Atomic Structure

Question 1

What is the degeneracy of the p; d; f orbitals

Answer 1

p orbital = 3 fold

d orbital = 5 fold

f orbital = 7 fold

Question 2

What are the reasons for the energy ordering

Answer 2

1. Repulsions among electrons
2. Relative amplitudes of the radial portions of the atomic orbital wave functions - compare the extent to which the radial portions of the various wave functions penetrate to the nucleus

Question 3

Explain Radial penetration of the wave functions

Answer 3

An electron in an atomic orbital with electron density close to the nucleus is stabilized by close interaction with the positive charge of the nucleus.

Question 4

Explain the stability ordering

Answer 4

• For any n: s-orbital most electron density close to the
  nucleus, then p, d, f.
• Order of stability: s > p > d > f

Question 5

Aufbau principle

Answer 5

• build up from the bottom
• orbitals of lowest energy first

Question 6

Hund’s rule

Answer 6

• Where orbitals are available in degenerate sets, maximum spin multiplicity preserved (electrons are not paired until each orbital in a degenerate set is half-filled)

Question 7

Pauli exclusion principle

Answer 7

• No two electrons may have the
  same set of 4 quantum numbers
• Two electrons in the same orbital
  must have opposite signs

ms = +½ and ms = −½

• maximum 2 electrons per orbital

Question 8

What is the orbital order

Answer 8

1s 2s. 2p. 3s. 3p. 4s. 3d 4p. 5s. 4d. 5p. 6s. 4f. 5d. 6p. 7s.

Question 9

What electron configuration does period 3 start with

Answer 9

[ Ne ] 3s^1

Question 10

Explain the electron configuration of each period

Answer 10

• Period 4 started with [ Ar ] 4s, then 3d and ended with 4p
• Period 5 starts with 5s, then 4d and ends with 5p.

Question 11

What is the electron configuration for 24Cr

Answer 11

[Ar] 4s13d5 (not [Ar] 4s23d4)

Question 12

What is the electron configuration for 29Cu

Answer 12

Ar] 4s13d10 (not [Ar] 4s23d9)

Question 13

What is the electron configuration for 42Mo:

Answer 13

[Kr] 5s14d5 (not [Kr] 5s24d4)

Question 14

What is the electron configuration for 47Ag:

Answer 14

[Kr] 5s14d10 (not [Kr] 5s24d9)

Question 15

What is the electron configuration for 41Nb

Answer 15

[Kr] 5s14d4 (not [Kr] 5s24d3)

Question 16

What is the electron configuration for 44Ru

Answer 16

[Kr] 5s14d7 (not [Kr] 5s24d6)

Question 17

What is the electron configuration for 45Rh

Answer 17

[Kr] 5s14d8 (not [Kr] 5s24d9)

Question 18

What is the electron configuration for 46Pd

Answer 18

[Kr] 5s04d10 (not [Kr] 5s24d8)

Question 19

What is the electron configuration for 78Pt

Answer 19

[Xe] 6s15d9 (not [Xe] 6s25d8)

Question 20

What is the electron configuration for 79Au

Answer 20

[Xe] 6s15d10 ( not [Xe] 6s25d9)

Question 21

What is the electron configuration for 64Gd

Answer 21

[Xe] 6s2 5d1 4f7 (not [Xe] 6s25d04f8)

Question 22

Explain Mendeleev

Answer 22

• Arranged in horizontal rows
• Related elements in vertical columns

Question 23

Explain Moseley

Answer 23

• Proper sequence: atomic number
• Vertical columns contain chemically similar elements and electronically similar atoms
• Periodic arrangement of electron configurations give the same result as what Mendeleev deduced from chemical observations.

Question 24

Explain Hund’s rule fully

Answer 24

1. Electrons make use of different orbitals as far as possible
   (within a set of degenerate orbitals)

A consequence of the charge of the electrons – electrons minimise repulsive forces by occupying different p orbitals

2. Parallel spins (same value of ms) are used until the
   Pauli exclusion principle requires pairing of spins
   Early pairing of spins leads to less stable arrangements

Question 25

Explain Most stable electronic state

Answer 25

The state with maximum spin multiplicity (largest number of (un)paired electron spins)

Question 26

What does Hund's rule imply

Answer 26

1. Spreading out of electrons as wide as possible in the space surrounding the nucleus 2. This spreading of electron density leads to extra stability

Question 27

What does the amount of nuclear effective charge felt by an electron depends on

Answer 27

1. The type of orbital in which the electron is housed 2. The ability of other electrons in more penetrating orbitals to screen (or shield) the electron from the nucleus

Question 28

When will an electron experience the greatest effective nuclear charge

Answer 28

- An electron will experience the greatest effective nuclear charge when housed in an s orbital (for any given n), than p, etc. (Reason for Aufbau) - Geometry and orientation of the p orbitals makes them poor at shielding one another from the nucleus.

Question 29

Explain poor electron shielding

Answer 29

Where screening of an electron is poor, the effective nuclear charge is high ( In terms of Hund’s rule: Electrons spread out into a degenerate set of orbitals in order to experience the maximum effective nuclear charge )

Question 30

Explain Slater’s screening constant, σ

Answer 30

- Slater proposed an empirical constant that represents the cumulative extent to which the other electrons of an atom shield (or screen) any particular electron from the nuclear charge - Z* is always smaller than Z (the electron is screened or shielded from Z by an amount σ). - The value of σ for the electron in question is the cumulative total provided by the other electrons of the atom– calculated using Slater’s rules.

Question 31

To conclude

Answer 31

Z* increase continually from left to right across the rows of the periodic table because of imperfect shielding