The Periodic Table

Development of the Periodic Table

Before scientists understood atomic structure, they tried to classify known elements by atomic mass (atomic weight). The table evolved through several key stages as new evidence emerged — a key example of how science works.

• Before 1860s

  Early Classification Attempts (no specific scientist names required)

  Before the discovery of protons, neutrons and electrons, scientists attempted to classify the elements by arranging them in order of their atomic weights. The early periodic tables were incomplete — not all elements had been discovered — and some elements were placed in inappropriate groups if the strict order of atomic weights was followed.

• 1864

  John Newlands — Law of Octaves (name not required by AQA)

  Newlands arranged the 56 known elements in order of atomic weight and noticed that every eighth element had similar properties — he called this the Law of Octaves. His table was criticised because:

  • The pattern broke down for heavier elements — some groups contained elements with very different properties
  • He mixed up metals and non-metals in the same groups
  • He left no gaps for undiscovered elements, so newly found elements did not fit
• 1869

  Dmitri Mendeleev name required

  Mendeleev overcame some of the problems with earlier tables by making two key changes:

  • He left gaps for elements he thought had not yet been discovered
  • In some places he changed the order based on atomic weights so that elements with similar chemical properties stayed in the same group

  Mendeleev was able to predict the properties of undiscovered elements from patterns in his table. Elements with the properties he had predicted were subsequently discovered and filled the gaps (e.g. gallium, germanium, scandium) — providing strong evidence for his arrangement, and leading to its acceptance.

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  💡 Exam tip — why Mendeleev's table was accepted

  Make sure you say both parts: the elements fitted the gaps he left, and their properties matched his predictions.

• Late 1800s – early 1900s

  Discovery of Subatomic Particles (see atomic model timeline — names required there)

  The discovery of electrons, protons and neutrons (covered in Section 5) revealed why elements in the same group behave similarly — they have the same number of outer shell electrons. This gave the periodic table a deeper theoretical basis.

• Modern

  The Modern Periodic Table — Ordered by Atomic Number (no specific scientist name required)

  In the modern periodic table, elements are arranged in order of atomic number (number of protons), not atomic mass. This resolves the occasional inconsistencies in Mendeleev's arrangement.

  ✅ The Role of Isotopes

  Knowledge of isotopes made it possible to explain why the order based on atomic weights was not always correct. Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons — they have different masses but belong in the same position on the table. Because of isotopes, average atomic mass does not always increase in the same order as atomic number.

  Example: argon (A_r = 39.9, atomic number 18) comes before potassium (A_r = 39.1, atomic number 19). Strict atomic mass ordering would swap them — placing potassium in the wrong group. Ordering by atomic number resolves this.

Groups and Periods in the Modern Table

Metals and Non-metals

Elements that react to form positive ions are metals. Elements that do not form positive ions are non-metals. The majority of elements are metals. Metals are found to the left and towards the bottom of the periodic table; non-metals are found towards the right and top.

The position of an element in the periodic table is linked to its atomic structure. Metals have few outer-shell electrons (typically 1–3) and tend to lose them, forming positive ions. Non-metals have more outer-shell electrons (typically 4–7) and tend to gain or share electrons.

Because reactivity depends on how readily an element gains or loses electrons, the reactions of elements are directly related to the arrangement of electrons in their atoms — and therefore to their atomic number and position in the table.