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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
1stIonization Energy (kJ/mol) 502 744 584 793 1017 1006 1257 >1526
Electronegativity (Pauling) 0.93 1.31 1.61 1.9 2.19 2.58 3.16 -
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' ( 3rdenergy 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.

  • 1st 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 1st 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 1 (IA) 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 1 (IA) 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

*Aluminium forms a covalent anydrous aluminium chloride, Al2Cl6, which forms ions in aqueous solution. Ionic aluminium chloride, AlCl3.6H2O, can be crystallised out of this solution.


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