XI - 1.10 Atomic and Molecular Mass - Video Lectures

XI -

1.10 Atomic and Molecular Mass - Video Lectures

1.9 Atoms and Molecules

Atoms are the smallest particle of an element which may or may not have independent existence, but it takes part in chemical reactions.

Molecule is the smallest particle of a substance (element or compound) capable of independent existence.

There are 112 elements in nature as per the present knowledge. Obviously there are 112 different types of atoms.

Molecules may have two or more atoms. Molecules are divided into two types:

1. Homoatomic molecules

2. Heteroatomic molecules

1.10 Atomic and Molecular Mass

Atomic Mass: An atom is such a small particle that its mass cannot be determined with the help of any available balance.  By an indirect method, the absolute mass of hydrogen atom has been found to be 1.66 X 10 to the power of -24 g.

Chemists have defined the atomic mass of hydrogen as one and expressed the atomic mass of other elements as multiples of hydrogen's atomic mass. The relative atomic masses expressed in terms of hydrogen as termed atomic weights.

Atomic weights of some common elements

Metals

Magnesium  24.3
Aluminium  27.0
Iron              55.8
Zinc             65.4
Tin             118.7

Gram Atomic Mass: Gram atomic mass is the quantity of an element whose mass in grams is numerically equal to its atomic mass.

Molecular Mass: Measuring the mass of a single molecule is not possible. Hence as in the case of atoms,  molecular mass is defined as the average relative mass of its molecule as compared to the mass of an atom of carbon (C12) having mass number 12.

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Teacher's Pet

XI - 1.8 Avogadro's Hypothesis Video Lectures

XI -
1.8 Avogadro's Hypothesis Video Lectures

1.8 Avogadro's Hypothesis

Avogadro's hypothesis or suggestion is that matter consists of two kinds of ultimate particles. These are atoms and molecules.

Atoms are the smallest particle of an element which may or may not have independent existence, but it takes part in chemical reactions.

Molecule is the smallest particle of a substance (element or compound) capable of independent existence.

The actual hypothesis is that under similar conditions of temperature and pressure, equal volumes of all gases contain equal number of molecules.

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Smart Learning

XI - 1.7 Dalton Atomic Theory - Video Lectures

XI -
1.7 Dalton Atomic Theory - Video Lectures


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Exam Fear Videos


1.7. Dalton’s atomic theory

To provide theoretical justification to the laws of chemical combination which are experimentally verified, John Dalton postulated a simple theory of matter. The basic postulates of Dalton’s atomic theory are:

a. Matter is made up of extremely small indivisible and indestructible ultimate particles called atoms.
b. Atoms the same element are identical in all respects ie., in shape, size, mass and chemical properties.
c. Atoms of different elements are different in all respects and have different masses and chemical properties.
d. Atom is the smallest unit that takes part in chemical combinations.
d. Atoms of two or more elements combine in a simpler whole number ratio to form compound atoms (molecules).
e. Atoms can neither be created nor destroyed during any physical or chemical change.
f. Chemical reactions involve only combinations, separation or rearrangement of atoms.

Modern atomic theory

As a result of new discoveries made after Dalton developed his postulates, some modifications were done to atomic theory. They are:

1. Atom is no longer considered to be indivisible: It is found that atom is made up of subatomic particles such as electrons, protons and neutrons. We now state how many electrons are there, protons are there in an atom.

2. Atoms of same element may not be similar in all respects. Atoms of same elements have different atomic masses. These different atoms are called isotopes.

3. Atoms of different elements may have similar one or more properties. Atomic mass of calcium and argon (40 a.m.u.) are same. So the property of atomic mass is same for atoms of different elements. Isobars or elements or atoms having the same atomic mass.

4. Atom is the smallest unit which takes part in chemical reactions. Though electrons and protons are there, it is atom which takes part in chemical reactions and electrons exchange takes place between atoms.

5. The ratio in which the different atoms combine may be fixed and integral but may not always be simple. For example in sugar molecule the ratio of C,H and O atoms is 12:22:11, which is not simple.

6. Atom of one elements may be changed into atoms of other element. Transmutation is the process by which atoms one element can be changed into elements of other elements by subjecting it to alpha rays.

7. The mass of atom can changed into energy. Mass and energy are convertible. The equation give for such conversion is E = mc². Hence we cannot say that mass is not destructible. But in chemical reactions, atom remains unchanged and its mass is not destroyed to liberate energy.

XII - 1.3 Heisenberg's Uncertainty Principle - Video Lectures

XII -

1.3 Heisenberg's Uncertainty Principle - Video Lectures

Heisenberg's Uncertainty principle

In 1927, Heisenberg put forward a principle known as Heisenberg’s uncertainty principle.

According it, “it is not possible to measure simultaneously both the position and momentum (or velocity) of a microscopic particle, with absolute accuracy.”

Mathematically, this principle is expressed as:

∆x * ∆p = h/4 π

Where
∆x = uncertainty in position

∆p = uncertainty in momentum

The constancy of the product of uncertainties means that, if the position of the particle is known with more accuracy, there will be large uncertainty in momentum and vice versa.

This uncertainty arises, as all observations are made by impact of light, the microscopic objects suffer a change in position or velocity as a result of impact of light. So there is a disturbance in them due to the measurement.

The principle does not affect the measurement of large objects as in these cases impact of light does not created any appreciable change in their position or velocity.

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AK Lectures


Heisenberg Uncertainty Principle Example # 1
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AK Lectures

Heisenberg Uncertainty Principle Example #2

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AK Lectures




Heisenberg Uncertainty Principle Example #3

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AK Lectures

XI - 1.6 Laws of Chemical Combination - Video Lectures

Class XI
1.6 Laws of Chemical Combination

1.6. Laws of chemical combination

a. Law of conservation of mass
During any physical or chemical change, the total mass of the products is equal to the total mass of reactants.

b. Law of constant proportions

A pure chemical compound always contains same elements combined together in the same definite proportion by weight.

c. Law of multiple proportions

When two elements combine to form two or more than two compounds, the weights of one of the elements which combine with a fixed weight of the other, bear a simple whole number ratio.

d. Law of reciprocal proportions
When two different elements combine separately with the same weight of a third element, the ratio in which they do so will be the same or some simple multiple of the ratio in which they combine with each other.

e. Gay Lussac’s law of combing volumes
Under similar conditions of temperature and pressure, whenever gases react together, the volumes of the reacting gases as well as products (if gases) bear a simple whole number ratio.
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Askiitians


Part 2
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Askiitians

XI - 1.5 Chemical Classification of Matter - Video Lectures

Class XI -

1.5 Chemical Classification of Matter - Video Lectures

1.5. Chemical classification of matter

1. Element
Further classification: Metals, non-metals, metalloids

2. Compound
Further classification: Inorganic and organic

3. Mixture
Further classification: Homogeneous mixtures - they are called solutions.
Heterogeneous mixtures: They have visible boundaries of separation between the different constituents and they can be easily seen with naked eye.

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Linda Hanson

XII - 1.2 Dual Nature of Matter - de-Broglie Equation - Video Lectures

1.2 Dual Nature of Matter - de-Broglie Equation
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Thomas Harrison