Development of the periodic table

This page covers a single Form 4 content standard: the development of the periodic table. It is a short but very examinable topic, because the story of how scientists built the table explains why the modern table is arranged the way it is. Learn the key contributions in order and you will be able to answer both the “who did what” questions and the “why proton number, not atomic mass” question that examiners like to ask.

Why chemists needed a periodic table

By the early nineteenth century, chemists had discovered many elements but had no organised way to arrange them. A good classification would group elements with similar properties together, so that knowing the properties of one element would let you predict the properties of its neighbours. The periodic table is the result of many scientists gradually improving on one another’s ideas until a reliable, predictive arrangement was reached.

The key contributions

You should be able to state, in order, what each scientist contributed:

• Antoine Lavoisier produced one of the first lists of known elements, classifying them broadly into metals and non-metals. This was a useful start but too rough to reveal any repeating pattern.
• Johann Döbereiner noticed that certain elements fell into groups of three with similar chemical properties, which he called triads. In a triad such as lithium, sodium and potassium, the relative atomic mass of the middle element is roughly the average of the other two. This was the first hint that properties were linked to atomic mass.
• John Newlands arranged the known elements in order of increasing atomic mass and found that every eighth element had similar properties, like the notes of a musical scale. He called this the Law of Octaves. It worked for the lighter elements but broke down for heavier ones, so it was not fully accepted at the time.
• Dmitri Mendeleev produced the first widely accepted periodic table in 1869. He arranged the elements in order of increasing atomic mass but placed elements with similar chemical properties in the same vertical column. Crucially, he left gaps for elements that had not yet been discovered and even predicted their properties. When elements such as gallium and germanium were later discovered and matched his predictions, his table was accepted.
• Henry Moseley later showed that the correct ordering property is the proton number (atomic number), not the atomic mass. Arranging elements by increasing proton number removed the few anomalies in Mendeleev’s table.

How the modern table is arranged

The modern periodic table is arranged in order of increasing proton number. Because the number of electrons equals the proton number in a neutral atom, this ordering makes elements with the same number of valence electrons line up in the same vertical group, so they share similar chemical properties. This is why arranging by proton number works so much better than arranging by atomic mass: it reflects the electron arrangement, which is what actually controls chemical behaviour. Mendeleev’s insight, group by properties, was correct; Moseley supplied the property that makes the grouping exact.

Worked example

Question. In the modern periodic table, argon (relative atomic mass 39.9) is placed before potassium (relative atomic mass 39.1). Explain why this order is correct, even though argon has the larger atomic mass.

Step 1, Identify the ordering rule. The modern table is arranged by increasing proton number, not by increasing atomic mass.

Step 2, Compare the proton numbers. Argon has proton number 18; potassium has proton number 19. So by proton number, argon (18) must come before potassium (19).

Step 3, Check against properties. Argon is an unreactive Group 18 noble gas, and potassium is a very reactive Group 1 metal. Placing argon in Group 18 and potassium in Group 1 matches their properties, confirming the proton-number order is correct.

Answer. Argon is placed before potassium because the modern table is ordered by increasing proton number (Ar = 18, K = 19), and this order also puts each element in the group that matches its chemical properties.

Practice question

Question. (a) State the property that Mendeleev used to arrange his periodic table. (b) State the property used to arrange the modern periodic table. (c) Give one reason why Mendeleev left gaps in his table.

Answer. (a) Increasing atomic mass (with elements of similar properties placed in the same column). (b) Increasing proton number (atomic number). (c) He left gaps for elements that had not yet been discovered, and he predicted their properties so the elements could be identified when found.

Exam tip

Examiners often ask you to link a scientist to the specific idea they contributed, so learn the pairs precisely: Döbereiner–triads, Newlands–octaves, Mendeleev–gaps and predictions, Moseley–proton number. The single most common higher-mark question is why the modern table uses proton number rather than atomic mass, the full answer is that proton number orders the elements so that those with the same number of valence electrons fall in the same group, and it removes the anomalies (such as the argon–potassium pair) that appear when atomic mass is used. Always mention electron arrangement in that answer, because that is the reasoning the marking scheme rewards.

Where this fits

This standard opens the The Periodic Table of Elements chapter and sets up everything that follows: once you accept that elements are ordered by proton number, the ideas of groups and periods, and the behaviour of Group 1, Group 17 and Group 18, all follow naturally. Revise the timeline with the revision notes and practise the reasoning questions with the worked examples. In our online 1-to-1 SPM Chemistry lessons, taught in English, from RM50/hr, our teachers make sure you can both recall each scientist’s contribution and explain the proton-number reasoning in full, which is exactly where marks are won on this topic. The whole arrangement underpins the rest of SPM Chemistry, so it is worth a few minutes to get word-perfect.

Quick recap

• Döbereiner grouped elements into triads; Newlands proposed the Law of Octaves.
• Mendeleev arranged elements by increasing atomic mass, grouped by properties, and left gaps with predictions.
• Moseley arranged elements by increasing proton number, which the modern table still uses.
• Proton-number order works because it lines up elements with the same valence electrons in the same group.