The definition of an atom was given by John Dalton. It was in the year 1808. He said atoms were the smallest and indivisible matter. He also was the first person to discover that an atom has sub-particles. We are all made up of atoms. Atoms are the fundamental unit of life. Atoms are split into three parts:

• Electrons
• Neutrons
• Protons

The neutrons and protons are responsible for the atomic mass of any element. Electrons define the properties of an atom. J.J.Thomson explored the structure of an electron. He discovered it with the help of cathode rays. He said electrons have a negative charge. It bears a charge of -1.6 x 10-19 C. It bears a mass of 9.11 x 10-31 kg. The scientist Rutherford discovered the proton. He found out about protons using an anode ray experiment. He said protons were positive particles. It carries a charge of 1.6 x 10-19.  1.007276 U is the mass of a proton. Chadwick discovered neutrons. It was in the year 1923. He said neutrons did not carry any charge. They were located in the nucleus. 1.008665 U is the mass of a neutron. Electromagnetic spectrum can be absorption or emission spectrum.

Emission spectrum-when a body absorbs a certain amount of energy and emits it

Absorption spectrum – when a body absorbs energy and leaves dark lines in the continuous spectrum

De Broglie discovered the duality of electrons. It comprises both particle and wave properties.

MCQs

• Which of these particles is most difficult to experimentally verify de Broglie’s relationship?

(a) A dust particle
(b) Proton
(c) Electron
(d) ⍺ particle

Solution: (a) – A dust particle

Who discovered electrons?

(a) J.J Thomson
(b) Neils Bohr
(c) James Chadwick
(d) Rutherford

Solution: (a)- J.J Thomson

• Consider an atom with atomic number 17 and mass number 37. What is the number of protons?

(a) 22
(b) 21
(c) 17
(d) 20

Solution: (c) 17

Who proposed atomic theory?

(a) Robert Millikan
(b) J.J Thomson
(c) Neils Bohr
(d) John Dalton
Solution: (a) Robert Millikan
Consider an atom with the atomic number 20. Which of the following is the correct electronic configuration?
(a) 2,6,8,2
(b) 2,8,8,2
(c) 2,6,8,2
(d) 2,4,6,2
Solution: (b) 2,8,8,2

Theory Questions

• What were the drawbacks of Bohr’s model?

Bohr’s model had a few drawbacks. They were:
•  It was inadequate to explain the atomic spectrum other than hydrogen. It could not explain multi-electron atoms
• It did not talk about the ability of atoms to create molecules by chemical bonding
• It also could not explain the shape of the molecules
• It did not match Heisenberg’s uncertainty principle
• Unable to elucidate spectral line splitting in a magnetic field
• Unable to demonstrate the duality of matter

• What is de Broglie’s principle? Explain with equations.

Louis de Broglie demonstrated the duality of electrons. He elucidated the particle and wave properties of electrons. He said every moving particle, either macroscopic or microscopic, will have a wave character. He called these matter waves. Then he formulated a relation between the momentum of a particle and velocity in a particular wavelength. This is de Broglie’s wavelength.
It is given by
E = hv = hc/𝛌
Or
E =  hc/𝛌 = mv2
𝛌 = h/mv – This equation is used to calculate de Broglie wavelength
λ = h / √2m * K E, where KE is kinetic energy
where, h = Plank’s constant
v = velocity of the particle
c = speed of light
𝛌 = wavelength
E = energy
m = mass of the particle
• Elucidate the main features of Rutheford’s model.

Rutherford is the scientist who discovered protons in an atom. Rutherford’s model includes:
• An atom is divided into the nucleus region and the outer nuclear region
• The nucleus embodies protons and neutrons. The outer nuclear region contains electrons
• Ninety-nine per cent of space in an atom is empty
• The whole mass and positive charges are condensed in the nucleus
• Electrons always revolve around the nucleus in specified paths called orbits

Explain the cathode-ray experiment by J.J.Thomson.

Cathode-ray experiment was demonstrated by J.J.Thomson. The negative particles were discovered by this experiment. These particles were electrons.
• The cathode-ray experiment consisted of a cathode ray discharge tube
• A cathode-ray tube embodied two pieces of metals called electrodes
• These electrodes were sealed inside a glass tube. The glass tube was attached to a vacuum pump. The pressure in the tube was 0.01mm
• When a fairly high voltage ( ex:10,000V) was applied across the electrodes, invisible rays were emitted
• These were cathode rays. After analyzing these rays, electrons were discovered

What are the postulates of Dalton’s Theory? Elucidate the demerits.

The postulates of Dalton Theory are:
• Everything around us (matter) is made of atoms
• These atoms cannot be divided into smaller units
• All elements have only one type of atom in them
• Every atom has a constant mass that varies from each element
• Atoms will undergo rearrangement during chemical reactions
• He showed that the Law of conservation of mass is also applicable to atoms, i.e. atoms can neither be created nor destroyed

Demerits:

• This theory did not explain isotope existence
• The structure of the atom was not explained properly
• It was later found out that atoms could be divided into smaller units

Numerical questions

Let us consider here an atom with an electron moving at a speed of 600 m/s. It has an accuracy of 0.005%. What is the certainty with which the position of the electron can be located? ( Given : me= 9.1 x 10-31 Kg and h= 6.6 x 10-34 Kgm2s-1)
We find  Δv by
Δv = 600 x 0.005/ 100 = 0.03
Δv x Δx = h/ 4ℼm
Rearrange
Δx = 6.6 x 10-34 /( 4 (ℼ x 9.1 x 10-31 x 0.03))
Δx = 1.92 x 10-3 m
• Consider an electron with velocity 1.6 x 106 m/s. What is de Broglie’s wavelength in this problem?
We use the equation   𝛌 = h/mv
Substitute values of h and m for an electron
𝛌 = 6.63 x 10-34/  (9.11 x 10-31 x 1.6 x 106 )  = 6.63 x 10-30 m
• Consider a lightwave with a period 2 x 10-10 s. Calculate the frequency, the wavelength and the wavenumber.
Frequency = 𝝂 = 1/ period = 1/ 2 x 10-10  = 5 x 109 s-1
Wavelength = 𝛌 = c/𝝂 = 3 x 108 / 5 x 109  = 6 x 10-2 m
Wavenumber = 1/ 𝛌 = 16.66 m-1

Conclusion

The topic of atomic structure is crucial in chemistry. It is essential to understand the concepts well. The atomic structure should be studied well if you are preparing for JEE mains or NEET. The atomic structure has both theory and numerical. Atomic structure elucidates the significance of electrons, protons, and neutrons. Atomic structure is the basis of chemistry. These important questions will help you score well. Practising important numericals will help you understand better. This also enables you to solve faster.