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Matter
Matter: Anything that has weight and occupies space is called matter.
Substance
Pure Substances
Element
Compound
Heterogeneous
Impure Substances (mixture)
Homogeneous
Element: An element is a substance, which cannot be split up into two or more, simplest
substances by any physical or chemical means. It is the simplest form of matter. For eg. O, H, N,
C, S, Fe, Cu, etc.
Classification of elements:
There are two types of elements-
1. Metals eg. Na, K, Fe, etc.
2. Non-metals eg. Cl, C, etc.
Metals can also be classified depending upon the physical state;
1. Solid eg. Fe, Cu, etc.
2. Liquid eg. Hg, Br, etc.
3. Gas eg. He, Ne, etc.
Compound: A compound is a pure substance formed from two or more elements combined
together in a definite proportion by weight.
OR
A compound is a pure substance that can be decomposed by chemical action into two or more
simple substances. Eg. NH3, CO2 , etc.
Properties:
1. The constituent elements of a compound cannot be separated by mechanical or physical
methods.
2. The properties of a compound are entirely different from those of its constituent elements.
3. In a compound, the constituent elements are present in a definite proportion by weight.
4. A compound has a fixed boiling point and a fixed melting point.
5. A compound is a homogenous substance, i.e. a compound is such a substance which is
same in its properties and composition.
Mixture: A mixture is a material containing two or more elements or compounds in any
proportion and the components of which can be separated by simple mechanical means. For
eg. Air, steam, etc.
Homogeneous mixture: It has a uniform composition throughout its mass. There are no visible
or sharp boundaries between the various constituents of a homogenous mixture. For eg. Salt
solution, sugar solution, etc.
Heterogeneous mixture: It does not have a uniform composition throughout its mass There are
visible and sharp boundaries between the various constituents of a mixture. For eg. Mixture of
iron filings and sulphur, mixture of water and sand, etc.
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Properties of a mixture:
1. A mixture may be homogeneous or heterogeneous.
2. The constituents of a mixture can be separated by physical methods like filtration,
evaporation, magnetic effect, etc.
3. In the preparation of a mixture, energy is neither given out nor absorbed.
4. The composition of a mixture is not fixed. The constituents of a mixture may be present in
any proportion by weight.
5. A mixture has no definite melting point and boiling point.
6. In the formation of a mixture no chemical reaction occurs.
Atomic Masses:
Hydrogen – 1
Oxygen – 16
Carbon – 12
Nitrogen – 14
Sulphur – 32
Chlorine – 33.5
Proportion by Number:
H2O
CO2
2:1
1:2
Proportion by Weight:
H2O
CO2
2*1:2*16 = 1:8
1*12:2*16 = 12:32 = 3:8
Laws of chemical combinations:
1. Law of conservation of mass: According to this law there is no detectable loss or gain in
total mass of substances during a chemical reaction.
2. Law of constant proportions: All pure specimens of a compound contain the same elements
in the same proportion by weight .for eg. Water is a compound of Hydrogen and Oxygen and
from no matter which source we take it , it always contains hydrogen and oxygen in the same
proportion by weight.;1:8
3. Law of Multiple Proportions: When one element combines with a second element, to form
two or more different compounds, then the weight of one of the elements which combines
with the constant weight of the other, bear a simple ratio to one another.
Dalton’s Atomic Theory of Matter
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Matter is composed of very small particles called atoms.
Atoms are indivisible.
Atoms can be neither created nor destroyed.
Atoms of the same element are same in all respects. For eg, size, shape, mass, properties, etc.
Atoms of different elements are different in properties.
Atoms of different elements combine in a simple whole number ratio to form compounds.
Atoms of the same element can combine in more than one ratio to produce more than one
compound.
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Drawbacks of Dalton’s Atomic Theory of Matter:
1. Atoms are no more indivisible. They have got their own structure as they are made up of
protons, electrons and neutrons.
2. Dalton’s atomic theory states that atoms of the same elements have same masses, but now it
is known that isotopes of the same elements have the same masses.for eg. Isotopes of
Carbon have got the masses of 12 and 14.
3. Dalton’s atomic theory states that atoms combine in a simple whole number ratio to form
compounds, but in certain cases this ratio is not simple.for eg. In sugar molecule
(C12H22O11) the ratio is 12:22:11 which is complex.
4. Dalton’s atomic theory states that atoms of the same element have same properties, but this
is not always the same.for eg. Diamond, Graphite and Charcoal are made up of carbon atoms
but they have different physical properties.
5. Atoms of the different elements have different masses, but since the discovery of isobars, it
is known that atoms of different elements have same masses.
