AS-Level Chemistry Chapter 3 Electrons in Atoms
Sub-shells and Atomic Orbitals
Principle quantum shell
1) Electrons are arranged outside the nucleus in energy levels or principle quantum shell, n.
2) The principal quantum shells are numbered according how far are they from the nucleus.
3) The lowest energy level, n = 1 is closest to the nucleus, the energy level n = 2 is further out, and so on.
4) The electrons in energy level further away from the nucleus have more energy and held less tightly by the nucleus.
5) Electrons do not move in fixed circular paths, they occupy a space called the atomic orbitals.
6) The total number of electrons that can occupy any principal shell is 2n², where n is the principal quantum number.
Quantum sub-shells
1) The principal quantum shells, apart from the first, are split into sub-shells. Each principle quantum shell contains a different number of sub-shells.
2) The first energy level contains one sub-shell, the second energy level contains two and so on.
3) The sub-shells are distinguished by letters s, p, d, f and so on.
4) The energy of electrons in the sub-shells increases in the order s < p < d < f.
The impossibility of drawing orbits for electrons
1) Heisenberg Uncertainty Principle says, you cannot know with certainty where an electron is and where it is going next.
2) This makes it impossible to draw out an orbit or pathway in which the electrons move.
Atomic orbitals
1) An atomic orbital is a region of space around the nucleus where the probability of finding a particular electron is maximum (>95%).
2) The sub-shells are split further into orbitals where the electrons are placed
3) The number of orbital in each sub-shell depends on the sub-shells.
s - one orbital {s}
p - three orbitals {px, py, pz}
d - five orbitals {dxy, dyz, dxz, dx²-y², dz²}
4) Orbitals having the same energy are called degenerate orbitals. For example, px, py and pz are degenerate orbitals.
5) The concept of orbitals arises from the fact that an electron has dual nature. It behaves as a particle as well as a wave.
6) In the nth principal quantum shell, there are n sub-shells, n² orbitals and a maximum of 2n² electrons.
The s orbital
1) All the s orbitals are spherical and non-directional.
2) The shaded region represents the region in which the chance of finding the s electron is more than 95%.
3) The size of the s orbital increases in the order 1s < 2s < 3s < 4s.
The p orbital
1) All the p orbitals are dumb-bell shaped and directional.
2) p orbitals are only available from the second principal quantum shell and onwards.
3) There are 3 types of p orbitals, px, py and pz. All 3 different types of p orbitals are perpendicular to each other along the x, y and z axes.
4) Going to a higher energy level, the 'lobes' of the p orbital become longer.
Orbitals and the Periodic Table
1) The elements in the Periodic Table can be divided into four blocks according to their valence shell electronic configuration.
2) The s-block elements have their valence electron(s) in the s orbital.
3) The p-block elements have their valence electrons in the p orbital.
4) The d-block elements have d orbitals filling.
5) The f-block elements have f orbitals filling.
Some useful facts
1) The valence electrons always appear at the end of the electronic configuration.
2) The valence electrons are large responsible for the chemical properties of an element.
3) The number of valence electron will indicate the group number of that element in the Periodic Table.
4) The outermost quantum shell number will indicate the period of that element in the Periodic Table.
Ionization Energy
What is ionization energy?
1) The 1st ionization energy, ΔHi1 is the energy needed to remove one electron from each atom in one mole of the atoms of the element in the gaseous state to form one mole of gaseous 1+ ions.
2) The general unit for ionization energy is kJ mol⁻¹.
Ca(g) → Ca⁺(g) + e⁻ ; ΔHi1 = +590 kJ mol⁻¹
3) If a second electron is removed from the gaseous 1+ ions, it is the 2nd ionization energy, ΔHi2.
Ca⁺(g) → Ca²⁺(g) + e⁻ ; ΔHi2 = +1150 kJ mol⁻¹
4) The 2nd ionization energy, ΔHi2 is the energy needed to remove one electron from each gaseous 1+ ion in one mole of the ions to form one mole of gaseous + ion.
5) The continues removal of electrons until the nucleus is left only will result in successive ionization energies.
Factors affecting the ionization energy
1) Charge on the nucleus (Number of proton)
- The greater the number of proton in the nucleus, the greater the amount of positive charge.
- The greater the positive charge, the greater the attractive force between the nucleus and outer electrons.
- More energy is needed to overcome the attractive force. So, the ionization energy is higher.
- The greater the nuclear charge, the higher the ionization energy.
2) Distance between nucleus and outer electrons (Size of atom/ion)
- The larger the size of the atom, the greater the distance between the nucleus and the outer electrons.
