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Trends Across Period 3 of the Periodic Table

Properties of Period 3 Elements

Name of Element Symbol	Sodium Na	Magnesium Mg	Aluminium Al	Silicon Si	Phosphorus P	Sulfur S	Chlorine Cl	Argon Ar
Atomic Number, z	11	12	13	14	15	16	17	18
Electronic Configuration	2,8,1	2,8,2	2,8,3	2,8,4	2,8,5	2,8,6	2,8,7	2,8,8
Atomic Radius (picometers)	186	160	143	118	110	102	99	192
1^stIonization Energy (kJ/mol)	502	744	584	793	1017	1006	1257	>1526
Electronegativity	0.9	1.3	1.6	1.9	2.2	2.6	3.2	-
Melting Point (^oC)	98	639	660	1410	44	113	-101	-189
Boiling Point (^oC)	883	1090	2467	2680	280	445	-35	-186
Metallic Character	metal	metal	metal	semi-metal (metalloid)	non-metal	non-metal	non-metal	non-metal

Trends across Period 3 of the Periodic Table

The modern Periodic Table is arranged in order of increasing atomic number. As one moves from one element to another on the right, one more proton is found in the nucleus, and one more electron is found in the same electron 'shell' (energy level). For this reason, all the elements in Period 3 have the first electron 'shell' (energy level) filled with 2 electrons and the second electron 'shell' (energy level) filled with 8 electrons (the electronic configuration of Neon). Sodium begins a new electron 'shell' ( 3^rdenergy level) with 1 electron, Magnesium has 2 electrons in the third electron 'shell' (energy level), Aluminium has 3 electrons in the third electron 'shell' (energy level) etc, until finally the third electron 'shell' (energy level) is filled with 8 electrons and the stable electronic configuration of the Noble Gas Argon is reached (2,8,8).
Atomic radius of the elements decrease across the Period from left to right. As we move from left to right across the period one more proton is added to the nucleus of each successive atom, and one more electron is added to the same electron 'shell' (energy level) of each successive atom. The increased positive charge in the nucleus of each successive atom attracts all the electrons in the atom more strongly, so they are drawn in more closely towards the nucleus.
1^st Ionization Energy (the energy required to remove an electron from the gaseous atom) increases across the Period from left to right. The further away from the positively charged nucleus that a negatively charged electron is, the less strongly the electron is attracted to the nucleus and so the more easily that electron can be removed. So, as the atomic radius decreases from left to right across the Period so the 1^st Ionization Energy increases.
Electronegativity (the relative tendency shown by an atom to attract electrons to itself) increases across the Period from left to right. Typically, metals have low electronegativity, little ability to attract electrons, while non-metals have high electronegativity, greater ability to attract electrons. As we move from left to right across the Period, the elements become less metallic in nature (more non-metallic).
In general metals are hard (EXCEPT Group I metals which are quite soft), have metallic lustre, high melting and boiling points (Except for mercury which is a liquid at room temperature, and the Group I metals which have low melting/boiling points compared to other metals) and good electrical conductivity. In general, non-metals are dull, brittle, have low melting and boiling points and are electrical insulators (non-conductors of electricity). Elements to the left of Period 3 exhibit metallic properties, elements to the right show non-metallic properties. Silicon is a semi-metal (metalloid).

Properties of Oxides and Chlorides of Period 3 Elements

Element	Sodium	Magnesium	Aluminium	Silicon	Phosphorus	Sulfur	Chlorine
Chloride formula Oxide formula	NaCl Na₂O	MgCl₂ MgO	AlCl₃ Al₂O₃	SiCl₄ SiO₂	PCl₃ P₄O₆	SCl₂ SO₂	Cl₂ Cl₂O₇
Chloride melting point (^oC) Oxide melting point (^oC)	801 920	712 2800	193 2045	-68 1700	-92 420	-80 17	-101 -92
Chloride bonding Oxide bonding	Ionic Ionic	Ionic Ionic	Covalent Ionic	Covalent Covalent	Covalent Covalent	Covalent Covalent	Covalent Covalent
Chloride conductivity of liquid Oxide conductivity of liquid	Good Good	Good Good	Poor Good	Nil Nil	Nil Nil	Nil Nil	Nil Nil
Chloride acid-base behaviour Oxide acid-base behaviour	Neutral Basic	Neutral Basic	Acidic Amphoteric	Acidic Acidic	Acidic Acidic	Acidic Acidic	Acidic Acidic