Atomic Structure: Electron, Proton, Neutron etc.

Atomic Structure: Electron, Proton, Neutron etc.

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, Ss etc.

Mole

A mole is a collection of 6.023 x 1023 particles.

It means that

• 1 mole = 6.023 x 1023

• 1 mole atom = 6.023 x 10 atoms

• 1 mole molecule = 6.023 x 10²³23 molecules

• 1 mole ion = 6.023 x 1023 ions

• 1 mole mango = 6.023 x 1023 mangoes

• 1 mole apple = 6.023 x 1023 apples

Avogadro’s number

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

Atomic Mass

It is the ratio of the mass of one atom of the element to the part of the mass of one atom of carbon-12.

Actual mass of 1 atom of an element = atomic mass in amu x 1.66 x 10⁻²⁴ g

It indicates how many times one molecule of a substance is heavier in comparison to the mass of one atom of carbon-12.

Constituents of an atom:

• Fundamental particles of an atom are electron, proton & neutron.

Electron:

(i) The electron had been discovered by J.J. Thomson.

(ii) The name 'electron' was given by Stoney.

(iii) Charge and Mass of Electron –

(iv) Charge / Mass (e/m)

ratio of electron = – 1.79 x 10^8 c/g

(v) An electron was obtained from the cathode ray experiment.

Proton:

(i) A proton had been discovered by Goldstein.

(ii) A proton was named by Rutherford.

(iii) Charge and mass of proton –

(iv) 

 ratio for proton = 9.58 x 10⁴ 

(v) A proton was obtained from the anode rays experiment.

Neutron:

(i) A neutron had been discovered by James Chadwick.

(ii) Charge and mass of Neutron –

(iii)  ratio of neutron = Zero. A neutron was obtained from the radioactivity phenomenon.

(iv) A neutron was obtained from the radioactivity phenomenon.

Atomic number (Z): The number of protons or electrons in an atom of the element is called the atomic number. It is denoted by Z.

Mass Number (A): The sum of the number of protons and neutrons in an atom of the element is called the mass number. It is denoted by A.

A = p + n, where p = no. of proton and n = no. of eutrons. Let,  Na,

In Na, Z = 11, A = 23, and
e = 11, p = 11,
n = A – p = 23 – 11 = 12

Isotopes: These are atoms of the elements having the same atomic number but different mass numbers.

Isobars: These are atoms of the elements having the same mass number but different atomic numbers. e.g.

Isoelectronic: These are atoms / molecules / ions containing the same number of electrons.
(i) O^2–, F^–, Ne, Na⁺, Mg²⁺       (ii) CN^{–, N2,} etc.

Thomson’s model of an atom:

• According to Thomson, an atom is treated as a sphere of radius 10^–8 cm in which positively charged particles are uniformly distributed and negatively charged electrons are embedded through them.

• This is also called the plum-pudding model of an atom or the watermelon model of an atom.

Rutherford’s model of an atom:

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

According to this model:

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

(ii) Radius of a nucleus = 10^–13 cm

     Radius of an atom = 10^–8 cm

The radius of an atom is 10⁵ times the radius of the nucleons.

So, the volume of an atom is 10¹⁵ times heavier than volume of a nucleus.

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

Nature of light & electromagnetic spectrum:

In 1856 James Clerk Maxwell stated that light, X-rays, gamma rays, heat, etc., emit energy continuously in the form of radiation 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).

1. Wavelength (λ):

The distance between two consecutive crests or troughs is called wavelength. It is denoted λ (lambda).

The SI unit of λ is metre (m), and the CGS unit is centimetre (cm).

1 A^o  = 10⁻¹⁰ m, 1 u (micron) = 10⁻⁶ m,

^{1nm =} 10^-9 m, 1pm = 10^-12 m,

2. Frequency (v):

The number of waves passing through a point in one second is called frequency. It is denoted by v (nu). The unit of frequency is cycle / second or sec⁻¹ or Hertz (Hz)

1 Hz = 1 cycle per second

3. Wave number (v^-):

The number of wavelengths which can be accommodated in one centimetre length along the direction of propagation is called the wave number. It is denoted by v^-. The SI unit of v is m⁻¹ and the CGS unit is cm⁻¹..

Wave number (v^-) = 1 / wavelength (λ)

4. The three relations

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

c = v λ                 Where, c 3 x 10⁸ m/sec

or, c = 3 x cm/sec

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

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

Planck's quantum theory of radiation:

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 known as photons. The energy of each quantum is directly proportional to the frequency of radiation.

i.e., ????∝???? 

or, E = hv = hc/λ - hcv^-

Where h is called Planck's constant. Its value is 6.626 x 10⁻³⁴ Js, and c is the velocity of light (c = 3 x 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 the spectrum.

Zeeman's effect:

• When spectral lines obtained from atomic spectra are placed in a magnetic field, they are split into a number of fine lines; this is called Zeeman's effect.

Stark's effect:

• When spectral lines obtained from atomic spectra are placed in an electric field, they are split into a number of fine lines; this is called Stark's effect.

Thomson's model: Plum pudding model (watermelon model)

Rutherford's model: Nuclear theory

Bohr's model: Concept of quantisation of energy.

Planck's quantum theory: photons & quanta.

Sommerfeld's model: Orbital-elliptical & spherical

de-Broglie's equation: Dual nature of electron

Heisenberg's Uncertainty principle: Exact position & momentum cannot be determined simultaneously

Schrödinger's wave equation: wave nature of electron.

Photoelectric Effect

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

Quantum Number

The set of four integers required to define an electron completely in an atom are called quantum numbers.

1. Principal quantum number (n): It describes the shape.

size and energy of the shell to which the electron belongs. It is denoted by n, where n = 1, 2, 3, 4 ... 8. Value of n: 1 2 3 4 5 6 7 Designation of Shell K L M N O P Q

2 Azimuthal quantum number (1) It determines the shape of the electron cloud and the number of subshells in a shell. The value of 'l' lies between 0 and n - 1, i.e., t = 0 to -1, the value of the T subshell.

3 P f The magnetic quantum number (m) / f * t determines the orientation of subshells. The value of m can range from - to +, including zero. i.e., m = 2l + 1, Spin quantum number(s): It represents the direction of electron spin around its own axis. It has S = 1/2 for the clockwise direction and S = -1/2 for the anticlockwise direction.

Pauli's Exclusion Principle

According to this principle, all four quantum numbers for any two electrons in an atom cannot be identical.

Hund's Rule

This rule states that the filling of electrons in orbitals first takes place singly; after that, the pairing of electrons takes place. For example, AA, AAA, PAVAA, PAVNA

Aufbau Principle

'Aufbau' is a German word meaning 'building up'. This principle states that electrons are filled in various orbitals in order of their increasing energies. An orbital of lowest energy is filled first. The sequence of orbitals in order of their increasing energy is. Is < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 6s < 4f < 5d < 6p < 7s < 3s

s-orbital 1 p-orbital = 3 d-orbitals, 5 f-orbitals, 7

The maximum number of electrons in a particular orbital is two. When two electrons are present in the same orbitals. Their spin is neutralised, i.e., the distribution of electrons in different shells is called electronic configuration.

Electronic configurations of an atom

4. Periodic classification of Elements

• Father of the periodic table, Mendeleev.

• The arrangement of the known elements in certain groups in such a way so that the elements with similar properties are grouped together is known as classification of elements