Colourful Solutions > The periodic table: Classification of elements > Electronic configuration, valency and formula

The Mad Science Lab

Standard level

The term electronic configuration refers to the arrangement of the electrons around the central nucleus from the inner energy shell to the outer energy shell.

Syllabus ref: S3.1.2

Structure 3.1.2 - The period number shows the outer energy level that is occupied by electrons.

  • Elements in a group have a common number of valence electrons.
  • Deduce the electron configuration of an atom up to Z = 36 from the element’s position in the periodic table and vice versa.

Guidance

  • Groups are numbered from 1 to 18.
  • The classifications “alkali metals”, “halogens”, “transition elements” and “noble gases” should be known.

Tools and links

  • Nature of science, Structure 1.2 - How has the organization of elements in the periodic table facilitated the discovery of new elements?

Electronic configuration

The number of outer electrons can be obtained from the periodic table using the group number, it is always the same. This does not apply to the transition metals.

  • Group 1 - has 1 electron in the outer shell
  • Group 2 - has 2 electrons in the outer shell
  • Group 13 - has 3 electrons in the outer shell
  • etc

The valency of an element from the main groups can be obtained from the group number. For elements in groups 1 & 2 the valency is the same as the group number. For elements in groups 15-17 the valency is equal to the 18 - group number

Example: What is the valency of selenium (group 16)

Groups 15 to 17 have a valency = 18 - group number

Valency of selenium = 18 - 16 = 2

^ top

Formula

Formulae of binary (two element) compounds can be found using the valencies of the elements. If the group numbers of the elements combining are known, then the formula is easy to determine.

Hydrogen (valency 1) combines with all of the elements from Na to Cl in period 3. The formula of the hydrides formed follows a pattern.

Group
1
2
13
14
15
16
17
Hydride
NaH
MgH2
AlH3
SiH4
PH3
H2S
HCl

Example: What is the formula of the compound formed between sodium (group 1) and selenium (group 16)

sodium valency = group number = 1
selenium valency = 18 - group number = 2

Formula of sodium selenide = Na2Se

^ top

Electronic configuration

The full electronic configuration is expected to include the sub-shells s, p, d, up to element 54. The configuration is written from the inner (lower energy) shells outwards.

Show all the configurations from 1-36

Hydrogen 1s1 Potassium 1s2 2s2 2p6 3s2 3p6 4s2
Helium 1s2 Calcium 1s2 2s2 2p6 3s2 3p6 4s2
Lithium 1s2 2s1 Scandium 1s2 2s2 2p6 3s2 3p6 4s2 3d1
Beryllium 1s2 2s2 Titanium 1s2 2s2 2p6 3s2 3p6 4s2 3d2
Boron 1s2 2s2 2p1 Vanadium 1s2 2s2 2p6 3s2 3p6 4s2 3d3
Carbon 1s2 2s2 2p2 Chromium 1s2 2s2 2p6 3s2 3p6 4s1 3d5
Nitrogen 1s2 2s2 2p3 Manganese 1s2 2s2 2p6 3s2 3p6 4s2 3d5
Oxygen 1s2 2s2 2p4 Iron 1s2 2s2 2p6 3s2 3p6 4s2 3d6
Fluorine 1s2 2s2 2p5 Cobalt 1s2 2s2 2p6 3s2 3p6 4s2 3d7
Neon 1s2 2s2 2p6 Nickel 1s2 2s2 2p6 3s2 3p6 4s2 3d8
Sodium 1s2 2s2 2p6 3s1 Copper 1s2 2s2 2p6 3s2 3p6 4s1 3d10
Magnesium 1s2 2s2 2p6 3s2 Zinc 1s2 2s2 2p6 3s2 3p6 4s2 3d10
Aluminium 1s2 2s2 2p6 3s2 3p1 Gallium 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p1
Silicon 1s2 2s2 2p6 3s2 3p2 Germanium 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p2
Phosphorus 1s2 2s2 2p6 3s2 3p3 Arsenic 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p3
sulfur 1s2 2s2 2p6 3s2 3p4 Antimony 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p4
Chlorine 1s2 2s2 2p6 3s2 3p5 Bromine 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p5
Argon 1s2 2s2 2p6 3s2 3p6 Krypton 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6

Each member of the same group has an identical outer electronic arrangement in terms of subshells, that can be represented by nsx npy.

