You are viewing a free preview of this lesson.
Subscribe to unlock all 10 lessons in this course and every other course on LearningBro.
The periodic table is the single most important organising idea in chemistry: a chart that arranges all the elements so that those with similar properties fall into the same column. But it did not arrive fully formed. It is the product of nineteenth-century chemists wrestling with incomplete data, and the story of how it was built — especially the leap of insight by Dmitri Mendeleev — is itself examined in Topic C2 of OCR Gateway Science A. This lesson traces that development, explains why the table is now ordered by atomic number rather than atomic weight, and shows how an element's position tells you about its properties.
By the end of this lesson you should be able to describe how the periodic table developed (early attempts, Newlands, Mendeleev), explain why Mendeleev left gaps and made predictions, explain why the modern table is ordered by atomic number, and use an element's position (group and period) to deduce its properties.
In the early 1800s only the atom was thought to define an element, and the only property chemists could measure to compare elements was their relative atomic weight. So the first attempts to organise the elements simply put them in order of increasing atomic weight. The trouble was that no one yet knew about protons, electrons or the structure of the atom, so weight was the only handle they had.
In 1864 John Newlands noticed that, when the known elements were listed in order of atomic weight, every eighth element seemed to have similar properties — rather like the repeating notes of a musical scale. He called this the law of octaves. It was a genuine insight that properties repeat periodically, and it worked for the lighter elements.
But Newlands' arrangement had serious flaws:
Because of these problems his ideas were not widely accepted at the time.
Exam Tip: The key fault of Newlands' octaves is that he left no gaps, so he had to force elements into groups and ended up with dissimilar elements together; the pattern also broke down for heavier elements.
In 1869 Dmitri Mendeleev produced a table that overcame these problems and is recognised as the ancestor of the modern one. He, too, began by ordering the elements by atomic weight, but he did two things that made his table work where Newlands' had failed.
He left gaps. Mendeleev realised that not all the elements had been discovered yet, so he left gaps in his table where he thought an element was missing. This meant the known elements could line up with others of genuinely similar properties, instead of being forced into the wrong group.
He swapped some pairs. Where strictly following atomic weight would put an element in a group with the wrong properties, he changed the order of a few pairs so that each element sat with others it actually resembled — trusting properties over a literal weight order.
The most powerful step was that Mendeleev used the gaps to predict the properties of undiscovered elements. For the gap below silicon he described a missing element he called "eka-silicon", predicting its atomic weight, density and the formula of its oxide. When germanium was discovered in 1886, its properties matched his predictions astonishingly well. The same happened for other gaps. These successful predictions were exactly what convinced chemists his table was correct — a table that predicts is far more than a list.
| Property of "eka-silicon" | Mendeleev predicted | Germanium (actual) |
|---|---|---|
| Approx. atomic weight | about 72 | about 73 |
| Appearance | grey metal | grey metal |
| Oxide formula | XO2 | GeO2 |
Exam Tip: Mendeleev's genius was not just listing elements by weight — it was leaving gaps for undiscovered elements and predicting their properties. When those elements (e.g. germanium) were found and matched, the predictions were strong evidence for his table.
Ordering by atomic weight worked well, but it left a few anomalies — pairs of elements that seemed to be in the "wrong" order, because following weight strictly put an element with the wrong group. Mendeleev had simply swapped these by hand to fit the properties. Why were they out of order?
The answer came in the early twentieth century, once the structure of the atom was understood. The discovery of protons showed that each element has a fixed number of protons — its atomic number — and the discovery of isotopes (atoms of the same element with different numbers of neutrons, and so different masses) explained why atomic weight did not always increase smoothly. When the elements are ordered by atomic number instead of atomic weight, the anomalous pairs fall into their correct places automatically, with no swapping needed.
Two classic examples:
So the modern periodic table is arranged in order of increasing atomic number, which is why Mendeleev's hand-made swaps are no longer necessary — the ordering principle now does the work for us.
Exam Tip: The modern table is ordered by atomic (proton) number, not atomic weight. Ordering by atomic number resolves the anomalies (like argon/potassium) that forced Mendeleev to swap pairs, because isotopes make atomic weight an imperfect guide.
The modern table arranges the elements into groups and periods, and this layout links directly to electronic structure (which you met in C1).
The table also divides into metals and non-metals. Metals are found on the left and centre (the large majority of elements); non-metals are on the right-hand side. A "staircase" line running down the right separates them, with the metals to its left.
Subscribe to continue reading
Get full access to this lesson and all 10 lessons in this course.