Structure of Atom

1. Structure of Atom By Prangav Singhal IX-C Roll no. 2

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

3. Charged particles in atom • Atoms are made up of electrons, protons and neutrons. • Electrons are negatively charged (e-). • Protons are positively charged (p+) • Neutrons are electrically neutral.

4. Structure of an atom • After 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: • J.J.Thomson • Ernest Ruthford • Neils Bohr

5. J.J.Thomson’s model of an atom Thomson 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

6. Ruthford’s model of an atom On 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

7. Bohr’s Model of an Atom In 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

8. 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.

9. 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.

10. 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.