(Atoms and Molecules) and (The Structure of an Atom) combined notes

 (Atoms and Molecules) and (The Structure of an Atom) combined notes  

The idea of divisibility of matter by Indian philosophers

Maharishi Kanad postulated that if we go on dividing the matter (called ‘padarth’), we shall get smaller and smaller particles.  And soon we will come across the smallest of particles beyond which further division will not be possible (called ‘parmanu’).

Pakudha Katyayama postulated that there are various forms of matter because the particles normally exist in a combined form.

The idea of divisibility of matter by Greek philosophers

Democritus and Leucippus –  suggested that when we go on dividing the matter, there comes a time when no more division of particles can take place. Such particles are called atoms which means being invisible.

Laws of Chemical Combination

There are two laws of Chemical Combination as established by Lavoisier and Joseph L. Proust.
(a)Law of Conservation of Mass
(b) Law of constant or definite proportion

Law of Conservation of Mass

It states that mass can neither be created nor destroyed in a chemical reaction. 

   OR

It states that during any chemical change, the total mass of reactants is equal to the total mass of the Product.

A + B -> C +D

Mass of Reactants=Mass of (A+B)
Mass of Products = Mass of (C+D)
Mass of Reactants = Mass of Products

Law of constant or definite proportion

It states that elements combine in their definite proportion of mass to give compounds. or In a chemical substance the elements are always present in definite proportions by mass
Example
For Water $H_{2}O$, the ratio of the mass of hydrogen to the mass of oxygen is always 1:8, whatever the source of water.

Dalton’s atomic theory

In 1808, John Dalton proposed an atomic theory that explained the Law of conservation of mass and the law of definite proportions.

 As per the theory, all matter whether an element, a compound, or a mixture is composed of small particles called ‘atoms’.

Postulates of Dalton’s Atomic Theory

• All matter is made of very tiny particles called atoms, which participate in a chemical reaction.
• Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.
• Atoms of a given element are identical in mass and chemical properties.
• Atoms of different elements have different masses and chemical properties.
• Atoms combine in the ratio of small whole numbers to form compounds.
• The relative number and kinds of atoms are constant in a given compound

What is an Atom?

• Atoms are the building blocks of matter. Just like bricks are the building blocks of a building.
• It is the smallest particle of an element that may or may not have an independent existence
• Atoms are very small, they are smaller than anything that we can imagine or compare with.
• The size of the atoms is measured by the Atomic Radius. Atomic radius is measured in nanometers ( $1nm={10}^{-9}m$)
• Atomic Radius of the Hydrogen is ${10}^{-10}m$.

Dalton’s atomic theory in suggested that an atom was indivisible. However, the discovery of two fundamental particles named as electrons and protons, inside the atom, led to the failure of Dalton’s atomic theory.

Fundamental particles of an atom:

Three particles electron, proton and neutron from which an atom is consisted of  are called fundamental particles of an atom or sub atomic particles.

Discovery of Electron:

By J. J. Thomson in 1897.

He carried a cathode ray experiment in which observed a stream of negatively charged particles coming out of cathode towards the anode. These particles were named as electrons.

Discovery of Proton:

By Ernest Goldstein in 1886.

He observed in the same gas discharge tube, with different situations that the anode emitted positive particles which he named as Canal Rays. His experiment led to the discovery of proton.

Discovery of Neutron:

By J. Chadwick in1932.

Neutron is present in the nucleus of all atoms.

Thomson’s Model of an Atom

• According to Thomson,(i) An atom consists of a positively charged sphere and the electrons are embedded in it. (ii) The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral
• The first model of an atom to be put forward and taken into consideration.
• He proposed a model of the atom be similar to that of a Christmas pudding/watermelon.
• The red edible part of the watermelon is compared with the positive charge in the atom.
• The black seeds in the watermelon are compared with the electrons which are embedded on it.

Limitations of Thomson’s Model:

→ It failed to explain how protons and electrons were arranged in atom so close to each other.

 Rutherford’s Model of Atom (By Ernest Rutherford in 1909)

In this experiment, fast-moving alpha (α)-particles were made to fall on a thin gold foil.

His observations were:

• A major fraction of the α-particles bombarded towards the gold sheet passed through it without any deflection, and hence most of the space in an atom is empty.
• Some of the α-particles were deflected by the gold sheet by very small angles, and hence the positive charge in an atom is not uniformly distributed.
• The positive charge in an atom is concentrated in a very small volume.
• Very few of the α-particles were deflected back; that is, only a few α-particles had nearly 180o angle of deflection. So the volume occupied by the positively charged particles in an atom is very small as compared to the total volume of an atom.

Rutherford’s Model of an Atom

Rutherford concluded the model of the atom from the α-particle scattering experiment as follows:

(i) There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus.

(ii) The electrons revolve around the nucleus in well-defined orbits.

(iii) The size of the nucleus is very small compared to the size of the atom.

