Periodicity (Edexcel A Level Chemistry)

• Elements in the periodic table are arranged in order of increasing atomic number and placed in vertical columns (groups) and horizontal rows (periods)
• The elements across the periods show repeating patterns in chemical and physical properties
• This is called periodicity

 The Periodic Table

All elements are arranged in the order of increasing atomic number from left to right

Melting point

• Period 2 and 3 elements follow the same pattern in relation to their melting points

Melting points of the elements across Period 3 table

Ions of Period 3 elements with increasing positive charge (metals) and increasing of number of outer electrons across the period

• A general increase in melting point for the Period 3 elements up to silicon is observed
• Silicon has the highest melting point
• After the Si element the melting points of the elements decreases significantly

• The above trends can be explained by looking at the bonding and structure of the elements

Bonding & structure of the elements table

• The table shows that Na, Mg and Al are metallic elements which form positive ions arranged in a giant lattice in which the ions are held together by a 'sea' of delocalised electrons 

 

Metal cations form a giant lattice held together by electrons that can freely move around

• The electrons in the ‘sea’ of delocalised electrons are those from the valence shell of the atoms
• Na will donate one electron into the ‘sea’ of delocalised electrons, Mg will donate two and Al three electrons
• As a result of this, the metallic bonding in Al is stronger than in Na
• This is because the electrostatic forces between a 3+ ion and the larger number of negatively charged delocalised electrons is much larger compared to a 1+ ion and the smaller number of delocalised electrons in Na
• Because of this, the melting points increase going from Na to Al
• Si has the highest melting point due to its giant molecular structure in which each Si atom is held to its neighbouring Si atoms by strong covalent bonds
• P, S, Cl and Ar are non-metallic elements and exist as simple molecules (P₄, S₈, Cl₂ and Ar as a single atom)
• The covalent bonds within the molecules are strong, however, between the molecules, there are only weak instantaneous dipole-induced dipole forces
• It doesn’t take much energy to break these intermolecular forces
• Therefore, the melting points decrease going from P to Ar (note that the melting point of S is higher than that of P as sulphur exists as larger S₈ molecules compared to the smaller P₄ molecule)

Atomic radius

• The atomic radius is the distance between the nucleus and the outermost electron of an atom
• The atomic radius is measured by taking two atoms of the same element, measuring the distance between their nuclei and then halving this distance
• In metals this is also called the metallic radius and in non-metals, the covalent radius

Atomic radii of period 3 elements

• You can see a clear trend across the period which also repeated in period 2

The graph shows a decrease in atomic radii of period 3 elements across the period

• Across the period, the atomic radii decrease
• This is because the number of protons (the nuclear charge) and the number of electrons increases by one every time you go an element to the right
• The elements in a period all have the same number of shells (so the shielding effect is the same)
• This means that as you go across the period the nucleus attracts the electrons more strongly pulling them closer to the nucleus
• Because of this, the atomic radius (and thus the size of the atoms) decreases across the period

The diagram shows that across period 3, the elements gain extra electrons in the same principal quantum shell

Ionisation energy across period 2 and 3

• The ionisation energy across a period generally increases due to the following factors:
  • Across a period the nuclear charge increases
  • This causes the atomic radius of the atoms to decrease, as the outer shell is pulled closer to the nucleus, so the distance between the nucleus and the outer electrons decreases
  • The shielding by inner shell electrons remain reasonably constant as electrons are being added to the same shell
  • It becomes harder to remove an electron as you move across a period; more energy is needed
  • So, the ionisation energy increases

Dips in the trend for period 2

• There is a slight decrease in IE₁ between beryllium and boron as the fifth electron in boron is in the 2p subshell, which is further away from the nucleus than the 2s subshell of beryllium
  • Beryllium has a first ionisation energy of 900 kJ mol^-1 as its electron configuration is 1s² 2s²
  • Boron has a first ionisation energy of 800 kJ mol^-1 as its electron configuration is 1s² 2s² 2p_x¹
• There is a slight decrease in IE₁ between nitrogen and oxygen due to spin-pair repulsion in the 3p_x orbital of oxygen
  • Nitrogen has a first ionisation energy of 1400 kJ mol^-1 as its electron configuration is 1s² 2s² 2p_x¹ 2p_y¹ 2p_z¹
  • Oxygen has a first ionisation energy of 1310 kJ mol^-1 as its electron configuration is 1s² 2s² 2p_x² 2p_y¹ 2p_z¹
• In oxygen, there are 2 electrons in the 2p_x orbital, so the repulsion between those electrons makes it slightly easier for one of those electrons to be removed

Dips in the trend for period 3 

• There is again a slight decrease between magnesium and aluminium as the thirteenth electron in aluminium is in the 3p subshell, which is further away from the nucleus than the 3s subshell of magnesium
  • Magnesium has a first ionisation energy of 738 kJ mol^-1 as its electron configuration is 1s² 2s² 2p⁶ 3s² 
  • Aluminium has a first ionisation energy of 578 kJ mol^-1 as its electron configuration is 1s² 2s² 2p⁶ 3s² 3p_x¹
• There is a slight decrease in IE₁ between phosphorus and sulfur due to spin-pair repulsion in the 3p_x orbital of sulfur
  • Phosphorus has a first ionisation energy of 1012 kJ mol^-1 as its electron configuration is 1s² 2s² 2p⁶ 3s² 3p_x¹ 3p_y¹ 3p_z¹
  • Sulfur has a first ionisation energy of 1000 kJ mol^-1 as its electron configuration is 1s² 2s² 2p⁶ 3s² 3p_x² 3p_y¹ 3p_z¹
• In sulfur, there are 2 electrons in the 3p_x orbital, so the repulsion between those electrons makes it slightly easier for one of those electrons to be removed