Colourful Solutions > The periodic table: Classification of elements > The structure of the periodic table

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Standard level

The periodic table is on show in most science laboratories. It is one of the most useful data tables for chemistry students and teachers.

Syllabus ref: S3.1.1

Structure 3.1.1 - The periodic table consists of periods, groups and blocks.

  • Identify the positions of metals, metalloids and non-metals in the periodic table.

Guidance

  • The four blocks associated with the sublevels s, p, d, f should be recognized.
  • A copy of the periodic table is available in the data booklet.

Tools and links

The arrangement of the elements

The periodic table is simply a list of the chemical elements arranged in order of increasing atomic number. The elements are also arranged in order of increasing relative mass, although in a couple of places this isn't followed.

Reading like a book from left to right and top to bottom, the atomic number increases in single units from 1 to above 110.

Although this all sounds simple enough, it was the result of investigations and thought by many notable scientists, most famously Mendeleev. The arrangement is such that similar elements are found next to one another in repeating cycles or periods. From this pattern comes the term 'The Periodic Table'.

However, the periodic table is more than just a list of elements. Its clever structure makes it one of the most useful datatables in all of science. Hence its ubiquitous place of honour on laboratory walls.

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Groups

The vertical columns of elements are groups, numbered from 1 to 18.

There are four blocks, or regions named according to the types of orbitals being filled in the outer energy shell. The transition metals,or 'd' block are not nowadays given group numbers. Neither are the 'f' block elements otherwise known as lanthanidees and actinides (not shown above).

The heavier elements above number 94 are artificial and radioactive. Their chemistry is of little importance as regards pre-university studies.

Some of the groups are also referred to by group names (in times past, they all had names).

Group number
Group name
1
alkali metals
2
alkaline earth metals
17
halogens
18
noble gases

Hydrogen is the only element of elements 1-20 that does not easily fit into any of the groups. For this reason it is usually to be found suspended in no-man's land, in the middle of the periodic table.

Groups share similar physical and chemical properties owing to the similarity of electronic configuration in terms of the outer (valence) electrons. For example, group 1 - the alkali metals all have one electron in the outer shell. They are all soft metals, which react with water making metal hydroxides.

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Periods

The horizontal rows, or periods, give the periodic table its name. Each period represents the elements in which the same energy level is being filled by electrons.

Thus, the second period has elements with an incomplete second energy level (except for neon, in which the second level is now full)

Note: The terms "lanthanoid" and "actinoid" have now been removed from the syllabus

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Electronic configuration

Putting together the group number and the period number it is possible to work out the electronic configuration for all of the main group elements. Main group excludes the 'd' block transition metals and the 'f' block (lanthanides and actinides).

Example: What is the electronic configuration of the element from period 3 in group 16?

Period 3 has a total of 3 occupied energy shells, i.e. two filled and one outer (valence shell)

Group 16 elements have an outer energy shell with 6 electrons

Therefore electronic configuration = 2, 8, 6

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Blocks

Blocks are not generally used to categorise the periodic table, nevertheless they are useful when considering the electronic configurations of the sub-shells.

The groups 1 and 2 are referred to as the 's' block, as the elements of these two groups are filling up an outer 's' orbital.

The transition metals (and scandium and zinc) are in the 'd' block as they all have 4d electrons as the outer shell.

The right hand side of the periodic table has the main groups elements, also known as the 'p' block. Here the 'p' orbitals are being filled up.

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Full Periodic Table

Worked examples

Q511-01 Which element should have properties most like those of phosphorus?
  1. Si
  2. S
  3. At
  4. Sb
Answer

Phosphorus is in group 15. From the choices given, only Sb, antimony is also in group 15.

Answer = Sb

Q511-02 Atoms of elements in a group on the Periodic Table have similar chemical properties. This similarity is most closely related to the atoms'
  1. number of principal energy levels
  2. number of valence electrons
  3. atomic numbers
  4. atomic masses
Answer

Chemical properties are mainly caused by the valence shell electrons. Atoms in the same group have the same number of valence shell electrons.

Answer = number of valence electrons

Q511-03 In which pair are the elements most similar in their chemical properties?
  1. B and N
  2. Li and Fr
  3. Mg and Al
  4. S and Cl
Answer

The only pair in which both elements belong to the same group is Li and Fr, group 1
Q511-04 For which element are the group number and the period number the same?
  1. Li
  2. Be
  3. B
  4. Mg
Answer
  • Lithium is in group 1 and period 2
  • Beryllium is in group 2 and period 2
  • Boron is in group 13 and period 2
  • Magnesium is in group 2 and period 3

Therefore the answer is Beryllium
Q511-05 Which is related to the number of electrons in the outer main energy level of the elements from the alkali metals to the halogens?
  1. I. group number
  2. II. period number
  1. I only
  2. II only
  3. Both I and II
  4. Neither I nor II
Answer

The outer electrons are always the same as the group number.

Correct answer = A

Q511-06 An element E, of mass number 40 has the simplified electronic configuration 2,8,8,2. Which statement regarding this element is not correct
  1. It belongs to group 2 of the periodic table
  2. It has 20 neutrons
  3. It belongs to period 4 of the periodic table
  4. The formula of its oxide is EO2
Answer

A configuration of 2,8,8,2 is indicative of group 2 and period 4. There are 20 protons as there are 20 electrons and the atomic number is 20. As the mass is 40 then there are also 20 neutrons. Elements in group 2 are expected to have a valency of 2, and so the incorrect answer is D - The formula of its oxide is EO2

Q511-07 An element is in group 13 and period 2. How many electrons are present in its outer shell?
  1. 2
  2. 3
  3. 5
  4. 6
Answer

Number of electrons in the outer shell = (group number-10) = 3

Q511-08 In which group of the Periodic Table are you most likely to find a metalloid?
  1. The alkali metal family
  2. The alkaline earth family
  3. The carbon family
  4. The halogen family
Answer

Metalloids are found on the border between the metals and the non-metals, response C
Q511-09 In which region of the periodic table would the element with the electronic structure shown below be located?

1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d6 5s2

  1. group 16
  2. noble gases
  3. s block
  4. d block
Answer

Examination of the configuration indicates that the 'd' orbitals are not filled, therefore the element belongs to the 'd' block.
Q511-10 Which one of the following properties of elements is best used as a basis for classifying them into groups on the periodic table?
  1. Electronic configurations
  2. Mass number
  3. Relative atomic mass
  4. Atomic sizes
Answer

The group number depends on the outer shell electrons therefore the correct response is A, electronic configurations
  Periodic Table Game

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