Structure of Atom

By

Prangav Singhal IX-C Roll no. 2

What is an atom?An atom is like a brick in a building. Like a

building is built with bricks, any matter in this world is made up of atoms.

Atoms in a crystal bricks in a wall

Charged particles in atomAtoms are made up of electrons, protons and

neutrons.Electrons are negatively charged (e-).Protons are positively charged (p+)Neutrons are electrically neutral.

Structure of an atomAfter the discovery of electrons and protons,

and thus the failure of Dalton's theory of an atom being indivisible, it became important to discover how these electrons and protons were arranged.

many scientists tried to explain the structure of atom. They were:

1. J.J.Thomson2. Ernest Ruthford3. Neils Bohr

J.J.Thomson’s model of an atomThomson proposed that:An atom consists of a positively charged

sphere and the electrons are embedded in it.The negative and positive charges are equal

in magnitude. So, the atom as a whole is electrically neutral.

Although Thomson’s model explained that atoms are electrically neutral, the results carried out by other scientists could not be explained by this model.

J.J.Thomson’s

model of an atom

Ruthford’s model of an atomOn the basis of his experiment of

passing a beam of alpha particles through a thin gold foil, he put forward a model of an atom:

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

The electrons revolve around the nucleus in well defined orbits.

The size of the nucleus is very small as compared to the size of the atom.

Ruthford’s experiment

Ruthford’s model of an atom

Bohr’s Model of an AtomIn order to overcome the objections

raised against Ruthford’s model of an Atom, Neils Bohr put forward the following prostulates about the model of an atom:

Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom.

While revolving in discrete orbits the electrons do not radiate energy. These orbits or shells are called energy levels.

Bohr’s Model of an Atom

Neutrons In 1932, J.Chadwick discovered another sub-

atomic particle which had no charge and a mass nearly equal to that of a proton .

It was eventually named neutron.Neutrons are present in the nucleus of all

atoms, except Hydrogen. In general, a neutron is represented as ‘n’.

The mass of an atom is thus given by the sum of masses of protons and neutrons present in the nucleus.

How are electrons distributed in different shells?

The distribution of electrons into different orbits of an atom was suggested by Bohr and Bury.

The following rules are followed for writing the number of electrons in different energy levels or shells:

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 that can be accommodated in the outermost shell is 8.

electrons are not accommodated in a given shell unless the inner shells are filled. That is the shells are filled in a stepwise order.

Electron shells for sodium(Na) atom(atomic no. 11, with 11 electrons.

What we have discussed Atoms are like the bricks in a building, every matter is made up of

atoms. Atoms have three different types of particles: electrons(-ve

charge), protons(+ve charge) and neutrons(neutral). Many scientists tried to explain the structure of an atom. J.J.Thomson said that electrons in an atom were embedded in a

positively charged sphere. Ruthford said that all the positive material is concentrated in the

centre of the atom and electrons revolve around the atom in fixed orbits.

Bohr said that only selected orbits are allowed in side the atom (discrete orbits) and electrons don’t radiate energy while in these orbits.