Atomic Structure

According to Dalton, atom is the smallest indivisible particle. But discharge tube experiments have proved that atom consists of some more smaller particles. Electrons, protons and neutrons are the fundamental particles of an atom.

Electron was discovered in cathode ray experiment. Cathode rays can be deflected in electric and magnetic field which shows that they are negatively charged particles. Cathode rays are negatively charged consisting of electrons.

Protons and neutrons are present in the nucleus and are called as nucleons. Protium contains only electron and proton. Except protium all the atoms contain electron, proton and neutron.

Proton was discovered in the anode ray experiment. Anode rays, also called as canal rays or positive rays, were discovered by E. Goldstein. Anode rays contain material particles obtained by the removal of one or more electrons from the gaseous atoms or molecules present in the tube.

The positively charged particles present in the anode rays produced when Hydrogen gas is present in the discharge tube were called protons by Rutherford (proton = first particle).

The ratio of charge to mass is called specific charge. Electron has the highest specific charge because of its negligible mass. The mass of electron increases with increase in velocity. Thus e/m of electron decreases with increase in velocity.

The number of electrons or protons present in an atom of an element is called its atomic number. A neutral atom contains equal number of electrons and protons. Atomic number is denoted by Z. Atomic number is equal to the nuclear charge of an element.

The sum of number of protons and neutrons in the atom of an element is called its mass number and it is denoted by A.

Number of neutrons = A - Z 

Atoms of elements having the same atomic number but different mass numbers are called isotopes. Isotopes of an element have the same number of protons and electrons but differ in the number of neutrons. Isotopes of an element have same chemical properties but different physical properties.

Rutherford's Atomic Model

This is called planetary model or nuclear model of atom. Rutherford's atomic model is based on the findings of α-ray scattering experiment.

An atom is a hollow sphere and the entire mass and positive charge are concentrated at the centre of the atom in the smallest region called nucleus. Electrons revolve around the nucleus in circular path.

This model failed to explain the stability of atoms and line spectra of atoms. As per the laws of electrodynamics an electron moving around the nucleus must radiate energy continuously and must spiral down into the nucleus. If an electron radiates energy continuously the atomic spectrum should be a continuous spectrum. The atom should collapse if it happens. But all the atoms give line spectra.

Bohr's Atomic Theory

Bohr recognized the relationship between the nature of the series of spectral lines and the arrangement of electrons in the atom. Bohr applied Planck's quantum theory to the electrons revolve around the nucleus. He retained the basic concept of Rutherford's model of atom that electrons revolve round the positively charged nucleus.

Bohr proposed his theory to explain the structure of atom. Electrons revolve around the nucleus with definite velocities in concentric circular orbits. These orbits are called stationary orbits as the energy of the electron remains constant. As long as the electron revolves in the same circular orbit it neither radiates nor absorbs energy.

Energy of the electron changes only when it moves from one orbit to another orbit. Energy is absorbed when an electron jumps from a lower orbit to a higher outer orbit. Energy is released when an electron jumps from higher orbit to a lower orbit.

Quantum Numbers

Principal quantum number (n)

It is proposed by Bohr and denoted by n. It represents the main energy level. It determines the size of the orbit and energy of the electron.

It takes all positive and integral values from 1 to n. The maximum number of electrons in a main energy level is 2n², and number of orbitals is n².

Azimuthal quantum number (l)

It is also known as angular momentum quantum number or orbital quantum number. To express the quantised values of the orbital angular momentum, azimuthal quantum number was proposed.

It takes values from 0 to n - 1. It determines the shape of orbitals.

Magnetic quantum number (m)

To explain Zeeman and Stark effects, Lande proposed magnetic quantum number. It is denoted by m. It represents the sub-sub energy level or atomic orbital. It determines the orientation of orbital in space.

When the atom is placed in an external magnetic field, the orbit changes its orientation. The number of orientations is given by the values of the magnetic quantum number m.

The m takes the values form -l to +l through 0. Total values of m for a given value of A = (2l + 1) values.

Spin quantum number (s)

In the fine spectrum of alkali metals pairs of widely separated lines are observed which are different from duplet, triplet, and quadruplets observed in the hydrogen spectrum.

To recognise and identify these pairs of lines, Goudsmit and Uhlenbeck proposed that an electron rotates or spins about its own axis. This results in the electron having spin angular momentum, which is also quantised.

The electron may spin clockwise or anti clockwise. Therefore, the spin quantum number takes two values +1/2 and -1/2. Clockwise spin or parallel spin is given +1/2 or ↑ and anti clockwise or anti parallel spin is given by –1/2 or ↓.