History of the periodic table: how to sort 100 elements

The problem

Imagine a kiryana store with a hundred items and no shelves. Rice next to soap, sugar next to batteries. You could find anything eventually, but you could never predict where something is. Chemistry looked like that for a long time.

The question was: which label do you sort by? Scientists tried properties, then groups of three, then atomic mass. In 1871 Mendeleev's table was so good that it left gaps for elements nobody had found yet, and predicted their properties.

Modern Periodic Law: if the elements are arranged in ascending order of their atomic numbers, their chemical properties repeat in a periodic manner.

Key terms

Triads

The German chemist Dobereiner put the known elements in groups of three, each group with similar properties. Al-Razi had classified substances even earlier, by their physical and chemical properties.

Law of Octaves

The English chemist Newlands arranged the 62 elements then known by increasing atomic mass. He noticed that every eighth element shared some properties with the first.

Periodic Law

The Russian chemist Mendeleev also sorted by atomic mass, into eight vertical groups and horizontal periods. He left gaps for undiscovered elements and predicted their properties. Germanium later matched his prediction.

Atomic number

After Moseley discovered atomic number in 1911, elements were sorted by atomic number instead of atomic mass. This removed several confusions in the old table and gave the modern periodic law.

Short questions with model answers

1. Q1. Check Newlands' “every eighth element” with lithium (Z = 3), sodium (Z = 11) and potassium (Z = 19). Does it still work for rubidium (Z = 37)?

   • 11 − 3 = 8 · 19 − 11 = 8
   • 37 − 19 = 18

   It works for Li → Na → K, then breaks: from K to Rb the gap is 18, not 8.

2. Q2. Mendeleev's table put Be, Mg, Ca, Sr and Ba in one vertical group with Zn, Cd and Hg. How does the modern table fix this?

   • By properties they are two different kinds of element.
   • group 2 (IIA): Be, Mg, Ca, Sr, Ba · group 12 (IIB): Zn, Cd, Hg

   It divides the group into group 2 (IIA: Be to Ba) and group 12 (IIB: Zn, Cd, Hg).

3. Q3. Why did germanium make scientists trust Mendeleev's table?

   • Mendeleev left a gap where an element should be.
   • From its position he predicted its properties.
   • A few years later germanium was found, and it matched remarkably well.

   His table predicted an unknown element's properties, and the prediction came true.

Common mistakes

• ✗ Writing that Mendeleev arranged the elements by atomic number.

  ✓ Mendeleev used atomic mass. Atomic number came only after Moseley's discovery in 1911. This is a fill-in-the-blank in the exercise.

• ✗ Saying the noble gases were in Mendeleev's original table.

  ✓ They had not been discovered yet. Group 18 (VIIIA) was added later, at the far right, as one of the improvements.

• ✗ Assuming the “every eighth element” rule works across the whole table.

  ✓ It fits the short periods (2 and 3, eight elements each). In periods 4 and 5 properties repeat after 18 elements, e.g. potassium 19 → rubidium 37.

MCQs

1. 1. Mendeleev arranged the elements in his periodic table according to their atomic:

   1. (a) numbers
   2. (b) masses
   3. (c) radii
   4. (d) volumes
   Show answer

   (b) Atomic masses. Arranging by atomic number came after Moseley's work in 1911.

2. 2. Dobereiner's groups of three similar elements were called:

   1. (a) octaves
   2. (b) periods
   3. (c) triads
   4. (d) blocks
   Show answer

   (c) Triads, from 1829. Octaves were Newlands' idea in 1864.

3. 3. How many elements did Newlands classify in 1864?

   1. (a) 8
   2. (b) 18
   3. (c) 62
   4. (d) 118
   Show answer

   (c) The 62 elements known at that time, in increasing order of atomic mass.

4. 4. True or false: in Mendeleev's periodic table, Be, Mg, Zn and Cd were placed in the same group.

   1. (a) True
   2. (b) False
   Show answer

   (a) True. That was one of the confusions; the modern table splits them into groups 2 and 12 (IIA and IIB).

5. 5. Which discovery led to the modern periodic law?

   1. (a) germanium
   2. (b) the noble gases
   3. (c) atomic number, by Moseley
   4. (d) the Law of Octaves
   Show answer

   (c) Moseley's atomic number (1911). Sorting by it instead of atomic mass removed the old confusions.

Quick revision

• Dobereiner (1829) made triads; Newlands (1864) found octaves in 62 elements; Mendeleev (1871) made the first regular periodic table.
• Mendeleev sorted by atomic mass, left gaps and predicted germanium; after Moseley (1911), atomic number became the sorting label.
• Improvements: atomic number instead of mass, A and B subgroups, and group 18 (VIIIA) for the noble gases.