The properties of the elements organised in the periodic table show trends which repeat themselves in each period. These trends are known as periodic trends and the study of these trends is known as periodicity in A-Level Chemistry.
Trends in period III
1. Atomic radius decreases across the period because nuclear charge increases without any significant increase in shielding, so the outer electrons are drawn closer to the nucleus since they are attracted more strongly.
2. Melting and boiling points increase across the period from Na to Al because there is increasing nuclear charge as the atomic radius decreases, plus there is an increased number of electrons in the delocalised cloud, and therefore increased electrostatic attraction so more heat energy is required to overcome this.
- Si is a macro-molecule so it has a high melting and boiling point due to the many strong covalent bonds which need to be broken.
- P, S and Cl are all simple molecular substances and the melting point of each depends on the strength of van der Waals’ forces, which is in turn determined by the size of the molecule (biggest and most polarisable will have a higher melting point). Each of these elements has a low melting point because van der Waals’ is weak and easily overcome (forces are never broken).
3. Ionisation energy generally increases across the period due to an increase in nuclear charge but with no significant increase in shielding, meaning the outer electrons are attracted more strongly.There is a slight decrease in ionisation energy from Mg to Al because the outerelectron in Mg is in a 3s orbital however in Al it has moved to a 3p orbital, meaning it is better shielded and therefore easier to remove. There is another decrease from P to S because the outermost electron in S is paired, leading to some repulsion.
Reactions of period III elements
With water
Of the period 3 elements, only sodium, magnesium and chlorine react with water.
1. Sodium reacts violently with cold water. A piece of sodium added to cold water fizzes, skates over the surface of the water and becomes molten from the heat of the reaction. Hydrogen is evolved and this may catchfire and burn with a yellow flame (characteristic of sodium). At the end of the reaction a colourless alkaline solution of sodium hydroxide remains:
2Na(s) + 2H2O(l) ⇨ 2Na+(aq) + 2OH-(aq) + H2(g)
2. Magnesium only reacts very slowly with cold water:
Mg(s) + 2H2O(l) ⇨ Mg(OH)2(s) + H2(g)
However it burns in steam when heated to give the white solid magnesium oxide and hydrogen:
Mg(s) + H2O(g) ⇨ MgO(s) + H2(g)
3. If chlorine is bubbled through water in the presence of bright sunlight, or a green solution of chlorine water is left in bright sunlight, a colourless gas is produced (oxygen) and the green colour (due tochlorine) fades. Under these conditions, chlorine oxidises water to oxygen and is itself reduced to chloride ions:
2Cl2 + 2H2O ⇨ 4H+ + 4Cl- + O2
Reactions with oxygen
The solid elements (Na to S) in period 3 all burn in air or oxygen when ignited. Sodium burns with a yellow flame, forming the oxide:
2Na(s) + ½O2(g) ⇨ Na2O(s)
Magnesium, aluminium, silicon and phosphorus burn when ignited, emitting a bright white light and white smoke of the oxides.
Mg(s) + ½O2(g) ⇨ MgO(s)
2Al(s) + 1½O2(g) ⇨ Al2O3(s)
Si(s) + O2(g) ⇨ SiO2(s)
P4(s) + 5O2(g) ⇨ P4O10(s)
Sulfur burns with a blue flame, but much less vigorously than the elements above, to form the pungent, colourless gas sulfur dioxide:
S(s) + O2(g) ⇨ SO2(g)
Acid-base properties of the oxides of period III elements
Physical properties, structure and bonding
Ionic lattices (Na, Mg, Al) are held together by strong electrostatic forces between ions, so that these lattices have high melting points. Macromolecular solids (Si) also have high melting points, because the atoms are held together by strong covalent bonds. Molecular solids (P, S, Cl) involve weak intermolecular dipole-dipole or van der Waals’ forces and have low melting points.
Across the period, the bonding of the oxide changes from ionic to molecular, the solutions of the oxides in water change from alkaline to acidic.
The basic oxides of sodium and magnesium react with acids to form salts. The acidic oxides of phosphorus and sulphur, however, react with bases to form salts. In the middle of the period, aluminium oxide being insoluble in water does not affect pH; it is, however, capable of reacting with and therefore dissolving in, both acids and bases.This behaviour is called amphoterism.
Drafted by Eunice Wong (Chemistry)
References:
- https://www.scienceskool.co.uk/ionisation-energies.html
- http://www.chemistryrules.me.uk/found/periodictable.htm
- https://www.slideshare.net/ctkhairani/elements-in-period-3-smkbbst-33322076
- https://www.google.com/imgres?imgurl=https%3A%2F%2Fslideplayer.com%2Fslide%2F10504956%2F35%2Fimages%2F7%2FBoardworks%2BA2%2BChemistry%2BPeriodicity%2Bin%2BPeriod%2B3.jpg&imgrefurl=https%3A%2F%2Fslideplayer.com%2Fslide%2F10504956%2F&tbnid=hSlYcH8Nq-cIsM&vet=12ahUKEwjawobi3b3xAhUNNRoKHcotBhgQMygNegUIARDAAQ..i&docid=4fluLOMVx3yqvM&w=960&h=720&q=period%203&client=firefox-b-d&ved=2ahUKEwjawobi3b3xAhUNNRoKHcotBhgQMygNegUIARDAAQ
- https://consent.youtube.com/m?continue=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3D9bAhCHedVB4&gl=GB&m=0&pc=yt&uxe=23983172&hl=en-GB&src=1
- https://mammothmemory.net/chemistry/the-metal-reactivity-series/potassium-sodium-lithium-and-calcium-reactions-with-steam/potassium-sodium-lithium-and-calcium-reactions-with-steam/magnesium-and-steam.html