Join now for JEE/NEET and also prepare for Boards Join now for JEE/NEET and also prepare for Boards. Register Now
Hey there! We receieved your request
Stay Tuned as we are going to contact you within 1 Hour
One of our academic counsellors will contact you within 1 working day.
Click to Chat
1800-1023-196
+91-120-4616500
CART 0
Use Coupon: CART20 and get 20% off on all online Study Material
Welcome User
OR
LOGIN
Complete Your Registration (Step 2 of 2 )
Sit and relax as our customer representative will contact you within 1 business day
Atoms are the basic building blocks of matter.
Different kinds of matter exist because there are different kinds of atoms present in them.
Whenever we rub two objects together, they become electrically charged. This is because atoms contain charged particles in them. Therefore, atoms can be divided further into particles i.e proton, electron and neutron.
Protons were discovered by Ernest Rutherford, in his famous gold foil experiment.
Electrons were discovered by J.J. Thomson, in his cathode ray tube experiment.
Neutrons were discovered by James Chadwick.
Atoms consist of protons and electrons in a balanced proportion.
Protons exist in the interiors of the atom and electrons exist in the exteriors of the atom. Therefore, electrons can be removed from an atom.
The postulates of the atomic theory by John Dalton
The matter is made up of tiny particles called Atoms that cannot be divided.
Atoms are never formed or destroyed during a chemical reaction.
Atoms of an element exhibit same nature. They have the same size, mass, and character.
Atoms of different elements exhibit variant nature. They do not have same characteristics.
Atoms form compounds by combining in a ratio of whole numbers.
A compound contains a constant number and kinds of atoms
Dalton suggested that atoms can neither be created nor destroyed and are indivisible. But the discovery of electrons and protons in atoms lead to failure of this aspect of Dalton’s theory.
According to J.J. Thomson, the structure of an atom can be compared to Christmas pudding where electrons are present inside a positive sphere.
An atom is composed of a positively charged sphere in which electrons are embedded.
Atom is neutral as the positive and negative charged are equal in proportion.
Rutherford’s Experiment
He experimented with thin gold foil by passing alpha rays through it.
He expected that the gold atoms will deflect the Alpha particles.
Alpha particles which had high speed moved straight through the gold foil
Atom contains a lot of empty space
Some particles got diverted a by slide angles
Positive charges in the atom are not occupying much of its space
Only one out of 12000 particles bounced back
The positive charges are concentrated over a particular area of the atom.
Thus, Rutherford gave the nuclear model of an atom based on his experiment which suggests that -
Atoms contain a lot of unoccupied space
There is a heavily positively charged substance present in the center of the atom which is called the nucleus
The nucleus contains an equal amount of positive and negative charge.
The nucleus id located at the center of the atom.
All the mass of the atom is because of the nucleus.
The electrons revolve around the nucleus in circular parts which are called Orbits
If we compare the size of the atom and nucleus, the nucleus is much smaller than the atom.
The Nuclear Model of the Atom failed to explain how an atom remains stable despite having positive and negative charges present in it. Maxwell has suggested a theory according to which if any charged particle moves in a circular motion it radiates energy. So, if electrons start moving in a circular motion around the nucleus they would also radiate some energy which would decrease at the speed of the electrons. As a result, they would fall into the nucleus because of its high positive charge.
What are nucleons? – Protons and Neutrons are collectively called as Nucleons.
Bohr suggested that –
Electrons spin around the nucleus in an individualized separate path or unattached orbit.
The electrons do not emit any energy while moving Indies special orbits.
These orbits are also called as Energy Levels.
They are represented using letters or numbers as shown in the figure below –
J. Chadwick discovered that there is another sub-atomic particle present in the atom. This particle carries no charge and is known as a Neutron. Therefore, we can conclude that atom consists of three types of particles -
which carry a negative charge
which carry a positive charge
they are neutral
If Orbit number = n
Then number of electrons present in an Orbit = 2n2
So, for n =1
Maximum electrons present in shell – K = 2 * (1)2 = 2
The outermost shell can contain at most 8 electrons.
The shells in an atom are filled in sequence.
Thus, until the inner shells of an atom are filled completely the outer shells cannot contain any electrons.
Valence Electrons – Electrons existing in the outermost orbit of an atom are called Valence Electrons.
The atoms which have completely filled the outermost shell are not very active chemically.
The valency of an atom or the combining capacity of an atom is given by the number of elements present in the outermost shell.
For Example, Helium contains two electrons in its outermost shell which means its valency is two. In other words, it can share two electrons to form a chemical bond with another element.
What happens when the outermost shell contains a number of electrons that are close to its maximum capacity?
Valency in such cases is generated by subtracting the number of electrons present in the outermost orbit from octet (8). For example, oxygen contains 6 electrons in its outermost shell. Its valency is calculated as: 8 – 6 = 2. This means oxygen needs two electrons to form a bond with another element.
Atomic Number of an Element
Atomic Number (Z) = Number of protons in an atom
Mass Number of an Element
Mass Number = Number of protons + Number of neutrons
The atoms of an element can exist in several forms having similar atomic numbers but varying mass numbers.
Isotopes are pure substances.
Isotopes have a similar chemical nature.
Isotopes have distinct physical characteristics.
1. The fuel of Nuclear Reactor – Isotope of Uranium
2. Treatment of Cancer – Isotope of Cobalt
3. Treatment of Goiter – Isotope of Iodine
Example: Consider two atomic species namely U and V. Are they isotopes?
From the above example, we can infer that U and V are isotopes because their atomic number is the same.
The atoms of several elements can have a similar mass number but distinct atomic masses. Such elements are called Isobars.
Signing up with Facebook allows you to connect with friends and classmates already using askIItians. It’s an easier way as well. “Relax, we won’t flood your facebook news feed!”
Dear , Preparing for entrance exams? Register yourself for the free demo class from askiitians.
Revision Notes on Matter in Our Surroundings What...
Revision Notes on Force and Laws of Motion How...
Revision Notes on Improvement in Food Resources...
Revision Notes on Gravitation What is the...
Revision Notes on Natural Resources Resource...
Revision Notes on Is matter around us Pure What is...
Revision Notes on Motion A Reference Point is used...
Revision Notes on Why do we Fall Ill? The...
Revision Notes on Diversity in Living Organisms...
Revision Notes on Work and Energy Work The...
Revision Notes on Tissues Quick Review Living...
Revision Notes on Atoms and Molecules The...
Revision Notes on Sound Sound energy is a form of...
Revision Notes on The Fundamental Unit of Life...