Less common C2.2

Arrangement of the Periodic Table

The modern Periodic Table arranges elements based on one fundamental property: their atomic number. This sequential ordering is the foundation for the table's structure of periods and groups, which in turn reveals patterns in element properties.

Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.

Key points

  • Elements in the Periodic Table are listed in order of increasing atomic number (Z).
  • The atomic number represents the number of protons in an atom's nucleus.
  • This ordering means that each element has one more proton than the element preceding it.
  • This principle replaced earlier models which attempted to order elements by atomic mass.

Diagram

Periodic table snippetOutline of the main groups with highlighted positions. Groups I - VIII
Periodic table outline showing elements ordered by increasing atomic number from left to right across periods and down groups. The layout visually represents the sequential organization principle.

Definitions

Atomic Number (Z)
The number of protons found in the nucleus of an atom. It is the unique whole number that identifies a chemical element.

Worked example

An atom of element X has 12 protons. An atom of element Y has a mass number of 23 and 12 neutrons. An atom of element Z has 13 electrons in its neutral state. What is the correct order of these elements as they would appear from left to right in the Periodic Table?

  1. 1

    The Periodic Table arranges elements by increasing atomic number, which is the number of protons.

  2. 2

    Determine the number of protons for each element.

  3. 3

    Element X:

    Given directly as 12 protons.

    So, Z = 12
  4. 4

    Element Y:

    Atomic number = Mass number - number of neutrons.

    Z = 23 - 12 = 11
  5. 5

    Element Z:

    In a neutral atom, the number of electrons equals the number of protons.

    Z = 13
  6. 6

    Arrange the elements in order of increasing atomic number (proton number):

    Y (11), X (12), Z (13).

Answer: Y, X, Z

Common mistakes

  • ×Mistaking atomic number for mass number. The table is ordered by the number of protons (atomic number), not the total number of protons and neutrons (mass number). For example, Argon (Z=18, Mass≈40) appears before Potassium (Z=19, Mass≈39) despite having a higher average atomic mass.

No-calculator tips

  • ✓You can instantly determine the number of protons (and electrons in a neutral atom) for any element just by finding its position. No calculation is needed. For example, the 15th element in the table is Phosphorus, which must have 15 protons.

Read this topic in the official UAT-UK ESAT guide →

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