Periodicity Flashcards
Why does silicon have a greater melting point than aluminium?
Silicon has a giant covalent structure. Each Si atom is covalently bonded to four other atoms; There are very many covalent bonds which are very strong; Breaking them requires a large amount of energy so silicon has high melting and boiling points.
Why do sodium, magnesium and aluminium have higher melting points than phosphorus, sulfur, and chlorine?
Na, Mg and Al contain metallic bonds which are very strong and require a lot of energy to break them; P, S, and Cl are simple molecular substances so only Van der Waals forces which are weak need to be overcome which requires a lot less energy.
Why does aluminium have a higher melting point than magnesium and sodium?
The outermost electrons in metals are delocalised which leaves behind positive cations; Aluminium has three delocalised electrons per atom, magnesium two and sodium one; The size of the cations decreases from sodium to aluminium so the charge density increases; This makes the metallic bonds stronger so they require more energy to break them;
Why does sulfur have a higher melting point than phosphorus?
P forms molecules of 4, S forms molecules of 8. Therefore, sulphur has more electrons so more van der Waals forces.
Why does aluminium have a lower melting point than Si but a higher boiling point?
The metallic bonds in liquid aluminium are stronger than the remaining covalent bonds in silicon. However, the structure is intact in solid form so silicon has more covalent bonds so is harder to melt.
Explain why the atomic radius decreases as we go across period 3.
Increased nuclear charge across period 3; same level of shielding across period 3; force of attraction between nucleus and the electrons increases; atomic radius decreases because the force of attraction has increased.
Explain the trend in first ionisation energies across period 3.
As we go across, it increases generally. This is due to an increased nuclear charge.
Why does aluminium have a lower first ionisation energy than magnesium?
Aluminium has an electron in 3p3 which is easier to remove due to it being higher in energy and Mg has a full shell which is harder to remove.
Describe the trend in melting points across group 3.
Describe the trend in boiling points across period 3.
