The Periodic Table

Arrangement of elements

• The periodic table lists every chemical element ever isolated — currently over 100
• Elements are arranged in order of increasing atomic number
• Each successive element has one more proton (and one more electron) than the one before it
• The columns are aligned so that elements with similar chemical behaviour sit above and below each other

Groups and periods

• Periods are the horizontal rows, numbered 1 to 7
• Groups are the vertical columns, numbered 1 to 7, plus Group 0 on the far right (sometimes called Group 8 in older notations; Edexcel uses Group 0)
• Two links between position and electron arrangement (covered in section 2):
  • Period number = number of occupied electron shells
  • Group number = number of electrons in the outer shell (for Groups 1 to 7)
• For example:
  • Period 2 elements have 2 electron shells; Period 3 elements have 3 shells
  • Group 4 elements have 4 outer electrons; Group 6 elements have 6 outer electrons

Atomic number as a fingerprint

• Every element has its own unique atomic number; no two elements share one
• The atomic number does not change during a chemical reaction; only electrons move