Arrangement of Elements in the Periodic Table
Senior 1 · Chemistry
Arrangement of Elements in the Periodic Table
Lesson 1 of 4
22 min
By 1860 chemists knew about 60 elements. Each had its own atomic mass, its own colour and state, and its own way of reacting. Imagine a shopkeeper in Kimironko market who receives 60 new kinds of goods at once: before she can sell anything she has to sort them, putting similar goods on the same shelf. Chemists needed a shelf system for the elements too.
The first attempt was simple. In 1789 the French chemist Antoine Lavoisier listed the elements known to him and divided them into metals and non-metals. That division is still useful, but it is far too coarse: sodium and iron are both metals, yet sodium fizzes on water while iron hardly reacts with it.
The German chemist Johann Döbereiner noticed that some similar elements came in groups of three. Chlorine, bromine and iodine (above) all react with metals to form salts such as sodium chloride, sodium bromide and sodium iodide. And when he arranged each group in order of atomic mass, something striking appeared: the atomic mass of the middle element was close to the average of the other two. Its properties, such as colour and state, were in between too.
Definition
Triad: a group of three chemically similar elements in which, arranged in order of atomic mass, the atomic mass of the middle element is roughly the average (mean) of the other two. Example: lithium, sodium, potassium.
Table 1.1: Three of Döbereiner's triads (modern atomic masses)
| Triad | Atomic masses | Average of first and third | Middle element |
|---|---|---|---|
| Lithium, sodium, potassium | 6.9, 23.0, 39.1 | Na: 23.0 | |
| Calcium, strontium, barium | 40.1, 87.6, 137.3 | Sr: 87.6 | |
| Chlorine, bromine, iodine | 35.5, 79.9, 126.9 | Br: 79.9 |
Worked example
Problem. Show that lithium, sodium and potassium form a Döbereiner triad. Their atomic masses are 6.9, 23.0 and 39.1.
Solution. The elements are similar: all three are soft, shiny metals stored under oil, and all react with water to give hydrogen and an alkali. The average of the first and third atomic masses is
This equals the atomic mass of sodium, the middle element, so the three form a triad.
Worked example
Problem. Before bromine was well studied, a chemist knew only that chlorine (atomic mass 35.5) is a gas and iodine (126.9) is a solid. Use the triad idea to predict bromine's atomic mass and its state at room temperature. How good is the prediction? (Bromine's real atomic mass is 79.9.)
Solution. Predicted atomic mass:
The error is , which is : a good prediction. For the state, the middle member should be "in between" a gas and a solid, so bromine should be a liquid, and it is: bromine is one of only two elements that are liquid at room temperature.
Döbereiner's idea was the first to link the properties of elements with their atomic masses, and that link turned out to be the key. But he found only a handful of triads. Most elements did not fit into any group of three, and some families clearly had more than three members: fluorine behaves like chlorine, bromine and iodine, so the "triad" is really a family of four or more.
The English chemist John Newlands tried a different approach. He wrote out all the known elements in order of increasing atomic mass and numbered them. He noticed that every eighth element had properties similar to the first, just as the eighth note of a musical scale (do, re, mi, fa, sol, la, ti, do) sounds like the first. Lithium is element 2 in his list, sodium is 9 and potassium is 16: they fall seven places apart, on the same "note".
Figure 1.1: Newlands' octaves drawn on a keyboard. Counting the first element as one, every eighth element falls on the same note.
Definition
Law of octaves: when the elements are arranged in order of increasing atomic mass, every eighth element has properties similar to the first. It was proposed by John Newlands in 1865.
Newlands set out his elements in columns of seven. The first two columns work well; the third starts to go wrong.
Table 1.2: The first three "octaves" in Newlands' table
| Note | 1st octave | 2nd octave | 3rd octave |
|---|---|---|---|
| do | H | F | Cl |
| re | Li | Na | K |
| mi | Be | Mg | Ca |
| fa | B | Al | Cr |
| sol | C | Si | Ti |
| la | N | P | Mn |
| ti | O | S | Fe |
Worked example
Problem. Use Table 1.2 to decide where the octaves work and where they break down. Consider (a) Li, Na and K; (b) O, S and Fe.
Solution. (a) Lithium, sodium and potassium are all soft, reactive metals that form ions with a charge of +1: the octave works. (b) Oxygen and sulfur are both non-metals that form ions with a charge of −2, but iron is a hard, magnetic metal, nothing like them: the octave breaks down. Newlands also had to squeeze two metals, cobalt and nickel, into a single place on the same note as fluorine and chlorine.
Other chemists laughed at the musical comparison, and in 1866 the Chemical Society refused to publish Newlands' paper. His law had real weaknesses:
Yet Newlands had seen something true. Today we know the repeat comes every eight elements (lithium 3, sodium 11, potassium 19). In 1865 the noble gases (helium, neon, argon) had not yet been discovered, so each of his "octaves" was one element short. In 1887 the Royal Society gave him its Davy Medal for the discovery.
Warning
Do not mix up the two rules. A triad is about three elements and an average atomic mass. An octave is about every eighth element in a long list ordered by atomic mass.
Tip
Two names, two numbers: Döbereiner, triads of 3, 1829; Newlands, octaves of 8, 1865. Both ordered the elements by atomic mass.
Freesciencelessons: Development of the Periodic Table (5 min). Watch the part on Döbereiner and Newlands, and note why each attempt was not accepted. The rest of the video leads into Lesson 2.
Practice
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
