Answer from question # 13.

Three rules govern the pairing of electrons in the orbital of an atom in its ground state.
(i)Aufbau's principle which states that, "The electrons in an atom are so arranged that they occupy orbitals in the order of their increasing energies."

(ii) Hund's rule which states that, "When more than one orbital of equal energies are present, then the electrons first occupy 

these orbitals separately with parallel spins. The pairing of electrons will only be after all the orbitals of a given sub-energy level are singly occupied."

(iii) Pauli's exclusion principle which states that, “No two electrons in an atom can have the same values for all the four quantum numbers”

(b) Calculate the energy associated with an electron moving in an orbital of energy level n=2.
Given
energy level,n=2
required, Energy associated with the orbital in this energy level say E₂.
solution.
From the transition equation generally;
E₂=_ 13.6eV
            n²
⇒ _ 13.6 × 1.6×E-19J
               2²
E₂=-5.44E−19J.
.: The energy associated with the orbital in this energy level, n=2 is -5.44E-19J.

(c)
(i) An isotope is one of the two or more species of atoms of a chemical element with the same atomic number and position in the periodic table and nearly identical chemical behaviour but differ in atomic masses and physical properties.

(ii) The conventional symbols for the two isotopes of X.
79                     81
35X and  35X


(iii)
Given.
Abundances, X₁=50.5%,X₂=49.5%
Atomic masses, M₁=79, M₂=81.
Required; the relative atomic mass of X, R.A.M
solution
Since,
R.A.M=(M₁•X₁)+(M₂•X₂)
                       100%
⇒(79×50.5%)+(81×49.5%)
                     100%
R.A.M=79.99.
.: The relative atomic mass of X is 79.99.

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