C2.1 Development of The Periodic Table

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: from lists of atomic weights to Mendeleev’s gaps and the modern table ordered by atomic number. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Describe how early periodic tables ordered the elements by atomic weight, and why they were incomplete
• Explain how Mendeleev overcame these problems by leaving gaps and changing the order of some elements
• Use Mendeleev’s approach to predict the properties of an undiscovered element
• Explain how the discovery of protons and isotopes showed why atomic weight order is not always right
• Explain how confirmed predictions give strong support to a scientific idea

Key words

• Atomic weight: The old name for the mass of an atom compared with other atoms. Today we call it relative atomic mass, Ar.
• Group: A vertical column of the periodic table. Elements in a group have similar chemical properties.
• Period: A horizontal row of the periodic table.
• Law of octaves: Newlands’ idea that, in order of atomic weight, every eighth element has similar properties.
• Atomic number: The number of protons in the nucleus of an atom. The modern periodic table is in order of atomic number.
• Periodic: Repeating at regular intervals. Similar properties turn up again and again in a regular pattern.
• Isotopes: Atoms of the same element with the same number of protons but different numbers of neutrons.

Ordering the elements by atomic weight

In the early 1800s chemists learned to measure the atomic weights of elements. Protons, neutrons and electrons had not been discovered, so atomic weight was the only sensible way to put the elements in a list. Some elements had not been found yet, and a few substances wrongly thought to be elements were really compounds, so every early table was incomplete.

Chemists also noticed families of elements with similar reactions. In 1829 Döbereiner described triads: three similar elements where the middle atomic weight is close to the mean of the other two.

Worked example: testing a triad

Chlorine, bromine and iodine are all coloured, reactive non-metals. Atomic weights: Cl = 35.5, I = 127.
Mean of chlorine and iodine = (35.5 + 127) ÷ 2 = 162.5 ÷ 2 = 81.25. The atomic weight of bromine is 80.

Answer: 81.25 is very close to 80, so Cl, Br, I behaves as a triad

Newlands and the law of octaves

In 1864–65 John Newlands arranged the known elements in order of atomic weight. He spotted that every eighth element seemed similar, like notes repeating in a musical scale.

His idea was not accepted, because:

• the pattern only worked for the lighter elements; after calcium, columns mixed very different elements (the metal iron with oxygen and sulfur; copper with lithium, sodium and potassium)
• he left no gaps for elements that had not been discovered
• he sometimes squeezed two elements into one box to keep the pattern going

Mendeleev’s breakthrough

In 1869 Dmitri Mendeleev also began with atomic weight order, but he let the properties of the elements decide where they went:

• he left gaps where no known element fitted, saying these elements were still to be discovered
• he swapped the order of a few pairs so that elements sat with others that behaved the same way (tellurium before iodine)
• he used the gaps to predict the properties of the missing elements

Worked example: predicting like Mendeleev

Estimate the atomic weight of the missing element between silicon (28) and tin (119) in Group 4.
Mean of the elements above and below = (28 + 119) ÷ 2 = 147 ÷ 2 = 73.5

Answer: about 73 to 74. Germanium (found 1886) has Ar = 72.6 – a very good match

Key fact: Gallium (1875), scandium (1879) and germanium (1886) were discovered with properties very close to Mendeleev’s predictions. This convinced other scientists that his table was right.

Mendeleev did not know about the noble gases. When they were discovered in the 1890s they fitted neatly into a new group (Group 0) without changing the rest of his table.

Why predictions matter. An idea that only explains facts already known is weaker than one that makes predictions that can be tested. If later results match, the idea is supported; if they do not, the idea is refuted and must be changed or dropped.

The modern periodic table

Once protons were discovered, the elements were arranged in order of atomic number (number of protons). This automatically places tellurium (52) before iodine (53), so no special swap is needed.

Isotopes explain the odd atomic weights. Most tellurium atoms are heavy isotopes, so its relative atomic mass (127.6) is higher than iodine’s (126.9) even though it has fewer protons. Argon (Ar 39.9, 18 protons) and potassium (Ar 39.1, 19 protons) are another pair like this.

Exam tip: Elements in the same group have the same number of electrons in their outer shell, which is why they react in similar ways. Mendeleev did not know this, but electronic structure explains his groups (see C2.2).

Common mistakes

• Saying Mendeleev ordered elements by atomic number. He used atomic weight; protons were not known in 1869.
• Saying the modern table is in order of relative atomic mass. It is in order of atomic (proton) number.
• Forgetting why the predictions mattered: newly found elements matched them, so scientists accepted the table.
• Saying a confirmed prediction “proves” an idea. Matching results support it.
• Saying Newlands left gaps. He did not; leaving gaps was Mendeleev’s idea.

Revision questions

Try these, then check your answers below each one.

Question 1. Why did Mendeleev place tellurium (atomic weight 127.6) before iodine (atomic weight 126.9)? (Choose from: So that each element sat in a group of elements with similar properties, Because tellurium was discovered first, Because he ordered the elements by atomic number, Because iodine is a gas and tellurium is a solid.)

Answer: So that each element sat in a group of elements with similar properties

Working: He put properties first: iodine reacts like chlorine and bromine, and tellurium like oxygen and sulfur. Atomic number had not yet been discovered.

Question 2. True or false? Newlands left gaps in his table for elements that had not been discovered yet.

Answer: False

Working: False. Newlands left no gaps, so some columns contained very different elements. Leaving gaps was Mendeleev’s idea.

Question 3. The diagram shows argon and potassium. Why does argon come before potassium in the modern periodic table, even though its relative atomic mass is higher? (Choose from: Argon has fewer protons in its nucleus, Argon has fewer neutrons in its nucleus, Argon is a gas and potassium is a metal, Argon was discovered before potassium.)

Answer: Argon has fewer protons in its nucleus

Working: The modern table is in order of atomic number (protons): argon 18, potassium 19. Argon atoms are mostly a heavy isotope, which makes its relative atomic mass higher.

Related lessons

• Previous lesson: C1.8 Electronic Structures
• Next lesson: C2.2 Electronic Structures and The Periodic Table
• In this chapter: C2.3 Group 1 – The Alkali Metals
• In this chapter: C2.4 Group 7 – The Halogens
• In this chapter: C2.5 Explaining Trends

Continue in the full course

This lesson is section C2.1 (AGCSC1901) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.