Atomic Number & The Periodic Table (AQA GCSE Combined Science: Synergy: Physical Sciences): Revision Note

Arrangement by atomic number

• Over 100 chemical elements 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 groups elements with similar chemical properties into the same vertical columns

Groups and periods

• The table is arranged in:

  • Vertical columns called groups 

  • Horizontal rows called periods

Groups

• The groups are numbered from 1 – 7, with a final group called Group 0 (instead of Group 8)

• The group number indicates how many electrons an atom has in its outer shell

  • For example, elements in Group 2 have 2 electrons in their outer shell

Periods

• The periods are numbered from 1 - 7

• The period number indicates the number of electron shells that an atom has

  • For example, elements in Period 3 have 3 electron shells

How does the electronic structure of an element relate to its location in the Periodic Table?

• There is a clear relationship between the electronic configuration and how the Periodic Table is designed

• The number of notations in the electronic configuration tells us the number of occupied shells

  • This tells us what period an element is in

• The last notation shows the number of outer electrons the atom has

  • This tells us the group an element is in

Electrons and shells

• An element's electronic configuration determines its position in the periodic table

• The number of occupied electron shells tells you which period the element is in

• The number of electrons in the outer shell tells you which group the element is in

• Elements in the same group have the same number of outer shell electrons

  • This is why they have similar chemical properties

• We can represent the structure of the atom in two ways:

  1. Using diagrams called electron shell diagrams 

  2. By writing out a notation called the electronic configuration

Electron shell diagrams

• Electrons orbit the nucleus in shells (or energy levels) and each shell has a different amount of energy associated with it

• The further away from the nucleus, the more energy a shell has

• Electrons fill the shell closest to the nucleus 

• When a shell becomes full of electrons, additional electrons have to be added to the next shell

  • The first shell can hold 2 electrons

  • The second shell can hold 8 electrons 

• For this course, a simplified model is used that suggests that the third shell can hold 8 electrons

  • For the first 20 elements, once the third shell has 8 electrons, the fourth shell begins to fill

• 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 

Electronic configuration

• The arrangement of electrons in shells can also be explained using numbers

• Instead of drawing electron shell diagrams, the number of electrons in each electron shell can be written down, separated by commas

• This notation is called the electronic configuration (or electronic structure)

  • The electronic structure of carbon is 6 electrons, 2 in the 1st shell and 4 in the 2nd shell

    • So its electronic configuration is 2,4

• Electronic configurations can also be written for ions

  • E.g. A sodium atom has 11 electrons, a sodium ion has lost one electron, therefore has 10 electrons; 2 in the first shell and 8 in the 2nd shell

    • Its electronic configuration is 2,8

Diagram showing the relationship between the electronic configurations

• Period: The red numbers at the bottom show the number of notations 

  • The number of notations is 3

  • Therefore chlorine has 3 occupied shells 

• Group: The last notation, in this case 7

  • This means that chlorine has 7 electrons in its outer shell 

  • Chlorine is therefore in Group 7

The Periodic Table showing the location of chlorine 

Mendeleev and the Periodic Table

Early Periodic Tables

• Before subatomic particles were discovered, scientists arranged elements in order of atomic mass

• Patterns appeared at regular intervals

  • This is why we use the term "periodic"

• Some elements were placed in the wrong group because chemical properties were overridden by strict mass ordering

• Tables were incomplete, with gaps that couldn't be filled by known elements

Mendeleev's arrangement

• In 1869, Dmitri Mendeleev organised elements into columns based on their chemical properties

• They were then further arranged horizontally by increasing atomic mass

• Where elements didn't fit, he left gaps rather than forcing them into place

  • He predicted that undiscovered elements would eventually fill the gaps

Mendeleev's Periodic Table

• When those elements were later discovered and matched his predictions, it confirmed his approach

Isotopes and atomic number

• Isotopes were unknown in Mendeleev's time

  • Since the same element can have atoms with different masses, ordering by atomic mass alone created unavoidable anomalies

• Once subatomic particles were discovered, atomic number was calculated for each element

• Reordering by atomic number (rather than mass) resolved the anomalies and gave us the modern periodic table