compound to
the mass of 1/12 th part of mass of one atom of carbon. Relative Molecular mass
is a pure ratio and has no unit. If the molecular mass of a given substance is
expressed in gram, it is known as gram molecular mass of that substance.
Molecular
mass is the sum of the masses of all the atoms present in one
molecule of the compound or an element.
Test your numerical skill:
Problem: Find the gram molecular mass of water (H2O)
Solution:
Þ 2(H)
= 2 x 1 = 2 and 1(O) = 1 x 16 = 16 ; Gram molecular mass of H2O = 2 + 16 = 18g
Problem: Find the
gram molecular mass of carbon dioxide
Solution: Þ (CO2)
1(C) = 1 x 12 = 12 and 2(O) = 2 x 16 =
32; Gram molecular mass of CO2 = 12 + 32 = 44 g
Calculate the percent by mass (weight) of sodium (Na) and chlorine (Cl) in sodium chloride (NaCl)
Relative molecular mass of NaCl = 22.99 + 35.45 = 58.44
The total mass of Na present:
1 Na is present in the formula, mass = 22.99
Tthe percent by mass (weight) of Na in NaCl:
%Na = (22.99 ÷ 58.44) x 100 = 39.34%
The total mass of Cl present:
Cl is present in the formula, mass = 35.45
The percent by mass (weight) of Cl in NaCl:
%Cl = (35.45 ÷ 58.44) x 100 = 60.66%
Here, 39.34 + 60.66 = 100.
The answers above are probably correct if %Na + %Cl = 100, that is,
Empirical formula: The empirical
formula is the simplest formula for a compound in which atoms of different
elements are present in simple ratio. It shows the relative number of atoms of
each element. For example CH2O is the empirical formula of Glucose C6H12O6
Molecular formula: It is the formula in which the actual number
of atoms of different element are
present. For example, if the empirical formula of benzene is CH where as molecular
formula is C6H6, etc.
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