Structure of the periodic table:
Elements are arranged in the periodic table in order of atomic number.
The row across the table are called periods. The vertical columns are
called groups. According to the modern periodic law, “the physical and
chemical properties of the elements are the periodic function of their
atomic numbers.”

Variation in size of atoms:
1) Covalent radius is half the internuclear distance between two like
atoms bonded by a single covalent bond.
2) Van der Waal’s radius is half the average distance between two
adjacent non-bonded atoms.
3) For example, the covalent radius of Cl2 is 0.099 nm while the van der
Waal’s radius of Cl2 is 0.180 nm.
Note: van der Waal’s radius is always larger than covalent radius.

Variation of atomic radius across the period 3
1. Across Period 3, the atomic radius decreases gradually. This is
because the Nuclear charge increases.
2. The outer electrons are more attracted towards the nucleus,
making the atoms smaller.
3. For comparison, metallic radii are used for Na, Mg and Al, covalent
radii are used for Si, P, S and Cl. For argon, van der Waal’s radius is
used (argon do not form any bonds)

Variation in ionic radius
Cations are formed when an atom loses electron(s). In Period 3, Na,
Mg, Al and Si form cations by losing electron(s) to achieve stable octet
electronic configuration. The ions formed are Na⁺, Mg2⁺, Al3⁺ and Si⁴⁺
respectively.
Cations are smaller than their respective atoms because a whole layer
of electrons are lost(shell is lost). The remaining electrons are attracted
more strongly towards the centre by the same nuclear charge.

1. Anions are formed when an atom gains electron(s). In Period 3, P, S
and Cl form anions by gaining electron(s) to achieve stable octet
electronic configuration. The ions formed are P3⁻, S2⁻ and Cl⁻
respectively.
2. Anions are bigger than their respective atoms because they have
more electrons than protons. The electrons are held less strongly by
the nucleus. Besides, a repulsion is created between the electrons
when a new electron is introduced and this causes the ion to
expand.
3. Anions are bigger than cations because anions have one more shell
of electrons compared to cations.

Isoelectronic means atoms having same no of
electrons but different nuclear charge.
• In the isoelectronic series(from Na⁺ to Si⁴⁺ and P3⁻ to Cl⁻), the ionic
radius decreases gradually. This is because the same number of
electrons are attracted more strongly by the increasing nuclear
charge.

Variation in melting and boiling points
Across a Period:
1. Melting point increases from Na to Al because the strength of the
metallic bond increases. Melting point of Si is highest because Si
has a giant covalent structure, a lot of energy is required to
overcome the strong covalent bonds.
2. Melting points of P, S, Cl and Ar are lower because these have
simple molecular structures, only weak van der Waal’s forces of
attraction exist between them. Melting point of S > P > Cl > Ar
because these elements exist as S8, P4, Cl2 and Ar respectively. S8
contains the most number of electrons, followed by P4, Cl2 and Ar.
Van der Waal’s forces get stronger with increasing number of
electrons.

Variation in electrical conductivity
Across the Period, the elements change from metals(Na to Al) to semi-
metal(Si) and then to non-metals(P to Ar).
Electrical conductivity is highest is metals, lower in semi-metals and
lowest in non-metals(Most non-metals do not conduct electricity at
all).
The electrical conductivity of Period 3 elements:

The electrical conductivity of Period 3 elements:
1. Increases from Na to Al because the number of electrons
contributed by per atom to the sea of delocalised electrons
increases from one in Na, two in Mg and three in Al. There are more
electrons to conduct electricity.
2. Decreases from Al onwards. Si is a semi-metal therefore it is a
semiconductor.
3. The remaining elements do not conduct electricity because there
are no mobile electrons.

Ionisation energy:
The 1st ionisation energy, (ΔHi1) is the energy needed to remove a(1)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.
The general unit for ionisation energy is kJ mol⁻1.
Ca(g) → Ca⁺(g) + e⁻ ; ΔHi1 = +590 kJ mol⁻1
If a second electron is removed from the gaseous 1+ ions, it is the 2nd ionisation
energy, ΔHi2.
Ca⁺(g) → Ca2⁺(g) + e⁻ ; ΔHi2 = +1150 kJ mol⁻1
The 2nd ionisation 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 2+ ion.
The continuous removal of electrons until the nucleus is left only will result in
successive ionisation energies.

Factors affecting the ionisation energy:
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 ionisation
energy is higher.
– The greater the nuclear charge, the higher the ionisation energy

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 ionisation
energy is lower.
– The greater the distance between nucleus and outer electrons, the
lower the ionisation energy.

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 ionisation
energy is lower.
– The greater the shielding effect, the lower the ionisation energy.

Pattern of ionisation down a Group
1) The first ionisation energy decreases down a Group. 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.

Pattern of ionisation energy across a Period
The general trend of ionisation energy across a Period is increasing. This is
because, across a Period, the number of proton in the nucleus increases by
one therefore the nuclear charge increases.
However, the distance between the nucleus and outer electrons decreases
across a Period and the outer electrons experience the same amount of
shielding.
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 ionisation energy is higher.

The drop between (Be-B) and (Mg-Al):
1) There is a slight decrease in first ionisation 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

The decrease in first ionisation 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).
1) There is a slight decrease in first ionisation energy between nitrogen-
oxygen.
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 : 1s22s22px
12py
12pz
1 O : 1s22s22px²2py
12pz
1

Successive ionisation 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.

1) The electronic configuration of chlorine is 1s22s22p⁶3s23px
23py
23pz
1.
2) Between the second and third ionisation energy, there is a slight increase in
difference in ionisation 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 ionisation energy being lower.
3) There is also a slight increase in difference in ionisation 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 ionisation 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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