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Trends Across Period 3 of the Periodic Table

Key Concepts

The general trends in the properties of period 3 elements are summarised in the table below:

  Group
1
Group
2
Groups
3 - 12
Group
13
Group
14
Group
15
Group
16
Group
17
Group
18
Period 3 Elements Na
sodium
Mg
magnesium
  Al
aluminium
Si
silicon
P
phosphorus
S
sulfur
Cl
chlorine
Ar
argon
Trend: Atomic Number (Z) low   high
Trend: No. Electrons low   high
Trend: Atomic Radius larger   smaller largest
general Trend: 1st Ionisation Energy lowest   highest
Trend: Electronegativity lowest   highest none
General Melting Point higher highest lower
Metallic Character metals semi-metal
(metalloid)
non-metals
Bonding: Oxides ionic covalent
(3-D network)
covalent molecular none
Bonding: Chlorides ionic covalent none

Properties of Period 3 Elements

The table below summarises the physical 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-1)
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

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General Trends in Physical Properties Across Period 3 of the Periodic Table

  • Atomic Number (Z) increases across the period from left to right.

    The modern Periodic Table is arranged in order of increasing atomic number.
    As you go across the period from left to right, 1 more proton is added to the nucleus of the atom which increases its atomic number (Z) by 1.

  • Electron configuration increases by 1 across the period from let to right.

    As one moves from one element to another on the right, one more proton (positive charge) is found in the nucleus, so 1 more electron (negative charge) must be added to an electron 'shell' in order to balance the charge.
    The electron is added to 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 increases across the Period from left to right.

    First ionisation energy is the energy required to remove an electron from the gaseous atom:

    M(g) → M+(g) + e-

    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 increases across the Period from left to right.

    Electronegativity is the relative tendency shown by a bonded atom to attract electrons to itself.

    Typically, metals have low electronegativity, little ability to attract electrons, while non-metals have high electronegativity, greater ability to attract electrons.

  • Elements become less metallic in nature (more non-metallic) across the period from left to right.

    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).

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Properties of Oxides and Chlorides of Period 3 Elements

The table below summarises the properties of the oxides and chlorides of Period 3 elements:

Element Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine
Formula
Chloride
Oxide

 
NaCl
Na2O

 
MgCl2
MgO

 
AlCl3*
Al2O3

 
SiCl4
SiO2

 
PCl3
P4O6

 
SCl2
SO2

 
Cl2
Cl2O7
melting point (oC)
Chloride
Oxide

 
801
920

 
712
2800

 
193
2045

 
-68
1700

 
-92
420

 
-80
17

 
-101
-92
Bonding:
Chloride
Oxide

 
Ionic
Ionic

 
Ionic
Ionic

 
Covalent
Ionic

 
Covalent
Covalent

 
Covalent
Covalent

 
Covalent
Covalent

 
Covalent
Covalent
 
Conductivity of Liquid
Chloride
Oxide

 
Good
Good

 
Good
Good

 
Poor
Good

 
Nil
Nil

 
Nil
Nil

 
Nil
Nil

 
Nil
Nil
Acid-Base Behaviour
Chloride
Oxide

 
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.

For more details about the properties of the oxides of period 3 elements,
go to the Trends in the Properties of Oxides tutorial.

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General Trends in Properties of Oxides and Chlorides Across Period 3

  • The bonding in oxides and chlorides of period 3 elements becomes more covalent across the period.

    Group 1 and 2 metallic elements form ionic oxides and chlorides.

    Aluminium (Group 13) forms an ionic oxide, but a covalent chloride.

    Silicon (Group 14 semi-metal or metalloid) forms covalent chlorides and oxides, but the oxide forms a giant 3-dimensional covalent network similar to that of diamond.

    The oxides and chlorides of the non-metals to the right of the period are all covalent, and the species exist as small, discrete, covalent molecules.

  • Melting point of oxides and chlorides decrease from left to right across Period 3.

    Ionic oxides of the Group 1, 2 and 13 metallic elements have higher melting points than the covalent molecular oxides of non-metals on the right hand side of the period.
    Note that the oxide of the semi-metal (metalloid) silicon is high because it forms a giant covalent network rather than discrete covalent molecules.

    Ionic chlorides of Group 1 and 2 metallic elements have higher melting points than the covalent chlorides of the rest of the period 3 elements.
    Note that the anhydrous chloride of aluminium is covalent and has a melting point lower than that of the ionic chlorides to the left of it in the period.

  • Molten ionic oxides and chlorides conduct electricity.

    When molten, the ions in the ionic compounds are free to move and therefore the liquid can conduct electricity.
    Note that solid ionic compounds will not conduct electricity.

    The covalent oxides and chlorides will not conduct electricity when solid or liquid because there are no "charge carriers" available.
    The exception is aluminium chloride because, when molten, there will be a few ions available to move, so it can conduct electricity but only very poorly.

  • Oxides become more acidic from left to right across period 3.

    Ionic oxides of Group 1 and 2 metallic elements are basic.

    Aluminium oxide is amphoteric, it is capable of acting as either an acid or a base.

    Oxide of the semi-metal (metalloid) silicon is only weakly acidic.

    Oxides of the non-metallic elements on the right of the period are all acidic.

  • Chlorides become more acidic across period 3 from left to right.

    Ionic chlorides of Group 1 and 2 metallic elements are neutral (neither acidic nor basic).

    The covalent chlorides are all acidic.


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