Why do we need to classify elements?
By the early 1800s chemists knew about 30 elements. Today we know 118. Learning each one separately would be impossible.
Classifying means putting similar things in the same group. When elements are grouped well:
- we study a whole family at once (for example, all alkali metals react with water);
- we can predict the behaviour of an element from its neighbours;
- we can spot gaps where a new element should exist.
So the aim was a single chart that shows patterns clearly.
History of the periodic table
Döbereiner's triads (1817–1829)
Johann Döbereiner found groups of three similar elements. When arranged by atomic mass, the middle one's mass was nearly the average of the other two. Examples: Li, Na, K and Cl, Br, I. Limit: only a few triads fitted.
Chancourtois (1862)
A.E.B. de Chancourtois wrote elements on a spiral round a cylinder in order of mass; similar elements fell on the same vertical line. It got little attention.
Newlands' law of octaves (1865)
John Newlands arranged elements by increasing mass and noticed that every eighth element had properties like the first, like the eighth note in music. Limit: it worked only up to calcium.
Lothar Meyer (1869)
He plotted physical properties such as atomic volume against atomic mass and got a repeating (periodic) curve.
Mendeleev's periodic law (1869)
Dmitri Mendeleev said: the properties of elements are a periodic function of their atomic masses. His strengths:
- He gave more weight to similar properties than to strict mass order. Iodine (127) was put after tellurium (128) so that it sat with fluorine, chlorine and bromine.
- He left gaps and predicted unknown elements: eka-aluminium (later gallium, 1875) and eka-silicon (later germanium, 1886). Their real properties matched his guesses closely.
- He corrected wrong atomic masses of some elements.
Limits: no fixed place for hydrogen; isotopes (same element, different masses) would need different places; some pairs (Ar–K, Co–Ni, Te–I) break the mass order.
Modern periodic law and the long form of the table
In 1913 Henry Moseley studied X-rays given out by elements. The frequency rose steadily with atomic number (Z), not with mass. So Z is the true identity card of an element.
Modern periodic law: the physical and chemical properties of the elements are a periodic function of their atomic numbers.
Arranging by Z removes Mendeleev's problems: isotopes share one place (same Z), and Ar (18) comes before K (19) naturally.
Long form of the periodic table
- Periods = horizontal rows. There are 7. The period number equals the highest shell number (n) in use.
- Groups = vertical columns. There are 18 (IUPAC numbering 1 to 18). Elements in a group have the same outer electron arrangement, so they behave alike.
- Number of elements per period: 2, 8, 8, 18, 18, 32, 32.
- Lanthanoids (Z = 58–71) and actinoids (Z = 90–103) are kept in two rows at the bottom to keep the table from getting too wide.
Why do properties repeat? Because the outer electron arrangement repeats after certain intervals (2, 8, 8, 18, 18, 32).
Names of elements with atomic number above 100
When a new heavy element is made, scientists may argue for years about who made it first. Until the official name is given, IUPAC uses a temporary name built straight from the atomic number.
| Digit | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
|---|---|---|---|---|---|---|---|---|---|---|
| Root | nil | un | bi | tri | quad | pent | hex | sept | oct | enn |
| Letter | n | u | b | t | q | p | h | s | o | e |
Steps: write the roots of the digits in order, then add -ium. The symbol is the first letters of the roots, first one capital.
- Two small spelling rules: "bi-ium" and "tri-ium" drop one i (bium, trium); "enn-nil" drops one n (ennil).
- Z = 101 → un-nil-un-ium = Unnilunium, Unu (official: Mendelevium, Md).
- Z = 104 → Unnilquadium, Unq (official: Rutherfordium, Rf).
- Z = 118 → Ununoctium, Uuo (official: Oganesson, Og).
All elements up to 118 now have official names, but exam questions still ask for the IUPAC temporary name and symbol.
Key formulas and definitions
- Triad rule: mass of middle ≈ (mass of first + mass of third) ÷ 2
- Modern periodic law: properties = periodic function of atomic number Z
- Elements per period: 2, 8, 8, 18, 18, 32, 32
- IUPAC name for Z > 100 = digit roots + "ium"; symbol = first letters of the roots
- Roots: 0 nil, 1 un, 2 bi, 3 tri, 4 quad, 5 pent, 6 hex, 7 sept, 8 oct, 9 enn
Worked examples
1. Check whether Li (7), Na (23) and K (39) form a Döbereiner triad.
Average of first and third = (7 + 39) ÷ 2 = 46 ÷ 2 = 23. This equals the mass of Na, and all three are soft reactive metals. Yes, it is a triad.
2. Cl = 35.5, Br = 80, I = 127. Estimate the atomic mass of Br from the triad rule.
(35.5 + 127) ÷ 2 = 162.5 ÷ 2 = 81.25. The real value 80 is close, so Cl, Br, I form a triad.
3. In Newlands' order (Li, Be, B, C, N, O, F, Na, …), which element is like Li?
Count Li as 1: Be 2, B 3, C 4, N 5, O 6, F 7, Na 8. The 8th element, Na, is like Li. Both are soft, reactive metals.
4. Write the IUPAC name and symbol of the element with Z = 107.
Digits 1, 0, 7 → un + nil + sept + ium = Unnilseptium. Symbol = U + n + s = Uns. (Official name: Bohrium, Bh.)
5. Write the IUPAC name and symbol for Z = 119 and Z = 120 (not yet made).
Z = 119: un + un + enn + ium = Ununennium, Uue. Z = 120: un + bi + nil + ium = Unbinilium, Ubn.
6. Ununhexium: find Z and the period and group it would belong to.
un = 1, un = 1, hex = 6 → Z = 116. Period 7 holds Z = 87 to 118, so it is in period 7. After Og (118) at group 18, count back: 117 is group 17, 116 is group 16. So period 7, group 16 (official name Livermorium, Lv).
7. Te = 127.6 and I = 126.9. Why did Mendeleev place Te before I, and how does the modern table justify this?
By mass I should come first, but I behaves like F, Cl, Br, so Mendeleev put it in their group and kept Te with O, S, Se. In the modern table Te has Z = 52 and I has Z = 53, so Te comes first anyway. Atomic number fixes the "anomaly".
Common mistakes
- Saying Mendeleev arranged by atomic number. He used atomic mass; Moseley and the modern law use atomic number.
- Writing "Unnilbiium" for Z = 102. The rule drops one i: Unnilbium (Unb).
- Mixing up periods and groups. Periods are rows (7), groups are columns (18).
- Thinking Newlands' octaves worked for all elements. They failed after calcium.