Periodicity Of Physical Properties Across Period 3

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Periodic patterns of atomic radii in period 3

Period 3 elementsodium (Na)magnesium (Mg)aluminium (Al)silicon (Si)phosphorus (P)sulphur (S)chlorine (Cl)argon (Ar)
Atomic radius in nm0.1570.1360.1250.1170.1100.1040.099

The table above shows the atomic (single covalent) radii of Period 3 elements in nanometres, where 1 nm = 10−9m.

The atomic (single covalent) radius is half the distance between the two nuclei of the same type of atom.

Atoms of the noble gases (Group 18 elements), such as argon, do not have a covalent radius because they do not form bonds with each other. In such cases, their atomic radii can be determined from their van der Waals’ radius which is found from dividing by 2 the distance between the nuclei of two neighbouring, touching atoms.

Periodicity Of Physical Properties Across Period 3 (A-level Chemistry)

As shown by the graph above, the atomic radius decreases across Period 3. The same pattern is the same in other periods too.

This is because the number of protons and the number of electrons, increases by one with each successive element across a period. This extra electron added occupies the same energy level as the subsequent ones. As a result, the shielding effect remains roughly constant. However an extra proton increases the nuclear charge, the increased attractive force exerted by the nucleus on the outer (valence) shell electrons pulls them in closer to the nucleus, reducing the atomic radius decreases across a period.

Periodic patterns of ionic radii in period 3

Ions of Period 3 elementsNa+Mg2+Al3+Si4+P3−S2−ClAr
Ionic radius / nm0.0950.0650.0500.0410.2120.1840.181

The table above shows the ionic radii of Period 3 elements (argon does not form ions).

Atoms of metallic elements form positively charged ions (called cations) and atoms of non-metallic elements form negatively charged ions (called anions).

The positively charged ions are formed when atoms lose their outer shell of electrons (the third principal quantum shell or energy level for period 3 atoms). As a result, the cations are much smaller than their atoms.

Going across the period, from Na+ to Si4+, the ions get smaller atomic due to the increasing nuclear charge increasing the attraction of the outermost (valence) to the nucleus.

The negatively charged ions are formed when atoms gain electrons in the outer shell. As the electrons are gained, the number of protons (and hence the nuclear charge) remains constant. The increase in the number of electrons causes an increasing in the repulsion between its electrons, but the nuclear charge remains constant. This increases the size of the anion compared with its atom. As a result, the anions formed are larger than their original atoms.

Going across the period, the anions decrease in size, going from P3 to Cl, as the nuclear charge also increases across the period.

Periodicity Of Physical Properties Across Period 3 (A-level Chemistry)


Periodic patterns of first ionisation energies in period 3

ElementFirst ionisation energy / kJ/mol
Sodium (Na) 494
Magnesium (Mg) 736
Aluminium (Al) 577
Silicon (Si) 786
Phosphorus (P)1060
Sulfur (S)1000
Chlorine (Cl)1260
Argon (Ar)1520

The table above shows the first ionisation energy of Period 3 elements in kilojoules per mole (kJ/mol).

Periodicity Of Physical Properties Across Period 3 (A-level Chemistry)

The nuclear charge increases as the number of protons in the nucleus increases across the period. All the Period 3 elements have 3 shells each and the electron to be removed comes from the same shell, which is the third shell.

The force of attraction between the positive nucleus and the
outer negative electrons increases across the period as

  • the nuclear charge increases
  • but the distance between the nucleus and the outer electron,
  • and the shielding by inner shells remains almost constant.

However from magnesium (1s2 2s2 2p6 3s2) to aluminium (1s2 2s2 2p6 3s2 3p1), there is a slight decrease in the first ionisation energy. This is due to the fact that even though aluminium has a greater nuclear charge, the outer electron of aluminium is in the 3p subshell, which is slightly further away from the nucleus than the 3s subshell from which the magnesium electron is removed. There is less attraction between the 3p electron and the nucleus than between the 3s electron and the nucleus, hence the decrease.

There is another slight decrease in first ionisation energy between phosphorus (1s2 2s2 2p6 3s2 3p3) and sulphur (1s2 2s2 2p6 3s2 3p4). Sulphur has a higher nuclear charge than phosphorus and the electron to be removed is in the same 3p subshell. An increase in first ionisation energy is expected, but however,

  • the electron removed from the phosphorus is from an orbital that contains an unpaired electron.
  • the electron removed from the sulphur is from the orbital that contains a pair of electrons.

The electron pair repulsion in the electron to be removed in sulphur results in less energy being needed to remove the electron, which makes the first ionisation energy of sulphur lower than expected.

Periodic patterns of melting points in period 3

Period 3 elementsodium (Na)magnesium (Mg)aluminium (Al)silicon (Si)phosphorus (P)sulphur (S)chlorine (Cl)argon (Ar)
Melting point / K371923932168331739217284

The table above shows the melting points of Period 3 elements measured in kelvin, K.

Period 3 contains different types of elements, from metallic solids on the left-hand side, through non-metallic solids in the middle to gases on the right-hand side.

The elements on the left-hand side, sodium, magnesium and aluminium are metals. Atoms of metals form positively charged ions which are bonded by a “sea” of delocalised electrons.

Periodicity Of Physical Properties Across Period 3 (A-level Chemistry)

For period 3 metals, the melting (and boiling) increase across the period as the number of electrons in the outer-shell increases. That is because each element in the outer-shell contributes to the strength of the metallic bond.

After the metals, there is silicon. Silicon is a metalloid because it has both metallic and nonmetallic properties. Its atoms are bonded in a giant covalent structure (similar to that of diamond), hence its high melting point.

After silicon there is phosphorus. Phosphorus is a non-metal made of groups of four atoms bonded by van der Waals forces. Its low melting point is due to the fact that to melt it, only the van der Waals forces have to be broken and van der Waals forces are relatively weak.

The next element is sulphur. Sulphur,is a non-metal made of groups of eight atoms bonded by van der Waals forces. Since its molecules (8 atoms) are bigger than those of phosphorus (4 atoms) in its molecules, there are larger van der Waals forces in sulphur than in phosphorus. Hence, sulphur has a higher melting point than phosphorus.

Next comes chlorine. A chlorine molecule has only two atoms. Hence the melting point of chlorine is lower than that of sulphur and phosphorus.

Then comes argon. Argon is a noble gas which is made up of single atoms with very low van der Waals forces. Hence, argon has the lowest melting (and boiling) point in period 3.

Periodic patterns of electrical conductivity in period 3

Sodium, magnesium and aluminium are good electrical conductors because of the “sea” of delocalised electrons they have. Silicon has partial conductivity and is known as a semi-conductor. The rest of the elements are electrical insulators.