Melting & Boiling Point Trends
- 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
