Atomic Structure

(a) Carbon - ¹²₆C

(b) Oxygen - ¹⁶₈O

(c) Phosphorus - ³¹₁₅P

(d) Argon - ⁴⁰₁₈Ar

(e) Calcium - ⁴⁰₂₀Ca

Formation of compounds from atoms by electron transfer

• Electronic configuration of Na = 2,8,1

• Nearest noble gas = Neon [2,8]

• Na loses one electron from outermost shell to attain stability

• Electronic configuration of Cl = 2,8,7

• Nearest noble gas = Argon [2,8,8]

• Cl gains one electron in outermost shell to attain stability.

Hence, the electron given by Na is gained by Cl, both attain a stable configuration and sodium chloride is formed.

1. An element 'X' has six electrons in its outer or valence shell. Its valency is -2.

2. An element 'Y' has electronic configuration 2, 8, 6. The element 'Y' is a non-metal.

3. A neutron is a sub-atomic particle with no charge and unit mass.

4. An element Z with zero valency is a noble gas.

5. Magnesium atom with electronic configuration 2, 8, 2 achieves stable electronic configuration by losing two electrons, thereby achieving stable electronic configuration of the nearest noble gas neon.

1. Isotopes are atoms of the same element.

2. Isotopes have same atomic number.

3. Number of electrons in an isotope of 'H' having mass no. = 2 is 1 as its atomic number is 1.

4. Number of neutrons in an isotope of 'H' having mass no. [A] = 3 is 2, because, atomic number [Z] is 1 so number of protons = number of electrons = 1 and number of neutrons [n] = A - Z = 3 - 1 = 2.

5. Number of electrons and protons are same in the 3 isotopes of 'H' as atomic number is the same.

1. Lithium [At. no. = 3, Mass no. = 7]

2. Carbon [At. no. = 6, Mass no. = 12]

3. Silicon [At. no. 14, Mass no. = 28]

4. Sodium [At. no. = 11, Mass no. = 23]

5. Isotopes of hydrogen [₁¹H, ₁²H, ₁³H]

William Crookes Experiment:

William Crookes passed an electric discharge through a discharge tube containing a gas at low pressure. Blue rays were emitted from the cathode [negative plate] which were called cathode rays.

J.J. Thompson's Experiment:

In his experiments, J.J. Thomson studied the characteristics and constituents of cathode rays and observed that the rays have kinetic energy & cast a shadow of an object in their path. They were deflected by a positively charged plate.

Discovery of electron:

This proved that the cathode rays consisted of negatively charged sub-atomic particles called electrons.

Goldstein's Experiment:

Goldstein used a modified cathode ray tube with a perforated cathode and observed new type of rays produced from the anode passing through the hole of the perforated cathode. These rays were called - Anode rays.

The positive rays were affected by electric & magnetic fields but in a direction opposite to that of cathode rays.

Goldstein concluded that anode rays or positive rays consist of positively charged particles called protons.

Lord Rutherford's Experiment:

Lord Rutherford projected alpha particles towards a thin gold foil in the path of the rays.

He observed that most of the alpha particles went straight through the foil but some were deflected slightly & some by large angles.

He concluded that an atom on the whole is relatively empty but consists of a concentrated positive mass in the centre which lead to the deflection of the alpha particles. Hence, Lord Rutherford discovered atomic nucleus.

The energy levels 1,2,3... are represented by integer 'n' or as K, L, M, N...

Below labelled diagram shows the representation of an atom as per the modern atomic theory:

The three sub-atomic particles in the atom are:

Protons and neutrons are found inside the nucleus of the atom. Electrons are distributed across energy levels outside of the nucleus.

Atomic number of an atom is equal to number of protons in the nucleus of an atom. As number of electrons = number of protons hence, atomic number is also equal to the number of electrons in an element.

If atomic number [Z] is 11, the number of electrons = number of protons = 11.

Mass number is equal to the total number of protons & neutrons in the atom of an element.

If,
mass number [A] = 35,
atomic number [Z] = 17 then
number of protons = number of electrons = 17
number of neutrons [n] = A - Z = 35 - 17 = 18 neutrons

As atomic weight is the number of times, one atom of an element is heavier than the mass of an atom of hydrogen or 1⁄12 the mass of an atom of carbon, hence, it is also termed as Relative Atomic Mass.

Electrons revolve around the nucleus in imaginary paths called orbits or shells. Different Orbits are K shell [first shell, n=1], L shell [second shell, n=2] M, N...... etc.

Rules governing the distribution of electrons are summarized below:

1. The maximum number of electrons in each shell or orbit is determined by the formula 2n², where n is the number of shell.
   K shell, n=1, no. of electrons = 2 x 1² = 2
   L shell, n=2, no. of electrons = 2 x 2² = 8
   M shell, n=3, no. of electrons = 2 x 3² = 18
2. Electrons are not accommodated in a given shell, unless the inner shells are filled. Shells are filled in step-wise manner.
3. The atom's outermost shell needs 2 electrons for stability (duplet rule) if it has only one shell. If it has multiple shells, it requires 8 electrons (octet rule) in the outermost shell for stability.

(i) As atomic number [Z] is 19 so electronic configuration = [2,8,8,1]

(ii) Number of valence electrons = 1

Postulates of Dalton's atomic theory : Matter consists of particles called - atoms, which are indivisible & cannot be created or destroyed.

1. Atoms of the same element are alike but differ from atoms of all other elements.
2. Compounds or molecules are formed when atoms combine with other atoms in simple whole number ratios.
3. An atom is the smallest unit of matter which takes part in a chemical reaction.

Contradiction :

In hydrogen chloride, one atom of hydrogen combines with one atom of chlorine so valency of chlorine in hydrogen chloride is 1.

(a) Stable electronic configuration — The atom's outermost shell needs 2 electrons for stability (duplet rule) if it has only one shell. If it has multiple shells, it requires 8 electrons (octet rule) in the outermost shell for stability.

(b) (i) The atoms of sodium, magnesium and aluminium donate 1, 2 and 3 electrons, respectively, in order to achieve a stable electronic configuration of 8 electrons (octet rule) in the outermost shell. Hence, their valencies are +1, +2 & +3.

(ii) The atoms of chlorine, oxygen and nitrogen accept 1, 2 and 3 electrons, respectively, in order to achieve a stable electronic configuration of 8 electrons (octet rule) in the outermost shell. Hence, their valencies are -1, -2 & -3.