Why elements need to be classified; Dobereiner's triads and Newlands' law of octaves
By the 1860s over 60 elements were known, so chemists grouped them. Dobereiner made triads; Newlands saw every eighth element repeat the first.
Why elements need to be classified
A chemist's shop holds hundreds of medicines, yet the chemist finds yours in seconds, because they are sorted into groups. Long ago, only a few elements were known, so each could be studied on its own. By the middle of the 1800s, more than 60 elements and a huge number of their compounds were known. Studying each one separately had become too hard. So chemists began to sort similar elements into groups. The first sorting was simply into metals and non-metals. It did not work for elements such as germanium and antimony, which behave partly like both. Chemists then turned to atomic mass, a property that does not change for an element.
Dobereiner's triads (1829)
| Triad | Atomic masses | Mean of the first and third | Middle element's mass |
|---|---|---|---|
| Lithium, sodium, potassium | 7, 23, 39 | (7 + 39) ÷ 2 = 23 | 23 |
| Calcium, strontium, barium | 40, 88, 137 | (40 + 137) ÷ 2 = 88.5 | 88 |
| Chlorine, bromine, iodine | 35.5, 80, 127 | (35.5 + 127) ÷ 2 = 81.25 | 80 |
Dobereiner's law and its limit
- The three elements of a triad are similar in physical and chemical properties.
- In order of increasing atomic mass, the middle element's atomic mass is close to the mean of the other two. Its properties also lie between theirs.
- Limit: only a few such groups of three could be found, so most elements stayed unclassified.
Newlands' octaves, set out like the notes sa re ga ma pa dha ni
| Sa | Re | Ga | Ma | Pa | Dha | Ni |
|---|---|---|---|---|---|---|
| Li (7) | Be (9) | B (11) | C (12) | N (14) | O (16) | F (19) |
| Na (23) | Mg (24) | Al (27) | Si (28) | P (31) | S (32) | Cl (35.5) |
| K (39) | Ca (40) | - | - | - | - | - |
Newlands' law: merits and limits
- Law: in order of increasing atomic mass, every eighth element is like the first, just as the eighth note of music (sa) repeats the first.
- Counting from lithium as the first, the eighth element is sodium. Both are soft, very reactive metals. Beryllium, magnesium and calcium also fall in one line.
- Merit 1: atomic mass was used as the basis of classification.
- Merit 2: it was the first to show periodicity, the repeating of properties at regular intervals.
- Limit 1: it worked only up to calcium (atomic mass about 40), so heavier elements did not fit.
- Limit 2: when the noble gases were found, the ninth element, not the eighth, matched the first. So the idea of octaves was given up.
Worked example: checking a triad
Sulphur (32), selenium and tellurium (127.6) formed one of Dobereiner's triads. Predict the atomic mass of selenium and compare it with its real value, 79.
- Selenium is the middle element of the triad.
- Predicted atomic mass = (32 + 127.6) ÷ 2 = 159.6 ÷ 2 = 79.8.
- The real value, 79, is very close to 79.8.
About 79.8, close to the real 79, so sulphur, selenium and tellurium form a triad
A kirana shop keeps all the pulses on one shelf and all the spices on another, so the shopkeeper finds anything quickly. Grouping elements by their properties does the same job for chemists.
2 marks: “State Dobereiner's law of triads. Show that calcium (40), strontium (88) and barium (137) form a triad.” Law: in a triad of similar elements in order of increasing atomic mass, the middle element's atomic mass is about the mean of the other two. Mean = (40 + 137) ÷ 2 = 88.5, close to strontium's 88, so they form a triad.
2 marks: “Give two reasons why Newlands' law of octaves was rejected.” (1) It held only up to calcium; heavier elements did not fit. (2) After the noble gases were discovered, the ninth element, not the eighth, was like the first.
1 marks: “What was the first, simplest classification of elements?” Into metals and non-metals.
Dobereiner: three in a row, middle = average. Newlands: eight, like music, failing after calcium.