The Periodic Table (Oxford AQA IGCSE Combined Science Double Award)

How the Elements are Ordered

How the Periodic Table has developed

• In 1869, Russian chemist Dmitri Mendeleev created his first draft of the Periodic Table

• He organised elements into columns based on their properties and the properties of their compounds

• He then arranged them horizontally in order of increasing atomic weight

• He noticed a pattern appearing where chemically similar elements fell into the same columns

  • There were exceptions, as some elements didn't fit the pattern of increasing atomic weight

• Mendeleev worked to include all the elements, but:

  • He didn't force an element to fit the pattern, instead he left gaps for elements that had not yet been discovered

  • He also switched the order of the elements to maintain consistency down the columns

Mendeleev's Periodic Table

• He used the properties and trends of other elements in the group with the gap to predict the properties of undiscovered elements

• Elements were later discovered that fitted Mendeleev's pattern and confirmed his theories

  • For example, Mendeleev predicted an element called “eka-silicon”

  • This element is now known as germanium

The current Periodic Table

• Over 100 chemical elements have been isolated and identified

• The Periodic Table arranges elements in order of increasing atomic number

  • Each element has one proton more than the element preceding it

  • This is done so that elements end up in columns with other elements which have similar properties

• The table is arranged in:

  1. Vertical columns called groups

  2. Rows called periods

• The name Periodic Table comes from these groups and periods

  • It reflects that there is a regular, or periodic, pattern of elements with similar properties

The Periodic Table

Periods

• The periods / horizontal rows on the Periodic Table are numbered from 1 - 7

  • Note: Period numbers are not always shown on the Periodic Table

• They show the number of energy levels / electron shells an atom has:

  • Atoms of a Period 2 element have two energy levels / electron shells

  • Atoms of a Period 3 element have three energy levels / electron shells

Groups

• The groups / vertical columns on the Periodic Table are numbered from 1 - 7, with Group 0 at the end

  • Note: The final group is normally called Group 0 instead of Group 8

• These group numbers show how many outer electrons / valence electrons an atom has:

  • Atoms of a Group 4 element have 4 electrons in their highest energy level / outermost shell

  • Atoms of a Group 6 element have 6 electrons in their highest energy level / outermost shell

• The number of outer electrons determines the chemistry of the element

  • So, all elements in a group have similar chemical properties

Group 0 - the Noble gases

• The elements in Group 0 of the Periodic Table are called the Noble gases

• The Noble gases are all non-metals that exist as single atoms

Noble Gases on the Periodic Table

• All of the Noble gases have a full highest energy level / outer electron shell

  • Electronic configurations of the Noble gases:

    • He = 2

    • Ne = 2, 8

    • Ar =  2, 8, 8

    • Kr =  2, 8, 18, 8

    • Xe = 2, 8, 18, 18, 8

• These electronic configurations make the Noble gases extremely stable and unreactive

  • This is because elements react to achieve a full / complete outer shell by losing, gaining, or sharing electrons

  • Group 0 elements do not need to do this as their outer shells are already full