Arrangement of Elements Notes

Cambridge IGCSE Chemistry: Revision notes

Key facts

  • Elements are in order of proton (atomic) number.
  • Groups (columns) share the number of outer electrons; periods (rows) share the number of shells.
  • Across a period, elements change from metals on the left to non-metals on the right.
  • Group I forms 1+ ions, Group II 2+, Group VI 2− and Group VII 1−.
  • Elements in a group react similarly because they have the same number of outer electrons.

How the table is arranged

Elements are in order of proton number, with groups as columns and periods as rows.

Elements with similar chemical properties fall in the same group because the pattern repeats periodically as proton number increases. Elements in a group have the same number of outer electrons; elements in a period have the same number of occupied shells.

1234567012345671H2He3Li4Be5B6C7N8O9F10Ne11Na12Mg13Al14Si15P16S17Cl18Ar19K20Ca21Sc22Ti23V24Cr25Mn26Fe27Co28Ni29Cu30Zn31Ga32Ge33As34Se35Br36Kr37Rb38Sr39Y40Zr41Nb42Mo43Tc44Ru45Rh46Pd47Ag48Cd49In50Sn51Sb52Te53I54Xe55Cs56Ba57La72Hf73Ta74W75Re76Os77Ir78Pt79Au80Hg81Tl82Pb83Bi84Po85At86Rn87Fr88Ra89Ac104Rf105Db106Sg107Bh108Hs109Mt110Ds111Rg112Cn113Nh114Fl115Mc116Lv117Ts118Og
Groups run down, periods run across.

What do elements in the same period have in common?

Across a period

Elements change gradually from metals on the left to non-metals on the right.

Metals tend to lose electrons easily; non-metals tend to gain them. The change is gradual, and elements near the dividing line show intermediate behaviour. The number of outer electrons rises by one across the period.

02468NaMgAlSiPSClArElementOuter electrons
Outer electrons across Period 3
1234567012345671H2He3Li4Be5B6C7N8O9F10Ne11Na12Mg13Al14Si15P16S17Cl18Ar19K20Ca21Sc22Ti23V24Cr25Mn26Fe27Co28Ni29Cu30Zn31Ga32Ge33As34Se35Br36Kr37Rb38Sr39Y40Zr41Nb42Mo43Tc44Ru45Rh46Pd47Ag48Cd49In50Sn51Sb52Te53I54Xe55Cs56Ba57La72Hf73Ta74W75Re76Os77Ir78Pt79Au80Hg81Tl82Pb83Bi84Po85At86Rn87Fr88Ra89Ac104Rf105Db106Sg107Bh108Hs109Mt110Ds111Rg112Cn113Nh114Fl115Mc116Lv117Ts118Og
  • Metals
  • Non-metals
  • Semi-metals
Metals are left of the staircase line; non-metals are right.

How many outer electrons does an aluminium atom (Period 3, Group III) have?

Group and ion charge

The group tells you how many electrons an atom loses or gains to form its ion.

Group I loses 1 electron (1+); Group II loses 2 (2+); Group VI gains 2 (2−); Group VII gains 1 (1−). In general, metals in Groups I to III form positive ions and non-metals in Groups V to VII form negative ions.

−3−2−10123IIIIIIVVIVIIGroupIon charge
Typical ion charge by group

What ion does a Group II element form?

Similar behaviour

They have the same number of outer electrons, and reactions mainly involve those electrons.

Elements in a group react in similar ways and form ions and compounds with similar formulae. So an element’s position can be used to predict its properties from trends in the same group or period.

Li3p 4n

Lithium

2,1

Na11p 12n

Sodium

2,8,1

K19p 20n

Potassium

2,8,8,1

One outer electron each: Group I.

Why do lithium, sodium and potassium react similarly?

Try an exam question

Explain why the elements in Group I of the periodic table have similar chemical properties.

[3 marks]

That's the notes covered.

Carry on to the next subtopic.