History of the Periodic Table

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The history of the periodic table can be traced back to the discovery of elements. In 1649, Henning Brand made history by being the first person to scientifically discover an element: Phosphorous. By the year 1869, scientists had discovered a total of 63 elements and documented their properties as well as their compounds. With the increase in the number of elements, scientists began to identify patterns in their properties and thus started to develop means of classifying them.

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By the year 1817, Johann Dobereiner observed that strontium, an element with the same characteristics as calcium and barium, had an atomic weight falling between that of calcium and barium. In 1829, the discovery of halogen triad (Cl, Br, and I) and the alkali metal triad (Li, Na, and K) made Dobereiner to conclude that nature is made up of triad of elements with the middle element having properties that are midway of the other two members when arranged by way of their atomic weights. He called this the law of triads.

In 1829 to 1858, other scientists would, later on, establish that the chemical relationships among elements went beyond the triad. This period saw the addition of fluorine to the family of halogens. Also, O, S, Se, and Te were grouped as a family. Similarly, N, P, As, Bi and Sb were grouped under a different family.

De Chancourtois, in the year 1862, ordered a list of elements on a cylinder by their increasing atomic weights. He constructed a cylinder in such a way that it was possible to write 16 mass units on the cylinder per turn. On the cylinder, he arranged elements which were closely related vertically. With this, he came to realise that the properties of elements reoccurred after every seven elements. Using this chart, he was also able to come up with a stoichiometry of many metallic oxides. The only disadvantage of this chart is that it combined elements, ions, and compounds.

In 1863, John Newlands advanced the work of De Chancourtois when he classified 56 elements into 11 groups. The basis of his classification was a similarity in physical properties. In 1864, he came up with his periodic table and a law of octaves which stated that an element will show analogous behaviour to the 8th element in the table.

The modern periodic table can be attributed to the work of a Russian chemist Dmitri Mendeleev. He gathered and catalogued many facts about the already discovered 63 elements. The characteristics enabled him to conclude that a group of elements possessed similarity in periodic properties. In his periodic table, he arranged elements based on increasing atomic masses and further into columns depending on the similarity in chemical properties. Based on the periodic properties, he was able to predict the properties of unknown elements. In the year 1913, Henry Moseley discovered that the proton number in the nucleus of specific atom always remained constant. He ordered the atoms according to increasing atomic number. The current periodic table is based on Moseleys work.

Some features of the periodic table

In the contemporary periodic table, the horizontal rows are referred to as periods while the vertical columns are known as groups. The groups are numbered IA to VIIIA. From the 1st to the 8th group, the elements are grouped as Alkali, Alkaline earth metals, Boron family, Carbon family, Nitrogen family, Oxygen family, Halogens, and Noble gases.

In the same group, elements have the similarity in chemical properties because they have the same number of electrons in their outermost energy level. Down the group, there is an increase in atomic in atomic radius because the number of energy levels is increasing. Across the periods, the atomic radii decrease due to shielding effect.

Atoms of elements in the periodic table are unstable, except those in group VIII. To gain stability (or noble state configuration), they either lose or gain electrons. Metallic atoms of elements (those of group I, II, and III) achieve their stability by losing electrons. On the other hand, non-metallic atoms of elements (those of group V, VI, and VII) achieve the noble state configuration by gaining electrons.

The particles formed when atoms gain or lose electrons are called ions. Ions are either positively or negatively charged. Positively charged ions are referred to as cations while negatively charged ones are called anions. Cations are formed when atoms of elements lose electrons. On the other hand, anions are formed as a result of a gain of electrons by atoms.

Generally, anions have bigger radii as compared to the radii of atoms they are derived from. This is because the addition of electrons makes the nuclear force of attraction to be weakened. On the contrary, cations have smaller radii as compared to the radii of the atoms they come from because the loss of electrons makes the nuclear force of attraction to be stronger. The ionic radii of elements, like the atomic radii, also increase down the groups.

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