- The greater the distance between the nucleus and the outer electrons, the weaker the attractive force between nucleus and outer electrons.
- Furthermore, the outer electrons experience greater shielding effect by the inner electrons.
- Less energy is required to overcome the attractive force. So, the ionization energy is lower.
- The greater the distance between nucleus and outer electrons, the lower the ionization energy.
3) Shielding effect by the inner electrons
- All electrons are negatively-charged, so they repel each other. Electrons in full inner shells will repel the outer electrons and so prevent the full nuclear charge being felt by the outer electrons. This is called shielding or screening.
- The greater the shielding effect, the weaker the attractive force between the nucleus and outer electrons.
- Less energy is required to overcome the attractive force. So, the ionization energy of lower.
- The greater the shielding effect, the lower the ionization energy.
Pattern of ionization down a Group
1) The first ionization energy decreases down a Group.
2) This is because the atomic size increases and hence the distance between the nucleus and outer electrons increases. The outer electrons also experience a greater shielding effect.
3) These two factors outweigh the increasing nuclear charge.
4) The above factors causes the attractive force between nucleus and outer electrons to decrease, less energy is required to overcome the weaker attractive force. Hence, the ionization energy is lower.
Pattern of ionization energy across a Period
1) The general trend of ionization energy across a Period is increasing.
2) This is because, across a Period, the number of proton in the nucleus increases by one therefore the nuclear charge increases.
3) However, the distance between the nucleus and outer electrons decreases across a Period and the outer electrons experience the same amount of shielding.
4) The above factors causes the attractive force between nucleus and outer electrons to increase, more energy is required to overcome the stronger attractive force. Hence, the ionization energy is higher.
The drop between (Be-B) and (Mg-Al)
1) There is a slight decrease in first ionization energy between beryllium-born and magnesium-aluminium.
2) This is because the fifth electron in boron is located in the 2p sub-shell, which is slightly further away from the nucleus. The outer electron in boron is shielded by the 1s² as well as 2s² electrons.
Be: 1s²2s² B : 1s²2s²2p¹
3) The decrease in first ionization energy between magnesium and aluminium has the same reason, except that everything is happening at the third energy level.
Mg : 1s²2s²2p⁶3s² Al : 1s²2s²2p⁶3s²3p¹
The drop between (N-O) and (P-S)
1) There is a slight decrease in first ionization energy between nitrogen-oxygen and phosphorus-sulphur.
2) This is because the electron being removed in oxygen is from the orbital which contains a pair of electrons. The extra repulsion between the pair of electrons results in less energy needed to remove the electron. This is called spin-pair repulsion.
N: 1s²2s²2px¹2py¹2pz¹ O : 1s²2s²2px²2py¹2pz¹
3) The decrease in first ionization energy between phosphorus and Sulphur has the same reason, except that everything is happening at the third energy level.
P: 1s²2s²2p⁶3s²3px¹3py¹3pz¹ S : 1s²2s²2p⁶3s²3px²3py¹3pz¹
Successive ionization energy
1) The following data can be obtained from successive ionization energy:
i. Total number of electrons in an atom.
- is equal to the number of separate ionization energies possessed by the atom.
ii. Number of principal quantum shells occupied and the number of electrons in each.
-by looking the big difference between two successive ionization energies.
iii. Number of sub-shells occupied and the number of electrons in each.
2) Successive ionization energies get larger because removing an electron from a positive ion with increasing positive charge is going to be more difficult due to the increasing attractive force.
3) There is a relatively big increase in ionization energy between the first and second electron being removed. This suggests that the second electron being removed is from a principal quantum shell closer to the nucleus.
4) The big jump occurs three times, so there are four principal quantum shells occupied by this atom.
5) After the big jumps, there is a steady increase in ionization energy, this suggests that the electrons being removed come from the same principal quantum shell.
6) The electronic configuration for this atom can be written as 2,8,8,1.
Electronic Configuration of Cl.
1) The electronic configuration of chlorine is 1s²2s²2p⁶3s²3px²3py²3pz¹.
2) Between the second and third ionization energy, there is a slight increase in difference in ionization energy. This is because the first two electrons being removed come from the orbitals which contain a paired electrons. The extra repulsion between the electrons result in the ionization energy being lower.
3) There is also a slight increase in difference in ionization energy between the fifth and sixth electron being removed. This is because the sixth electron being removed comes from the 3s sub-shell, which is slightly closer to the nucleus.
4) The drastic increase in ionization energy between the seventh and eighth electrons suggests that the eighth electron comes from a principal quantum shell closer to the nucleus.
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