Lavoisier classified elements into metals and nonmetals. This classification was based on certain distinctive physical properties such as hardness, malleability and luster. On the basis of these properties, sodium and lead were classed together as belonging to the group of metals.
- Hardness, malleability and luster were found to be the only common properties of sodium and lead, otherwise the two elements were entirely different.
- In such a classification there was no place for elements with properties resembling those of metals as well as nonmetals.
There fore, Lavoisier’s classification was found to be inadequate.
Law of triads In 1817, German chemist Johann Dobereginer classified elements having similar chemical properties into groups of three. These groups were called triads. He proposed a law known as Dobereiner’s law of triads. According to this law, when elements are arranged in the order of increasing atomic mass in a triad, the atomic mass of the middle element was found to be approximately equal to the arithmetic mean of the atomic masses of the other two elements.
|Set I||Set II||Set-III|
|Element||Atomic mass||Element||Atomic mass||Element||Atomic mass|
|Average of the atomic masses of calcium and barium||Average of the atomic masses of lithium and potassium||Average of the atomic masses of chlorine and iodine|
|Atomic mass of strontium = 87.5||Atomic mass of sodium = 23||Atomic mass of bromine = 80|
The classification of elements into triads was very successful in predicting the atomic mass and properties of the middle element. Further, this classification showed that there exists some relationship between the properties of elements and their atomic masses. This paved the way for future attempts at classification of elements.
Limitation : All the elements could not be grouped into triads.
Law of octaves In 1864, John Newlands, and English chemist, showed that when elements are arranged in the order of their increasing atomic masses, the eighth element, starting from a given element, was a kind of repetition of the first one, like the eighth note in an octave of music, i.e.,
sa re ga ma pa dha ni sa,
where the first and the eighth note are same.
A part of Newlands’ classification is given below where the figures under the symbols show the atomic masses
|Octaves of music and Newlands’ arrangement of elements|
|Newlands’s arrangement of elements with atomic masses
|Co and Ni
58.93 and 58.71
|Ce and La
140.12 and 138.91
Starting from lithium (Li) the eight element is sodium (Na). The eight element starting from sodium is potassium. The properties of lithium, sodium and potassium are similar. The properties of beryllium, magnesium and calcium are similar too.
- This law worked well for lighter elements (up to calcium), but it could not be applied to heavier ones (elements of higher atomic masses) because starting from calcium every eight element was found to have properties different from those of the first element.
- Newlands emphatically said that only 56 elements do exist in nature and no more element is likely to be discovered in future. But this concept was later on found to be untrue with the discovery of many new elements which defined the law of octaves.
- In arranging elements in the form of a table, Newlands clubbed two elements together at the same place and in the same column. Not only this, he also placed some dissimilar elements in the same column. For example, cobalt (Co) and nickel (Ni) were clubbed together in the column of fluorine (F), chlorine (Cl) and bromine (Br) (under sa/do). We know that cobalt and nickel have properties entirely different from those of fluorine, chlorine and bromine. It is also known that cobalt and nickel have properties similar to those of iron. But iron (Fe) was placed in a column (under ni/ti) different from the column of cobalt and nickel.
However, this law support to the idea that the properties of elements depend upon the atomic masses. It also showed that the properties of elements are repeated after a certain interval, i.e., the properties of elements are periodic in nature.