Example: The electronic configuration of the elements in group 14 are as follows:

  • Carbon = [He] 2s2 2p2
  • Silicon = [Ne] 3s2 3p2
  • Germanium = [Ar] 3d10 4s2 4p2

In all cases the outer energy shell can be expressed as ns2 np2

^ top

Worked examples

Q512-01 The most likely molecule to be formed by the reaction of boron and chlorine is
  1. BCl
  2. BCl3
  3. BCl4
  4. BCl6
Answer

Boron is in group 13, hence it has a valency of 3

Chlorine is in group 17, hence it has a valency of (18-17) = 1

Therefore three chlorine atoms are needed for each boron atom = BCl3
Q512-02 What is the formula of an ionic compound formed by element X (group 2) and element Y (group 16)?
  1. X3Y
  2. X2Y
  3. XY2
  4. XY
Answer

Element X is in group 2, hence it has a valency of 2

Element Y is from group 16, hence it has a valency of (18-16) = 2

The valencies balance out therefore = XY

Q512-03 Atom T has 3 valence electrons and atom S has 6 valence electrons. The formula expected for an ionic compound of T and S is
  1. T2S3
  2. T3S2
  3. TS3
  4. T2S
Answer

Element T has a valency of 3

Element S has a valency of (8-6) = 2

Therefore three S atoms are needed for every two T atoms, therefore = T2S3
Q512-04 What is the formula of the compound formed by calcium and nitrogen?
  1. CaN
  2. Ca2N
  3. Ca2N3
  4. Ca3N2
Answer

Calcium is in group 2 and therefore has a valency of 2

Nitrogen is in group 15 and therefore has a valency of (18-15) = 3

Therefore three Ca atoms are needed for every two N atoms, therefore = Ca3N2
Q512-05 The formula that one would expect for the fluoride of gallium, Ga, is
  1. GaF
  2. GaF2
  3. GaF3
  4. Ga2F3
Answer

Gallium is in group 13 and therefore has a valency of 3

Fluorine is in group 17 and therefore has a valency of (18-17) = 1

Therefore three fluorine atoms are needed for every gallium atom, therefore = GaF3
Q512-06 Based on its position in the periodic table, which of the following compounds would germanium be expected to form with hydrogen?
  1. GeH4
  2. GeH3
  3. GeH2
  4. GeH
Answer

Germanium is in group 14 and therefore has a valency of 4

Hydrogen is element number 1, it has a valency of 1

Therefore four hydrogen atoms are needed for every germanium atom, therefore = GeH4
Q512-07 Based on its position in the periodic table, which of the following compounds would tellurium be expected to form with fluorine?
  1. TeF4
  2. TeF3
  3. TeF2
  4. TeF
Answer

Tellurium is in group 16 and therefore has a valency of (18-16) = 2

Fluorine is in group 17 and therefore has a valency of (18-17) = 1

Therefore two fluorine atoms are needed for every tellurium atom, therefore = TeF2
Q512-08 What is the formula of the compound formed by calcium and silicon?
  1. CaSi4
  2. CaSi3
  3. Ca2Si
  4. Ca2Si2
Answer

Calcium is in group 2 and therefore has a valency of 2

Silicon is in group 14 and therefore has a valency of 14

Therefore two calcium atoms are needed for every silicon atom, therefore = Ca2Si
Q512-09 Based on its position in the periodic table, which of the following compounds would arsenic be expected to form with hydrogen?
  1. AsH4
  2. AsH3
  3. AsH2
  4. AsH
Answer

Arsenic is in group 15 and therefore has a valency of (18-15) = 3

Hydrogen is element number 1, it has a valency of 1

Therefore three hydrogen atoms are needed for every arsenic atom, therefore = AsH3
Q512-10 Based on their positions in the periodic table, which of the following compounds would polonium be expected to form with aluminium?
  1. Al3Po4
  2. Al2Po3
  3. Al3Po2
  4. Al3Po
Answer

Aluminium is in group 13 and therefore has a valency of 3

Polonium is in group 16 and therefore has a valency of (18-16) = 2

Therefore three polonium atoms are needed for every two aluminium atoms, therefore = Al2Po3
  Now test yourself

^ top