The Periodic Table

This lesson covers: 

  1. How early chemists grouped elements by properties and atomic mass
  2. The contributions of Mendeleev and Moseley to the development of the periodic table
  3. The arrangement of the modern periodic table into blocks, periods and groups
  4. How to use the periodic table to determine the electron configuration of elements

Early attempts at organising elements

In the early 1800s, chemists could only categorise elements in two ways:

  • By physical and chemical properties
  • By relative atomic mass
  • Johann Döbereiner identified triads of elements with similar properties and relative atomic masses in between the others.
  • John Newlands organised elements by increasing atomic mass and noticed a pattern of similar elements recurring every 8 elements. He called this the law of octaves.


Döbereiner’s triads:

Triad 1Triad 2Triad 3
LiCaCl
NaSrBr
KBaI

Newland’s octaves:

Octave 1LiBeBCNOF
Octave 2NaMgAlSiPSCl

However, these early attempts had limitations and did not reliably categorise all known elements.

Mendeleev's periodic table

In 1869, Dmitri Mendeleev created a better periodic table by:

  • Primarily organising elements by increasing atomic mass
  • Swapping the order of some elements to keep those with similar properties in the same group
  • Leaving gaps where elements should fit based on trends in properties
  • Predicting new elements to fit in these gaps


When the predicted elements were discovered and found to fit his table, it confirmed that Mendeleev had arranged elements logically based on their properties. His decision to prioritise properties over strict atomic mass order was later validated by the discovery of isotopes, which explained why some elements had different atomic masses but similar chemical behavior.

Group 1Group 2Group 3Group 4Group 5Group 6Group 7
Period 1H
Period 2LiBeBCNOF
Period 3NaMgAlSiPSCl
Period 4K, CuCa, Zn*, *Ti, *V, AsCr, SeMn, Br
Period 5Rb, AgSr, CdY, InZr, SnNb, SbMo, Te*, I

The positions marked * in the periodic table represent gaps that Mendeleev left for elements that had not yet been discovered at the time he created his periodic table.

Moseley improved the table based on atomic number

In 1914, Henry Moseley reorganised the periodic table by:

  • Arranging elements in order of increasing atomic (proton) number.
  • This properly sequenced the elements.

The modern periodic table is based on Moseley's version organised by atomic number.

Periodic table structure

The periodic table is organised into periods (rows) and groups (columns):

  • Periods: Elements in the same period have the same number of electron shells. As you move down a period, the number of electron shells increases.
  • Groups: Elements in the same group have similar electron configurations in their outer shell. As a result, they exhibit similar physical and chemical properties.


The periodic table can be further divided into three blocks based on the type of subshell being filled in the outermost electron shell:

Diagram showing the structure of the periodic table with s-block, d-block, and p-block elements.
  1. s-block: Elements in the s-block have their outermost electrons in an s orbital.

For example, the electronic configuration of lithium (Li) is 1s2 2s1.


  1. p-block: Elements in the p-block have their outermost electrons in an p orbital.

For example, the electronic configuration of oxygen (O) is 1s2 2s2 2p4.


  1. d-block: Elements in the d-block have their outermost electron in a d orbital.

For example, the electronic configuration of iron (Fe) is 1s2 2s2 2p6 3s2 3p6 4s2 3d6.

Determining electron configurations

You can use the table to deduce the electron configuration of elements:

  1. Identify the period - This shows how many electron shells
  2. Note which block it falls in - This shows which subshell is being filled in the outer electron shell.
  3. Write out the complete configuration by building up from the 1s orbital

Worked example 1 - Determining the electron configuration of vanadium

Write the electronic configuration of an atom of vanadium, V.


Step 1: Identify which period vanadium belongs to

Vanadium is located in period 4, so it has 4 electron shells.


Step 2: Identify which block vanadium belongs to

Vanadium is part of the d-block, so its outer shell includes a partially filled d subshell.


Step 3: Write the complete electronic configuration

The complete configuration is:

Period 1:  1s2

Period 2:  2s2p6

Period 3:  3s3p6

Period 4:  4s3d3

Therefore, the electron configuration for vanadium is 1s2 2s2 2p6 3s2 3p6 4s2 3d3.

The abbreviated configuration is [Ar] 4s2 3d3.