Electronic Structure (OCR Gateway GCSE Chemistry: Combined Science)

How the Periodic Table links to Electronic Structure

• There are over 100 chemical elements which have been isolated and identified
• Elements are arranged on the periodic table in order of increasing atomic number
  • Each element has one proton more than the element preceding it
  • This means that each element also starts with one electron more than the element preceding as a neutral atom starts with a number of electrons equal to the number of protons
• The table is arranged in vertical columns called groups and in rows called periods
  • Periods:
    • These are the horizontal rows that show the number of shells of electrons an atom has
    • They are numbered from 1 - 7
    • For example, elements in Period 2 have two electron shells, elements in Period 3 have three electron shells
  • Groups:
    • These are the vertical columns that show how many outer electrons each atom has
    • They are numbered from 1 - 7, with a final group called group 0 (instead of Group 8)
    • For example, Group 4 elements have atoms with 4 electrons in the outermost shell, Group 6 elements have atoms with 6 electrons in the outermost shell
    • Many groups also have alternative names. Group 1 is also known as the 'alkali metals', Group 7 is also known as the 'halogens' and Group 0 is also known as the 'noble gases'

Electron Shell Diagrams

The electron shells

• We can represent the electronic structure of atoms using electron shell diagrams
• Electrons orbit the nucleus in shells and each shell has a different amount of energy associated with it
  • The further away from the nucleus, the more energy a shell has, and the harder the shell is to fill
  • Electrons first occupy the shell closest to the nucleus which can hold a maximum of 2 electrons
  • When a shell becomes full of electrons, additional electrons have to be added to the next shell
  • The second shell and third shell can hold a maximum of 8 electrons each
  • The outermost shell of an atom is called the valence shell and an atom is much more stable if it can manage to completely fill this shell with electrons
• In most atoms, the outermost shell is not full. This means that these atoms react with other atoms in order to achieve a full outer shell of electrons in order to make them more stable
• • Atoms can gain electrons in order to complete their existing outer shell - non-metals do this - to form anions
  • Atoms can lose electrons to entirely empty this outer shell so that the next shell below becomes the (full) outer shell - metals do this - to form cations
  • Atoms can also share electrons to gain full outer shells - to form covalent bonds

Electron Configuration - Number Notation

• The arrangement of electrons in shells can also be explained using numbers
• This method consists of writing down the number of electrons in each shell and separating each number with a comma or full stop, starting with the innermost shell that is closest to the nucleus
• There is a clear relationship between the number of outer shell electrons and the group number - Group 2 elements have 2 electrons in the outer shell
• How many numbers are present in the electronic configuration will show the number of shells of electrons the atom has, showing the period the element is in - 2.8.2 would mean that the element was in Period 3 (and Group 2)
• Elements in the same group have the same number of outer shell electrons
  • Therefore electronic structure of 2.2 and 2.8.2 and 2.8.8.2 all show Group 2 elements as they all end in '2'

The electronic configuration for chlorine

• Period: The red numbers at the bottom show the number of notations which is 3, showing that a chlorine atom has 3 shells of electrons
• Group: The last notation, in this case 7, shows that chlorine has 7 outer electrons, and therefore is in group 7

The position of chlorine on the Periodic Table

Electronic Configuration of the First 20 Elements Table