Development of the periodic table: Before the discovery of protons, neutrons and electrons, scientists attempted to classify the elements by arranging them in order of their atomic weights.

The idea

Before protons, neutrons and electrons were discovered, scientists had no concept of atomic number. They tried to classify elements by arranging them in order of their atomic weights, which they could measure from chemical reactions. Early attempts, such as Newlands' law of octaves, forced elements into groups of eight and left no gaps, so they failed when new elements were found. Mendeleev's periodic table was more successful because he arranged elements mainly by atomic weight but left gaps for undiscovered elements and placed some elements out of strict weight order to keep similar properties together. For example, he predicted the properties of gallium and germanium, which were later discovered and matched his predictions. The modern periodic table orders elements by atomic number, which explains the patterns that early scientists could only observe.

What earns the marks

• State that before protons, neutrons and electrons were discovered, scientists had no knowledge of atomic number.
• Describe how early classification attempts arranged elements in order of their atomic weights.
• Explain that early tables, such as Newlands' law of octaves, forced elements into groups and left no gaps, causing problems when new elements were discovered.
• Describe Mendeleev's approach: arranging elements mainly by atomic weight but leaving gaps for undiscovered elements and sometimes reversing the order to keep similar properties together.
• Explain that Mendeleev's predictions of the properties of missing elements, such as gallium and germanium, were later confirmed, which supported his table.
• Recognise that the modern periodic table orders elements by atomic number, which resolved the inconsistencies in early weight-ordered tables.

Common mistakes and how to put them right

• Thinking Mendeleev arranged elements strictly by atomic weight.

  Put it right by stating that he left gaps and sometimes placed elements out of weight order to group similar properties together.

• Believing early scientists knew about protons and neutrons.

  Put it right by explaining that these subatomic particles were discovered later, so early tables could not use atomic number.

• Assuming all early periodic tables were equally successful.

  Put it right by contrasting Newlands' table, which left no gaps, with Mendeleev's table, which left gaps and made successful predictions.

• Confusing atomic weight with atomic number.

  Put it right by defining atomic weight as the mass of an atom relative to a standard, while atomic number is the number of protons in the nucleus.

Exam tips

• Name the scientist and describe the specific feature of their table, such as Mendeleev leaving gaps, rather than making a general statement.
• Explain why a feature was an advantage or disadvantage, for example gaps allowed predictions of new elements to be made and tested.
• Use a named example, such as gallium or germanium, to show how Mendeleev's predictions were confirmed.
• Link the development of the table to the discovery of subatomic particles, showing how atomic number replaced atomic weight as the ordering principle.