Atom:
1. An atom is the smallest particle which can take part in a chemical reaction.
2. Atoms of many elements are very reactive and cannot stay in the free state.for eg.Na, K, O,
N, etc.
3. Only the atoms of noble gases like He, Ar, Kr, etc. can exist in the free state because they are
unreactive.
Molecule:It is the smallest particle of a substance which has the properties of that substance and
can exist freely.
Molecules of an element: Hydrogen, Nitrogen, Oxygen, etc.
Molecules of a compound: Water, Hydrochloric acid, etc.
Molecules are of two types:
1. Homoatomic: These molecules are made up of the same elements. For eg. Hydrogen,
Nitrogen, Oxygen are diatomic, Ozone is triatomic, Phosphorous is tetraatomic and Sulphur
is Polyatomic.
2. Heteroatomic: These molecules are made up of different kinds of atoms. For eg. Water,
Carbondioxide, etc.
Atomicity of a element: It is the number of atoms present in one molecule of an element. Eg.
Oxygen-2, Hydrogen-2, Ozone-3, etc.
Atomicity of a compound: It is the number of atoms present in one molecule of a compound.
For eg. Carbondioxide- 1+2=3
Sulphuric acid- 2+1+4=7
Isobars: Atoms of different elements having the same mass.
Relative atomic mass: The atomic mass of an element is the relative mass of one atom of the
element relative to the 1/12 of the c-12 atom.
Relative atomic mass =
mass of one atom of an element
1/12 of the mass of the C-12 atom
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Atomic mass unit (amu): 1/12 of the mass of the C-12 atom. 1 amu is also called 1 Dalton.
Atomic mass unit scale: The scale of relative mass of atoms is called atomic mass unit scale.
For eg. Atomic mass of Mg is 24 which indicates that Mg is heavier than 1/12 of the mass of the
C-12 atom.
Relative molecular mass: The molecular mass of a substance is the relative mass of the
substance as compared to 1/12 of the mass of the C-12 atom.
For eg. Relative molecular mass = mass of one molecule of the substance
1/12 of the mass of the C-12 atom
Gram Atomic mass: atomic mass expressed in grams is called gram atomic mass. Eg. Atomic
mass of oxygen is 16 so the gram atomic mass will be 16g.
Gram molecular mass: The molecular mass expressed in grams is called gram molecular mass.
Eg. Molecular mass of oxygen is 32 so the gram molecular mass will be 32g.
Structure of Atoms:
Discharge Tube Experiments: J.J Thompson conducted many experiments which led to the
discovery of cathode rays which consist of negatively charged particles called electrons.
Discharge tube consists of a hard glass tube closed at both ends; it is fitted with metallic plated
at the two ends called electrodes. The electrode connected to the negative terminal is called
cathode while the electrode, which is connected to the positive terminal, is called anode. The
discharge tube is connected to a vacuum pump to reduce the pressure. The discharge tube is
filled with a gas under study. After connecting the electrodes to high voltage it was observed
that when the pressure in the tube was decreased to 0.0001 atmospheres, the glass of the tube at
the anode side begins to fluoroscence (emits light) when the rest of the tube is dark.. This shows
that an invisible radiation originates from the cathode which are called cathode rays.
Properties of cathode rays:
1. Cathode rays travel in a straight line: When the solid object ids placed in the path of the
cathode rays it casts a shadow on the wall opposite to it. This suggests that cathode rays
travel in a straight line.
2. Cathode rays produce mechanical effect: When a small pedal wheel is placed between the
electrodes, it rotates. This suggests that cathode rays consist of particles having kinetic
energy and can therefore produce mechanical effect.
3. Cathode rays consist of negatively charged particles: When cathode rays are passed through
a strong magnetic field, they get deflected towards the positively charged plate. This
suggests that cathode rays consist of negatively charged particles.
4. The cathode rays upon striking glass or certain other materials cause them to glow (produce
fluoroscence).
5. These rays penetrate through thin sheets of aluminium or other metals.
6. The nature of the rays does not depend upon the nature of the rays taken in the tube.
7. For every negatively charged particle of the cathode rays, the ratio of the charge (e) to the
mass (m) is constant.
Discovery of anode rays:
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Goldstein in 1896 repeated the discharge tube experiment using perforated cathode. When the
pressure in the tube was decreased, it was observed that in addition to the cathode rays, new
kinds of rays are also formed which came through the perforations of the cathode. These rays
travel in the opposite direction to the cathode rays and because they moved from the anode,
towards the cathode, these were called anode rays. Therefore, the anode rays consist of
negatively charged particles.
Properties of anode rays:
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Anode rays travel in a straight line.
Anode rays produce mechanical effect.
