CIE AS Chemistry (9701) 2019-2021

Revision Notes

2.1.1 Periodicity of Physical Properties of the Elements in Period 3

Properties of the Elements in Period 3

  • Elements in the periodic table are arranged in order of increasing atomic number and are 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

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 table

The Periodic Table - Table 1_Properties of the Elements in Period 3, downloadable AS & A Level Chemistry revision notes

  • Across the period, the atomic radii decrease
  • Note that radii is the plural of radius

Ionic radius

  • The ionic radius is the distance between the nucleus and the outermost electron of an ion
  • Metals produce positively charged ions (cations) whereas nonmetals produce negatively charged ions (anions)
  • The cations have lost their valence electrons which causes them to be much smaller than their atoms
  • Because there are fewer electrons, this also means that there is less shielding of the outer electrons
  • Going across the period from Na+ to Si4+ the ions get smaller due to the increasing nuclear charge attracting the outer electrons in the second principal quantum shell nucleus (which has an increasing atomic number)
  • The anions are smaller than their original atoms because each atom has gained one or more electrons in their third principal quantum shell
  • This increases the repulsion between electrons while the nuclear charge is still the same
  • Going across P3- to Cl the ionic radii decreases as the nuclear charge increases across the period and less electrons are gained by the atoms (P gains 3 electrons, S 2 electrons and Cl 1 electron)

Ionic radii of ions of Period 3 elements table

The Periodic Table - Table 2_Properties of the Elements in Period 3, downloadable AS & A Level Chemistry revision notes

Melting point

Melting points of the elements across Period 3 table

The Periodic Table - Table 3_Properties of the Elements in Period 3, downloadable AS & A Level Chemistry revision notes

  • The above trends can be explained by looking at the bonding and structure of the elements which are summarised in the table below

Bonding & structure of Period 3 elements table

The Periodic Table - Table 4_Properties of the Elements in Period 3, downloadable AS & A Level Chemistry revision notes

  • 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 around them:
    • The electrons are free to move around and are not bound to an atom 
  • 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
  • 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 are 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 single atoms)
  • 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)

Electrical conductivity

  • Electrical conductivity refers to how well a substance can conduct electricity

Trends in electrical conductivity across Period 3 table

The Periodic Table - Table 5_Properties of the Elements in Period 3, downloadable AS & A Level Chemistry revision notes

  • As mentioned before, going from Na to Al, there is an increase in the number of valence electrons that are donated to the ‘sea’ of delocalised electrons
  • Because of this, there are more electrons available in Al to move around through the structure when it conducts electricity, making Al a better electrical conductor than Na
  • Due to the giant molecular structure of Si, there are no delocalised electrons that can freely move around within the structure
  • Si is therefore not a good electrical conductor and is classified as a semimetal (metalloid)
  • The lack of delocalised electrons is also why P and S cannot conduct electricity
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