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2.1 Development of the Periodic Table

GCSE Chemistry · AQA · Chapter 2: The Periodic Table

A free GCSE Chemistry lesson for AQA (8462) from OBA School: how chemists sorted the elements, from atomic weights to Mendeleev’s gaps and the modern periodic table. 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 scientists ordered the elements by atomic weight, and why their tables were incomplete
  • Explain how Mendeleev overcame these problems by leaving gaps and changing the order in places
  • Use Mendeleev’s method to predict the properties of an undiscovered element
  • Explain how the discovery of protons and isotopes showed why ordering by atomic weight is not always right
  • Describe how the modern periodic table is arranged

Key words

  • Atomic weight: The old name for the mass of an atom compared with other atoms. Today we use 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. In the periodic table, 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.

Sorting the elements by atomic weight

By the early 1800s chemists could measure the atomic weights of many elements. Nobody yet knew about protons, neutrons or electrons, so atomic weight was the obvious way to put elements in order.

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

Worked example: checking a triad

Lithium, sodium and potassium are all soft, reactive metals. Atomic weights: Li = 7, K = 39.
Mean of Li and K = (7 + 39) ÷ 2 = 23. The atomic weight of sodium is 23.

Answer: the middle element’s atomic weight matches the mean, so Li, Na, K is a triad

Newlands and the law of octaves

In 1864–65 John Newlands listed the known elements in order of atomic weight. He noticed that every eighth element seemed to have similar properties, like notes in a musical octave.

Other scientists did not accept his ideas because:

  • the pattern broke down after calcium, so some columns mixed very different elements (for example the metal iron with the non-metals oxygen and sulfur)
  • he left no gaps for elements still to be discovered, and he sometimes put two elements in one box
  • his table was incomplete, so some elements were placed in groups where they did not fit

Mendeleev’s breakthrough

In 1869 Dmitri Mendeleev also started with atomic weights, but he put the properties of the elements first:

  • he left gaps where no known element fitted the pattern, saying these elements had not been discovered yet
  • he changed the order in a few places so that elements sat in groups with similar properties (for example, tellurium before iodine)
  • he used the gaps to predict the properties of the missing elements

Worked example: predicting like Mendeleev

Predict the atomic weight of the element in the gap below silicon. Its neighbours are Si (28) above, Sn (119) below, Ga (70) to the left and As (75) to the right.
Mean of the four neighbours = (28 + 119 + 70 + 75) ÷ 4 = 292 ÷ 4 = 73

Answer: about 73. Germanium was found in 1886 with Ar = 72.6, very close to the prediction

Key fact: When gallium (1875) and germanium (1886) were discovered, their properties closely matched Mendeleev’s predictions. This convinced scientists 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), which supported his table again.

How testing a prediction works. Explaining facts you already know is not enough to convince other scientists. A strong idea makes predictions that can be checked. If later results match, the idea is supported; if they do not match, the idea is refuted and has to be changed or dropped.

The modern periodic table

In the early 1900s scientists discovered protons, neutrons and electrons. The elements are now arranged in order of atomic number (number of protons). This puts tellurium (52) before iodine (53) without any special swap.

Isotopes explain the odd order of atomic weights. Most tellurium atoms are heavy isotopes (tellurium-128 and tellurium-130), so its relative atomic mass (127.6) is higher than iodine’s (126.9), even though it has fewer protons. Argon (Ar 39.9) and potassium (Ar 39.1) 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.

Common mistakes

  • Saying Mendeleev ordered elements by atomic number. He used atomic weight; protons had not been discovered.
  • 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 prediction “proves” an idea. Matching results support it; results that do not match refute it.
  • Saying Newlands left gaps. He did not; that was Mendeleev’s key idea.

Revision questions

Try these, then check your answers below each one.

Question 1. How did Mendeleev deal with elements that had not yet been discovered? (Choose from: He left gaps for them in his table, He ignored them and listed only known elements in order, He put two elements in the same box, He ordered the elements by atomic number instead.)

Answer: He left gaps for them in his table

Working: Mendeleev left gaps where no known element fitted, and predicted the properties of the missing elements. Atomic number was not known in 1869.

Question 2. Lithium (atomic weight 7), sodium and potassium (atomic weight 39) form a triad: the middle atomic weight is the mean of the other two. Use this to estimate the atomic weight of sodium.

Answer: 23

Working: (7 + 39) ÷ 2 = 46 ÷ 2 = 23. Sodium’s atomic weight really is 23.

Question 3. True or false? The elements in the modern periodic table are arranged in order of relative atomic mass.

Answer: False

Working: False. They are arranged in order of atomic number (number of protons). That is why tellurium (52) comes before iodine (53) even though it is heavier.

Related lessons

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