Mendeleev's Periodic Table

Introduction

In 1869 the Russian chemist Dmitri Mendeleev produced a table of the elements that changed chemistry. Others had noticed patterns, but Mendeleev did something bolder: he trusted the pattern of properties more than the list of known elements. Where the pattern demanded it, he left gaps for elements nobody had yet found and predicted what they would be like. When those elements were discovered with the properties he had described, his table was accepted around the world.

Core explanation

Ordering by atomic mass. Like Newlands, Mendeleev began by listing the roughly 63 known elements in order of increasing atomic mass. Atomic number had not yet been discovered, so mass was the best ordering tool available.

Grouping by properties. Mendeleev then arranged the elements so that those with similar chemical properties fell into the same vertical group. He paid special attention to the formulae of their oxides and chlorides. For example, elements that formed oxides with the general formula R₂O were placed together, as were those forming RO₂.

Leaving gaps. Sometimes the next element by mass did not fit the properties of the next group. Rather than forcing it in, Mendeleev left a gap and moved the element along to the group where it matched. He argued that each gap belonged to an element still waiting to be discovered.

Predicting new elements. Mendeleev used the properties of neighbouring elements to predict the properties of the missing ones. He named them using the prefix "eka-", meaning "one beyond": eka-aluminium, eka-boron and eka-silicon.

Predicted Discovered as Year --- --- --- Eka-aluminium Gallium 1875 Eka-boron Scandium 1879 Eka-silicon Germanium 1886

Swapping pairs. In a few places Mendeleev reversed the order by mass. Tellurium (relative atomic mass 127.6) is heavier than iodine (126.9), yet iodine clearly belongs with chlorine and bromine, and tellurium with sulfur and selenium. Mendeleev placed tellurium first, suspecting its mass had been measured wrongly. In fact the masses were correct; the true explanation came later with atomic number.

Why it was accepted. A theory is convincing when it makes predictions that come true. The discovery of gallium, scandium and germanium, each with properties close to his forecasts, persuaded chemists that Mendeleev's table reflected something real about nature. Newlands' octaves had made no such testable predictions.

What it lacked. Mendeleev's table had no group for the noble gases, which were unknown in 1869. When argon was discovered in 1894, a whole new group was added — and it fitted neatly, which further strengthened the table.

Step-by-step reasoning

How Mendeleev placed an element:

1. Take the next element in order of atomic mass. 2. Compare its properties and oxide formula with each group. 3. If it matches the next group, place it there. 4. If it does not, leave a gap and place it in the group it matches. 5. Use the neighbours of each gap to predict the missing element's properties.

Visual explanation

Picture a bookshelf where each shelf holds one type of book. Working through a pile sorted by weight, you place each book on the right shelf. When a shelf is skipped because the next book belongs higher up, a visible empty space remains, clearly waiting for a book you have not yet bought.

Real-world analogy

A football sticker album has numbered spaces. Even before you own sticker 42, you know from the players on either side roughly what it shows. Mendeleev treated his table like an album with missing stickers and described the missing pictures before they were found.

Real-world example

Gallium, one of Mendeleev's predicted elements, is now used to make gallium arsenide and gallium nitride, semiconductors found in LEDs, laser diodes and mobile-phone electronics. An element once known only as a gap in a table is now in billions of devices.

Why?

Why was leaving gaps such a clever decision? Because it put the pattern of properties first. By refusing to break a group just to keep the mass order unbroken, Mendeleev kept similar elements together, and the table made sense. The gaps also turned his table into a set of testable predictions.

Common misconception

"Mendeleev ordered the elements strictly by atomic mass." He mostly did, but he deliberately broke the mass order when properties demanded it, as with tellurium and iodine, and he left gaps rather than pushing elements into the wrong group.

Worked example

Question: Mendeleev predicted eka-silicon would have a relative atomic mass of about 72 and a density of about 5.5 g/cm³. Germanium has a relative atomic mass of 72.6 and a density of 5.35 g/cm³. Calculate the percentage error in his density prediction.

Reasoning: Error = 5.5 − 5.35 = 0.15 g/cm³. Percentage error = 0.15 ÷ 5.35 × 100 ≈ 2.8%.

Answer: About 3% — a remarkably accurate prediction.

Quick check

1. Why did Mendeleev place tellurium before iodine even though tellurium has the greater atomic mass? Answer: Because their properties matched the groups in that order: iodine resembles chlorine and bromine, and tellurium resembles sulfur and selenium.

Exam focus

Common questions ask why Mendeleev's table was accepted. Key points: he left gaps for undiscovered elements, predicted their properties, and these predictions were confirmed when gallium, scandium and germanium were discovered. Also mention that he swapped some elements to keep groups of similar properties together.

Advanced insight

Mendeleev was not alone. The German chemist Lothar Meyer published a similar table in 1870, based largely on physical properties such as atomic volume. Mendeleev is usually given the greater credit because he made bold, specific predictions that were later confirmed — a good example of how testable predictions earn a theory scientific respect.

Summary

Mendeleev arranged the elements in order of atomic mass but grouped them by similar properties. He left gaps for undiscovered elements, predicted their properties, and swapped some pairs such as tellurium and iodine to keep groups consistent. The discovery of gallium, scandium and germanium with the predicted properties led to his table being widely accepted.

Practice questions

1. What property did Mendeleev use to order the elements? Answer: Atomic mass (relative atomic mass). 2. Why did Mendeleev leave gaps in his table? Answer: Because the next element by mass did not match the properties of the next group; he believed the gaps belonged to elements that had not yet been discovered. 3. Name one element discovered after Mendeleev predicted it, and give the name he used. Answer: Germanium, which he called eka-silicon (or gallium as eka-aluminium, or scandium as eka-boron). 4. Explain why Mendeleev's table was accepted, whereas Newlands' octaves were not. Answer: Mendeleev's table made predictions that were confirmed by later discoveries, and it kept similar elements together by leaving gaps; Newlands left no gaps and forced unlike elements into the same rows.