Drawbacks of Rutherford’s Model

• He explained that the electrons in an atom revolve around the nucleus in well-defined orbits. Particles in a circular orbit would experience acceleration.
• Thus, the revolving electron would lose energy and finally fall into the nucleus.
• But this cannot take place as the atom would be unstable, and the matter would not exist in the form we know.

Bohr’s Model of an Atom

Bohr came up with the following postulates to overcome the objections raised against Rutherford’s model.

• Electrons revolve around the nucleus in stable orbits without the emission of radiant energy. Each orbit has a definite energy and is called an energy shell or energy level.
• An orbit or energy level is designated as K, L, M, and N shells. When the electron is in the lowest energy level, it is said to be in the ground state.
• An electron emits or absorbs energy when it jumps from one orbit or energy level to another.
• When it jumps from a higher energy level to a lower energy level, it emits energy, while it absorbs energy when it jumps from a lower energy level to a higher energy level.

Electron Distribution in Different Orbits

The distribution was suggested by Bohr and Bury.

• The maximum number of electrons present in a shell is given by the formula 2n2, where ‘n’ is the orbit number or energy level index, 1,2,3,….
• The maximum number of electrons in different shells are as follows: the first orbit will have 2 x12=2, the second orbit will have 2 x22=8, the third orbit will have 2 x 32=18, the fourth orbit 2 x 42=32 and so on.
• The shells are always filled in a step-wise manner from the lower to higher energy levels. Electrons are not filled in the next shell unless previous shells are filled.

IUPAC – International Union Of Pure & Applied Chemistry

• The IUPAC approves the names of elements.
• Generally, 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 (upper-case) and the second letter as a small letter (lower-case).
• There are a few elements where the symbols were taken from names of an element in Latin, German & Greek.

• Sodium (Na – Natrium)
• Potassium (K – Kalium)
• Iron (Fe – Ferrum)
• Copper (Cu – Cuprum)  
• Gold (Au – Aurum)
• Silver (Ag – Argentum)
• Tin (Sn – Stannum)
• Mercury (Hg – Hydrargyrum)   
• Lead (Pb – Plumbum)

Atomic Mass

The atomic mass of an element is the relative mass of its atoms as compared with the mass of an atom of carbon -12 isotope taken as 12 units.

 Atomic mass = Mass of 1 atom of the element 112 of the mass of an atom of Carbon-12 

• The mass of one atom is called atomic mass.
• Atomic mass of an atom is measured in amu and is written as 'u.
• For example, If we say that the atomic mass of Sulphur is 32, it means that sulphur is 32 times heavier than 1/12th of a carbon atom.

Molecule

• A molecule is a group of two or more atoms that are chemically bonded together.

• These atoms are held together by certain forces of attraction.

• A molecule can be defined as the smallest particle of an element or a compound that is cable of an independent existence and show all the properties of that substance.  

Molecules of Elements

• The molecules of an element are formed by combinations of similar types of atoms.

• Atomicity – the number of atoms in a molecule of an element is called its atomicity.

• • Monoatomic – when an element comprises a single atom. For example – all metals

  • Diatomic – when an element comprises two atoms. For example – all gases

  • Triatomic – when an element comprises of three atoms. For example – ozone

  • Tetra-atomic – when an element comprises of four atoms. For example – P4

  • Poly-atomic – when an element comprises of more than two atoms. For example – S8

Molecules of Compounds

Molecules of compounds constitute atoms of different elements that combine together in a fixed proportion. For example, water comprises of two atoms of hydrogen and one atom of oxygen.

What is an ion?

An atom or a group of atoms that contains positive or negative charges are called ions.

• A positively charged ion called ‘cation’.
• A negatively charged ion is called an ‘anion’.
• The ions consisting of only single atoms are called monoatomic ions.
• The ions consisting of a group of atoms is called a polyatomic ion.

 Radical: An atom or a group of atoms carrying positive or negative charges that behaves as a single unit in a 

chemical reaction.

Valency

The combining power (or capacity) of an element is known as its valency.

chemical formula

The chemical formula of a compound is a symbolic representation of its composition. 

Rules for writing a chemical formula 

(a) The valency of the elements or ions and this must be balanced in formula
(b) when a compound consists of a metal and a non-metal, the name or symbol of the metal is written first
(c) in compounds formed with polyatomic ions, the ion is enclosed in a bracket before writing the number to indicate the ratio. In case the number of polyatomic ion is one, the bracket is not required.

Binary compound:  Simplest compounds made  up of two different elements. Eg- HCl.

Molar mass: Molar mass is the sum of the atomic masses of the elements present in a molecule. It is obtained by multiplying the atomic mass of each element by the number of its atoms and adding them together. This is  expressed in terms of u.

Gram atomic mass of a substance – the atomic mass of a substance when expressed in grams is known as its gram atomic mass.

Formula unit mass

• formula unit mass is used for those substances whose constituent particles are ions
  
• it is a sum of the atomic masses of all atoms in a formula unit of a compound.
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