What you'll learn
The periodic table we use today took decades to develop, and the breakthrough came from a scientist who was willing to leave gaps for elements nobody had found yet. For AQA GCSE Chemistry you need to know how early attempts at classification worked, why Newlands' law of octaves failed, how Mendeleev overcame those problems, and why the table was later reordered by atomic number once protons were discovered. This guide explains each stage, the reasoning behind Mendeleev's gaps and swaps, and how the modern table is arranged. By the end you should be able to explain why Mendeleev's table was accepted and how the discovery of isotopes and protons completed the story.
Key terms and definitions
Periodic table — A table of all the elements arranged in order of atomic number, in groups and periods.
Atomic weight (relative atomic mass) — The mass of an atom compared with other atoms; used to order the earliest tables.
Atomic number — The number of protons in the nucleus of an atom; the basis for ordering the modern table.
Group — A vertical column in the periodic table; elements in a group have similar chemical properties.
Period — A horizontal row in the periodic table.
Law of octaves — Newlands' idea that every eighth element had similar properties.
Isotope — Atoms of the same element with the same number of protons but different numbers of neutrons.
Prediction — A statement about what will be found, used to test whether a scientific idea is correct.
Core concepts
Early attempts at classification
In the early 1800s, scientists knew of many elements but had no system to organise them. The only property they could measure reliably was atomic weight, so the first attempts arranged the elements in order of increasing atomic weight. At this time, protons had not been discovered and many elements were still unknown, which made a complete and correct arrangement very difficult.
Newlands' law of octaves (1864)
John Newlands arranged the known elements in order of atomic weight and noticed that every eighth element seemed to have similar properties, which he called the law of octaves — an analogy with musical scales. His idea contained an important truth about repeating patterns, but it had serious problems:
- He assumed all elements had been discovered and left no gaps.
- To keep his pattern, he sometimes placed two elements in the same box.
- As a result, some elements with very different properties ended up grouped together — for example, metals were placed with non-metals.
Because his groups contained elements that clearly did not belong together, other scientists rejected his work.
Mendeleev's table (1869)
Dmitri Mendeleev also arranged the elements in order of atomic weight, but he made two crucial decisions that made his table work:
- He left gaps for elements that had not yet been discovered, rather than forcing the known elements to fit.
- He occasionally swapped the order of elements, placing an element out of strict atomic-weight order so that it sat with others of similar properties.
These choices meant that elements with similar chemical properties lined up in the same groups. Mendeleev then used the gaps to predict the properties of the missing elements, including their atomic weights and how they would react.
Why Mendeleev's table was accepted
When the missing elements were later discovered — such as germanium and gallium — their properties matched Mendeleev's predictions closely. This was powerful evidence that his arrangement reflected something real about the elements, and his table became widely accepted. It is a good example of how a scientific idea gains acceptance: it makes testable predictions, and those predictions turn out to be correct.
The problem of the swaps
Mendeleev could not explain why some elements had to be swapped out of atomic-weight order. The answer came later with the discovery of isotopes. Because an element's relative atomic mass is an average across its isotopes, an element with a large proportion of heavier isotopes can have a higher atomic weight than the element that should follow it. Ordering by atomic weight therefore occasionally puts elements in the wrong place.
Ordering by atomic number
When protons were discovered in the early twentieth century, scientists could order the elements by atomic number — the number of protons — instead of atomic weight. This removed the need for swaps entirely: ordering by atomic number automatically placed every element in a group with elements of similar properties. The modern periodic table is arranged in order of increasing atomic number, with elements in the same group having the same number of electrons in their outer shell, which is why they react in similar ways.
The modern table
Today the table has groups (columns) of elements with similar properties and periods (rows) showing repeating patterns. Metals are on the left and non-metals on the right, and the noble gases — which were unknown to Mendeleev because they are so unreactive — form Group 0 on the far right. Their later discovery did not break his table, because there were places for them.
Worked examples
Example 1: Why Newlands' work was rejected
Give two reasons why other scientists did not accept Newlands' law of octaves. First, he left no gaps for undiscovered elements, so later elements could not be fitted in. Second, to keep his pattern he grouped elements with very different properties together, such as placing metals alongside non-metals, which made the groups meaningless.
Example 2: Explaining the gaps
Explain why leaving gaps was so important in Mendeleev's table. Leaving gaps meant known elements were not forced into the wrong groups, so each group genuinely contained elements with similar properties. The gaps also allowed Mendeleev to predict the properties of undiscovered elements, and when those elements were found and matched his predictions, it provided strong evidence that his table was correct.
Example 3: Explaining a swap
Tellurium has a higher atomic weight than iodine, yet Mendeleev placed tellurium first. Explain why this was correct. Tellurium's properties match the group it was placed in, so ordering by properties required the swap. We now know this happens because relative atomic mass is an average over isotopes, and tellurium has a higher proportion of heavy isotopes. Ordering by atomic number puts them in the correct order automatically.
Example 4: The effect of discovering protons
How did the discovery of protons change the periodic table? It allowed elements to be ordered by atomic number instead of atomic weight. This placed every element in the correct group without any need for swapping, and explained why elements in the same group behave similarly — they have the same number of outer-shell electrons.
Common mistakes and how to avoid them
A frequent mistake is saying Mendeleev ordered his table by atomic number. He could not — protons had not been discovered. He used atomic weight, with gaps and occasional swaps.
Students often mention the gaps but not the predictions. The gaps mattered most because they let Mendeleev predict properties of undiscovered elements, and the confirmation of those predictions is what convinced other scientists.
Another error is describing Newlands' work as simply "wrong". It correctly spotted a repeating pattern; it failed because of no gaps and forced pairings that put dissimilar elements together. Give the specific reasons.
Be careful with the isotope explanation. Isotopes explain why ordering by atomic weight occasionally gives the wrong sequence — they are not the reason the modern table works. The modern table works because it is ordered by atomic number.
Finally, do not say Mendeleev "knew about" the noble gases. They had not been discovered; their later discovery fitted his structure without breaking it.
Exam technique for "Development of the periodic table"
Questions here usually ask you to compare early tables with the modern one, or to explain why a particular scientist's work was accepted or rejected. Structure your answer around three things: what the scientist did, what problem it had or solved, and what evidence settled it.
For Mendeleev, the marking points are almost always: ordered by atomic weight, left gaps for undiscovered elements, swapped some elements so properties matched, and made predictions that were later confirmed. Learn those four as a set.
When asked about the modern table, mention ordering by atomic number and link similar properties in a group to the same number of outer-shell electrons. This connects the history to the chemistry and often earns the final mark in extended answers.
Quick revision summary
- Early tables were ordered by atomic weight, because protons were unknown.
- Newlands' law of octaves spotted a repeating pattern but left no gaps and forced dissimilar elements together, so it was rejected.
- Mendeleev left gaps for undiscovered elements and swapped some out of weight order so properties matched.
- He predicted the properties of missing elements; when they were discovered and matched, his table was accepted.
- Isotopes explain why atomic-weight order sometimes needs swapping.
- The modern table is ordered by atomic number; elements in a group share the same number of outer-shell electrons.