Atomic Structure

In 1803 to 1808 atomic theory had been given by john Dalton. The main points of Dalton’s atomic theory are –

1. Elements consist of minute, indivisible, indestructible particles called atoms.

2. Atoms of an element are identical to each other. They have same mass and size.

3. Atoms of different elements differ in properties and have different mass and sizes.

4. Atoms can not be created, destroyed or transformed into atoms of other elements.

Atom: The smallest particle of an element is called an atom. An atom can take part in chemical combination and does not occur free in nature. The atom of the hydrogen is the smallest and lightest. Example-Na, K, Ca, H etc.

Molecule : A molecule is the smallest particle of an element or compound that can have a stable and independent existence. Example-O₂, N₂, Cl₂, P₄, S₈ etc.

Mole : A mole is a collection of 6.023 × 10²³ particles. It means that

1 mole = 6.023 × 10²³ (just like 1 pair = 2)

1 mole atom = 6.023 × 10²³ atoms

1 mole molecule = 6.023 × 10²³ molecules

Avogadro’s Number : The number 6.023 x 1023 is called Avogadro’s Number.

If there is one million Mg²⁺ ions in MgCl₂ then number of Cl^– ions will be two millions.

Atomic Mass : The atomic mass of an element is a number which states that how many times the mass of one atom of an element is heavier than 1/12th mass of one atom of carbon-12.

Atomic mass of an element = mass of one atom of the element / 1/12 × mass of one atom of carbon-12

Actual mass of 1 atom of an element = atomic mass in amu × 1.66 × 10^-24 g

Molecular mass: The molecular mass of a substance is a number which states that how many times mass one 1/12 molecule of a substance is heavier in comparison to th mass of one atom of Carbon-12.

Constituents of an atom : Fundamental particles of an atom are Electron, Proton & Neutron.

1. Electron had been discovered by J.J. Thomson.

2. The name of electron was given by Stoney.

3. The relative charge on electron is-1 unit and its absolute charge is -1.6 × 10^-19 coulomb or – 4.8 × 10^-10 (electrostatic unit) e.s.u.

4. The relative mass of an electron is 0.000543 amu and its absolute mass is 9.1 × 10^-28 g or 9.1 × 10^-31 kg.

5. The charge/mass (e/m) ratio of an electronis-1.76 × 10⁸ Coulomb / gram

6. An electron was obtained from Cathode rays experiments.

1. A proton had been discovered by Goldstein

2. A proton was named by Rutherford.

3. The relative charge on proton is +1 unit and its absolute charge is +1.6 × 10^-19 Coulomb or + 4.8 × 10^-10 e.s.u.

4. The relative mass of proton is 1.00763 amu and its  absolute mass is 1.673 × 10^-24 gram or 1.673 × 10^-27 kg.

5. The charge/mass (e/m) ratio for a proton is = 9.58 × 10⁴ Coulomb / gram

6. An proton was obtained from anode rays experiment.

1. A neutron had been discovered by James Chadwick.

2. Charge on neutron is zero

3. The relative mass of neutron is 1.00863 amu and its absolute mass is 1.675 × 10^-24 gram or 1.675 × 10^-27 kg.

4. The charge/mass for a neutron is zero.

5. Aneutron was obtained from radioactivity phenomenon.

Isoelectronic These are atoms/molecules/ions containing the same number of electrons.

1.    O^2-, F^–, Ne, Na⁺, Mg²⁺        2 . CN^–, N₂, O₂²⁺ etc.

When a helium atom loses two electrons it becomes a positively charged helium ion He – 2e → He⁺⁺

This doubly positively charged He⁺⁺ is called alpha (a) particle.

Thomson’s model of an atom According to J.J. Thomson, an atom is treated as sphere of radius 108 cm in which positively charged particles are uninformally distributed and negatively charged electrons are embedded through them. This is also called Plum-Pudding model of an atom or water-melon model of an atom.

Rutherford’s model of an atom: On the basis of scattering experiment, Rutherford proposed a model of the atom which is known as nuclear atomic model.

1. An atom consists of a heavy positively charged nucleus where all protons and neutrons are present. Protons & neutrons are collectively called nucleons. Almost whole mass of the atom is contributed by these nucleons.

2. Radius of a nucleus = 10^-13 cm

Radius of Radius of an atom = 10^-8 cm

Radius of an atom = 10⁵ times of the radius of the nucleons.

3. Volume of an atom is 1015 times heavier than volume of a nucleus.

4. Electrons revolve around the nucleus in closed orbits with high speed. This model is similar to the solar system, the nucleus representing the sun and revolving electrons as planets. The electrons are therefore, generally referred as planetary electrons.

Rutherford’s a-scattering experiment was responsible for the discovery of proton. i.e., positively charged nucleur at the centre of atom.

Nature of light & Electromagnetic Spectrum : In 1856 James Clark Maxwell stated that light, X-ray, y-rays and heat etc emit energy continuously in the form of radiations or waves and the energy is called radiant energy. These waves are associated with electric as well as magnetic fields and are therefore known as electromagnetic waves (or radiations).

4. The relation between velocity of light (c), frequency (v) and wavelength (λ) is

c = vλ          Where, c = 3 × 10⁸ m/sec

or, c= 3 × 10¹⁰ cm/sec

Different types of electromagnetic waves (or radiation) differ with respect to wavelength or frequency. The wavelength of electromagnetic spectrum increases in the following order.

Cosmic rays <y-rays < X-rays < Ultraviolet rays < Visible < Infrared < Microwaves < Radiowaves.

Planck’s quantum theory of Radiations : In 1900 Max Planck put forward a theory which is known as Planck’s quantum theory. According to this theory radiant energy is emitted or absorbed in the form of small energy packets, called quanta. In case of light, these energy packets are known as photons. The energy of each quantum is directly proportional to the frequency of radiation.

i.e., Ε ∝ ν

or, E = hv = hc/λ = hcv

Where h is called Planck’s constant. Its value is 6.626 × 10^-34 Js and c is the velocity of light (c = 3 × 10⁸ m/s).

Spectrum: When white light is allowed to pass through a prism, it splits into seven colours. The seven coloured band is called spectrum.

Zeeman’s effect: When spectral lines obtained from atomic spectra are placed in a magnetic field, they are splitted into number of fine lines, this is called Zeeman’s effect.

Stark’s effect: When spectral lines obtained from atomic spectra is placed in electric field, they are splitted into number of fine lines this is called Stark’s effect.

When radiations with certain minimum frequency strike the surface of a metal, the electrons are ejected from the surface of the metal. This phenomenon is called photoelectric effect. The electrons emitted are called photoelectrons.

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