• 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

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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?2px1
• There is a slight?decrease?in?IE1?between?nitrogen?and?oxygen?due to?spin-pair repulsion?in the 3px?orbital of oxygen
  • Nitrogen?has a first ionisation energy of?1400 kJ mol-1?as its electron configuration is 1s2?2s2?2px1?2py1?2pz1
  • Oxygen?has a first ionisation energy of?1310 kJ mol-1?as its electron configuration is 1s2?2s2?2px2?2py1?2pz1
• In oxygen, there are 2 electrons in the?2px?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?3px1
• There is a slight?decrease?in?IE1?between?phosphorus?and?sulfur?due to?spin-pair repulsion?in the 3px?orbital of oxygen
  • Phosphorus has a first ionisation energy of 1012?kJ mol-1?as its electron configuration is 1s2?2s2?2p6?3s2?3px1?3py1?3pz1
  • Sulfur has a first ionisation energy of?1000 kJ mol-1?as its electron configuration is?1s2?2s2?2p6?3s2?3px2?3py1?3pz1
• In sulfur, there are 2 electrons in the 3px?orbital, so the repulsion between those electrons makes it slightly easier for one of those electrons to be removed