The nature of anode rays depends upon the nature of the gas taken in the discharge tube.
Anode rays are positively charged particles.
Particles
Nature
Relative mass
Absolute mass
Relative charge
Absolute charge
Electron
Negatively charged
1/1840 amu
9.109  10-28 g
-1
1.602 X 10-19 C
Proton
Positively charged
1 amu
1.67 X 10-24 g
+1
1.602 X 10 -19 C
Neutron
No charge
1 amu
1.67 X 10-24 g
No charge
No charge
Rutherfor’d Experiments- Discovery of Nucleus:
Rutherford carried out a classic experiment in which he used Alpha particles from radioactive
source like Radium, Pulonium, Uranium, etc. Alpha particle is a positively charged particle.
When alpha particles are allowed to strike a very thin gold foil it is found that;
1. Most of the alpha particles pass straight through the gold foil without any deflection from
their original path.
2. A few alpha particles are deflected through small angles and a few alpha particles are
deflected through large angles.
3. A few alpha particles completely rebound on hitting the gold foil and turn back their path.
Explanations:
1. Since, most of the alpha particles pass straight through the gold foil without any deflection,
it shows that there is a lot of empty space in an atom.
2. The observation that a few alpha particles are deflected through small and large angles, it
shows that there is a centre of positive charge which repels the positively charged alpha
particles and deflects them from their original path.
3. The observation that very few alpha particles completely rebound on hitting the gold foil,
shows that, the nucleus is hard and dense and does not allow alpha particles to pass through
it.
Coclusions:
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4.
The atoms consist of positively charged centre called nucleus.
Most of the mass of the atom is concentrated in the nucleus.
The nucleus of an atom is very small as compared to the atom as a whole.
The nucleus is surrounded by negatively charged electrons, which balance the positive
charge of the nucleus.
5. The electrons are not stationary but they are revolving around the nucleus at very high speed.
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6. Since, the atom is electrically neutral, the number of protons in the nucleus and the number
of electrons moving around the nucleus must be equal.
Niel Boher’s – Model of an atom:
1. An atom is made of three particles, electrons, protons and neutrons; electrons have negative
charge and protons have positive charge.
2. The protons and neutrons are located in the small nucleus at the centre of the atom. Due to
the presence of the protons, nucleus is positively charged.
3. The electrons revolve rapidly around the nucleus in fixed circular paths, called energy levels
or shells. (K, L, M, N, O, P)
4. The maximum number of electrons that can be accommodated in any shell is given by 2n2 .
K=1
L =2
M=3
2 X 12 =
2
2 X 22 =
8
2 X 32 =
18
2
N=4
2X4 =
32
5. Each energy level is associated with fixed amount of energy. The nearest the shell the lower
the energy.
6. There is no change in the energy of an electron as long as they keep on revolving in the same
energy level.
7. In the outermost shell of any atom, the maximum number of electrons possible is 8 even if it
has a capacity to accommodate more electrons.
Atomic Number (Z): The number of protons present in an atom of an element is its atomic
number. If the atom is neutral, the number of protons = number of electrons.
For eg. Atomic number of Oxygen is 8.
Mass Number (A): Number of protons + number of neutrons in an atom is its mass number.
Eg. Mass number of Oxygen is 16.
Valence electrons: Number of atoms present in the outermost shell which take part in a chemical
reaction.
Neutron = Mass Number – Atomic Number
A-Z
Element
Atomic Number
Na
Mg
Al
Si
P
S
Cl
Ar
K
Ca
11
12
13
14
15
16
17
18
19
20
Radioactivity:
Electronic
configuration
2,8,1
2,8,2
2,8,3
2,8,4
2,8,5
2,8,6
2,8,7
2,8,8
2,8,8,1
2,8,8,2
No. of electrons
11
12
13
14
15
16
17
18
19
20
Valence
Electrons
1
2
3
4
5
6
7
8
1
2
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(Henry Becquerel in 1896) The spontaneous emission of invisible radiation of invisible radiation
is called radioactivity and the substances which emit these substances are called radioactive
substances. Eg. Radium, Pulonium, Uranium, etc.
Cause of Radioactivity:
When the neutrons in the nucleus becomes too large then the nucleus becomes unstable. The
unstable nucleus of an atom emits radiation in the form of charged particles, alpha, beta, energy
(gama) so that a more stable nucleus of another type forms. This act of unstable nucleus to
change into a stable nucleus is called radioactivity.
n/p = 1 – for stable nuclei
eg. Ca: Protons = 20; Electrons = 20; Neutrons = 20
Therefore, n/p = 20/20 = 1
n/p >1 – for unstable nuclei
eg. U: Protons = 92; electrons = 92; neutrons = 146
Therefore, n/p = 146/92 > 1
Hence, it is radioactive.
