Chem 1 - Periodicity

Classification of elements, trends and properties.

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  • Created by: Kaimlee
  • Created on: 21-02-11 16:12


Group - a vertical column of elements, they have simular properties, elements in the same group have the same number of electrons on the outer shell.
Period - Horizontal row of elements which have trends in physical and chemical properties as move accross the row.

S - block - all elements that have their highest energy electrons in s - orbital (outer electrons)

P - block - all elements that have p - orbital electrons on outer shell

D - block - all elements have outer electrons on d - orbital

Placing hydrogen and helium

Helium - placed in noble gases - cause simular properties, but s orbital not p.

Hydrogen - sometimes placed in group 1 - cause can form +ve ion, although different properties (gas) sometimes placed with halogens

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Trends in Period 3 Elements

Physical Properties                                    Boiling and Melting Points

Na ---> Ar

Generally increase from Na --> Si then decrease to Ar


Na ---> Al (all metals - have metallic bonding)

charge increases on the metal ions Na+, Mg+2, Al+3 = stronger attraction and more delocalised e-/+ve ion so more energy needed to overcome.

Si (a metalliod) giant covalent structure, many strong covalent bonds need to be broken = lots of energy needed to overcome.

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Period 3 - m/b Points

Lower melting/boiling points

Phosphorous, Sulphur, Chlorine & Argon

  • (non - metals) small, seperate molecules
  • held together by van der waals' forces (weaker)
  • little energy needed to overcome

Phosphorous exists as P(4) molecules

Sulphur exists as S(8) molecules (so has higher b/m.point)

Chlorine exists as Cl(2) molecules (Ar exists as 1) decreases because VDWf's weakens as less e-'s (smaller molecules/atoms)

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Period 3 - m/b Points - Questions

Q1.) From Na ---> P which has the highest melting point? Explain why?

Q2.)  State the trend in boiling points from Na ----> Si? Explain why?

Q3.) State and explain the trend of boiling point from P ---> Ar?

Q4.) Explain why there is a large increase in boiling point from Na and Mg to Al

Q5.) Explain why Si has a much higher melting point compared to sulphur?

Q6.) State and explain the boiling points of Na ---> Al ?

Q7.) Explain why sulphur has a higher boiling point than phosphous and why phosphous has a higher boiling point than chlorine?

Q8.) From phosphous to Argon state the element that doesn't follow the decreasing pattern? explain why?

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First Ionisation Energy - Period 3

Generally Increases

The first ionisation energy is the enthalpy change when one mole of gaseous atoms forms one mole of gaseous ions, with a single +ve.

Endothermic   (gaseous so energy isn't used changing it's state)

General Equation =   X (g) ---> X+ (g) + e-            (state symbols)

  • more protons in nucleus - nuclear charge increases
  • therefore more attraction between nucleus & outer e-
  • negligible sheliding - e- on same energy level
  • so more enery needed to remove outer e-

Mg ---> Al Al outer e- on p higher e.level = more energy. Mg outer e- on orbital s.

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First Ionisation Energy - Period 3

Phosphous ---> Sulphur

P = unpaired electrons on 3P orbital         more energy

S = 2 paired electrons on 3P orbital          less energy


Theres some repulsion between paired electrons in the same sub level, this reduces the force of attraction to the nucleus - so less energy needed to remove a pair electron

than needed to remove unpaired electron from phosphous

therefore increase in ionisation energy of phosphous compared to drop in sulphur.

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First Ionisation Energy - Questions

Q1.) Al has a higher ionisation energy compared to Na, explain why?

Q2.) Explain why sulphur has a lower ionisation energy compared to phosphous?

Q3.) Will Na or Cl have a higher ionisation energy explain why?

Q4.) Does a Na atom have a higer or lower ionisation energy than Na+ ion? Explain?

Q5.) Which has the higher ionisation energy Na atom or Na- ion? Explain?

Q6.) From Na ---> P the trend increases in ionisation energies, why does Al not follow the trend and is lower than expected?

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Trend in Atomic Radius - Period 3

Na ---> Ar               Decrease

  • Number of protons increase
  • so nuclear charge increases
  • more electrons
  • but negligible shielding - on same sub level
  • so force of attraction between the nucleus & outer e- increases
  • therefore atomic radius decreases

Q1.) State and explain the trend in atomic radius Na ---> Cl ?

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