Week 5/Chapter 6 - Electron structure of atoms

Question 1

Planck’s constant/energy of a quantum (E)

Answer 1

Planck determined than the energy of a quantum (E) is the amount that can be emitted (or absorbed) in the form of electromagnetic radiation where E = h v where h is Planck’s constant (6.63 x 10-34 Js)

Question 2

Bohrs model of the atom

Answer 2

Each shell is associated with a particular energy level. Shells further away from the nucleus have higher potential energy and therefore it is possible for electrons to be lost from the outermost shell to form an ion.

Question 3

Bohr Theory

Answer 3

The Bohr model of the atom proposes that energy levels of electrons are discrete and that electrons revolve in stable orbits around the atomic nucleus but that they can jump from one orbital (or energy level) to another. He also suggested that electrons could be described both as particles and as a wave.

His theory has 5 components:

Electrons in atoms occupy discrete orbitals
Energy is not radiated continually
Absorption of fixed quantities of energy occurs
Emission of fixed quantities of energy occurs
Electrons have spin (more about this later).

Question 4

Quantum numbers

Answer 4

The solution to Schrodinger’s equation provides us with three integer values which describe the spatial distribution of each electron in each atom; you can think of them a bit like your postcode. These are known as quantum numbers. The fourth number we use to describe electrons is used to determine how electrons of the same charge can occupy similar areas in three-dimensional space.

Quantum Numbers
1 Principal Quantum Number - n

2 Angular Momentum Quantum Number - l

3 Magnetic Quantum Number - ml

4Spin Quantum Number - ms

Question 5

Principal quantum number

Answer 5

The Principal quantum number (n = 1, 2, 3…), corresponds to the energy levels of an electron within an atom. It is an integer value.

The n = 1 shell can hold up to 2 electrons

The n = 2 shell can hold up to 8 electrons

The n = 3 shell can hold up to 18 electrons

The periods (rows) of the Periodic Table represent the Principal Quantum numbers; row 1 (hydrogen and helium) have the Principal Quantum number n = 1 and those in row 2 (lithium to neon) have n = 2.

Question 6

Shell structure

Answer 6

The shell structure is qualitatively the same for all atoms. As the value of n increases the distance of the shell from the nucleus also increases. An increase in the value of n also increases the potential energy of electrons in the shell.

Energy changes between shells:

ΔE(2-1) &raquo_space; ΔE(3-2) > ΔE(4-3)

Question 7

Degenerate Orbitals

Answer 7

For hydrogen atoms, the energies of all orbitals in the same shell are the same; known as degenerate:

1s &laquo_space; (2s = 2p) < (3s = 3p) < (4s = 4p = 4d) etc.

This however only works for one-electron systems (hydrogen) as it does not take account of inter-electron interactions which occur in all other element atoms.

Question 8

The Aufbau principle

Answer 8

Electrons will fill orbitals with the lowest energy first, but how can we remember what that filling order is? The Aufbau principle allows us to determine which electrons are filled in the atomic orbitals stating that electrons are filled into atomic orbitals from the lowest energy first (ground state) and the available atomic orbitals with the lowest energy levels are occupied before those with higher energy levels.

Question 9

Shielding and Penetration: s electrons

Answer 9

s electrons penetrate more deeply into the nucleus than do p, d or f electrons
s electrons experience higher attraction to the nucleus.
The potential energy of s electrons is lower than p, d or f electrons
The distribution of electron density in each s, p, d and f orbital has a different shape and orientation determined by the particular values of l and ml.

Question 10

What does the symbol Ψ (capital psi) mean

Answer 10

energy and wavefunction

Question 11

Pauli Exclusion principle

Answer 11

Within one particular atom no two electrons can have exactly the same set of four quantum numbers (Pauli Exclusion Principle). Thus the four quantum numbers: n, l , ml and ms allow us to uniquely identify any electron in an atom.

Question 12

systematically placing electrons in orbitals guidelines

Answer 12

1
No more than two electrons can occupy one orbital.

2
Electrons occupy the lowest energy orbitals available. They enter a higher energy orbital only when the lower orbitals are filled. For the atoms beyond hydrogen, orbital energies vary as s < p < d < f for a given value of n.

3
Each orbital in a sublevel is occupied by a single electron before a second electron enters. For example, all three p orbitals must contain one electron before a second electron enters a p orbital (Hund’s Rule).

Question 13

Hunds rule

Answer 13

Within a sub-shell, electrons do not pair up unless all orbitals in that subshell already contain one electron.

                                                    OR  

The most stable arrangement of electrons in the sub-shells is the one which maximises the number of parallel spins.

Sometimes known as the Bus Seat Rule - generally people boarding a bus will choose an empty seat rather than sitting next to someone already seated on the bus!

Question 14

What are valence electrons

Answer 14

The electrons in the outermost energy level of an atom are called the valence electrons.

For example, oxygen, which has the electron configuration of 1s22s22p4, has electrons in the first and second energy levels. Therefore, the second principal energy level is the valence level of oxygen and therefore both the 2s and 2p electrons are the valence electrons; oxygen has 6 valence electrons.