(Here, n is the number of neutrons and p is the number of protons.)
Properties of Alpha rays:
Velocity: Alpha rays travel with 1/10th of the speed of light, that is 10000 miles per second.
Ionising power: Alpha rays ionise the gas through whichthey pass.
Action on photographic plate: They affect the photographic plate feebly.
Penetrating power: They have the least penetrating power and can be stopped by 0.1 mm
thick aluminium foil or a sheet of paper.
5. Effect of electric field: In an electric field they are deflected towards the negative plate. This
shows that Alpha rays are positively charged.
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Properties of Beta Rays
1. Velocity: Beta rays travel with the velocity ranging from 1% to 99% of the speed of light.
2. Ionising power: Their ionising power is much less than that of Alpha particles.
3. Effect on photographic plate: They affect a photographic plate more strongly than alpha
rays.
4. Penetrating power: Their penetrating power is 100 times greater then that of Alpha rays.
They can be stopped by 2 cm. thick sheet of aluminium.
5. Effect of electric field: In an electric field they are deflected towards the positive plate. This
shows they are negatively charged particles.
Properties of Gama Rays
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Nature: They are high frequency electro-magnetic waves.
Velocity: Gama rays have velocity equal to that of light.
Ionising power: They ionise the gas through which they pass.
Effect of electric field: In an electric field they are not deflected at all. This shows Gama
rays are neutral.
5. Penetrating power: These rays have more penetrating power than Alpha or Beta rays. They
can be stopped by 5 cm. thick lead.
6. Action on photographic plate: They darken a photographic plate deeply.
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Note: When a nucleus emits alpha particles the resulting daughter nucleus will have two units
less atomic number and four units less atomic mass. When a nucleus emits Beta particle the
resulting daughter nucleus will have 1 unit more atomic number and same atomic mass.
Half-life period: The half-life period of a radioactive substance is defined as the time required
for the radioactive substance to disintegrate to one-half of its original amount.
Eg. Half-life of radium is 1700 years.
Eg. Half-life of Carbon-14 is 5800 years.
Radio Carbon dating or Radio isotope dating:
Willard Libby in 1960 developed the technique of Radio Carbon dating to determine the age of a
plant or animal fossil.
C-14 produced in earth’s atmosphere by the bombardment of cosmic rays. Being similar in
properties C-14 and C-12 get mixed together. Living organisms use carbon-di-oxide. A small
part of it has radioactive Carbon. This 14CO2 is consumed by plants and in turn by animals. In a
living organism the ratio of C-14 and C-12 remains constant. When a plant dies this ratio begins
to decrease continuously due to continuous decrease of C-14 in the plant with the emission of
Beta particles.
14
6C
 157N + 0-1e
Therefor, a dead plant after 5800 years will have only half as much C-14 as a living plant. Thus
by knowing the ratio of C-14 and C-12 in the living as well as the dead plant, we can determine
how long back the plant died. This technique is known as radio isotope dating or radio carbon
dating.
Isotopes
Isotopes are the atoms of the same element having same atomic number but different atomic
mass number. Difference in the masses of isotopes is due to different number of neutrons in their
nuclei. All the isotopes of an element have identical chemical properties.
Explanation:
1. All the isotopes of an element contain the same number of electrons so they have the same
electronic configuration with the same number of valence electrons.
2. Since the chemical properties of an element are determined by number of valence electrons,
hence all the isotopes of an element have identical chemical properties.
3. Isotopes have different nuclear properties because of the different number of neutrons
present in the nuclei.
Examples of Isotopes:
12
6C
14
6C
P=6
E=6
Tritium
N=6
E.C. = 2,4
P=6
E=6
N=8
E.C. = 2,4
1
1H
Protium
2
1H
3
1H
Deuterium
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V.E. = 4
V.E. = 4
Ions
An ion is an electrically charged atom or a group of atoms. These are of two types:
a) Cations
b) Anions
Cations: They are positively charged ions like Na+, Mg2+,etc.
Anions: They are negatively charged ions like Cl-, O2-, etc.
X-rays
X-rays are produced when fast moving electrons (cathode rays), hit a metal. For eg. Tungsten
(W), etc.
They are in fact electro-magnetic waves of a short wavelength and high frequency. X-rays travel
in a straight line and are not bent in an electric or magnetic field. They can pass through opaque
objects like black paper, etc.
Uses:
X-rays are used to take pictures of fractured bones. Other problems like infections in lungs, can
be diagnosed with the help of X-rays.
- Kshitij Misra