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History of Periodic Table
What you need to know
Reflections and Exam tips
The quest for a systematic arrangement of the elements started with the discovery of individual elements.
The Law of Triads
German chemist Johann Dobereiner (1780-1849) grouped elements based on similarities.
Law of Triads - the middle element in the triad had atomic weight that was the average of the other two members.
Calcium (atomic weight 40), strontium (atomic weight 88), and barium (atomic weight 137) possess similar chemical prepares. Dobereiner noticed the atomic weight of strontium fell midway between the weights of calcium and barium: Ca (40), Sr (88), Ba (137) = (40 + 137) ÷ 2 = 88.
The law of triads worked for alkali metal triad (Li/Na/K) and the halogen triad (Cl/Br/I) but couldn't be applied to all other elements.
Law of Octaves
English chemist John Newlands (1837-1898), having arranged the 62 known elements in order of increasing atomic weights, noted that after interval of eight elements similar physical/chemical properties reappeared.
Law of Octaves - elements exhibit similar behaviour to the eighth element following it in the table.
Newlands was the first to formulate the concept of periodicity in the properties of the chemical elements.
Problem of law of octaves
Mendeleev's Periodic Table
In 1869, Russian chemist Dimitri Mendeleev (1834-1907) proposed arranging elements by atomic weights and properties (Lothar Meyer independently reached similar conclusion but published results after Mendeleev).
Mendeleev's table exhibited similarities not only in small units such as the triads, but showed similarities in an entire network of vertical, horizontal, and diagonal relationships.
Mendeleev’s ordered the elements by their relative atomic mass.
In order to make similar elements line up in the same group:
Arranging the elements according to increasing atomic numbers and not atomic masses eliminated some of the inconsistencies associated with Mendeleev's table.
Group 1 (alkali metals) elements
Properties of alkali metals
Reactions with halogens
Alkali elements react with non-metals to form white, soluble, crystalline ionic compounds which have high melting points due to the strong attraction between the metal cations (1+) and the negatively charged non-metal anions.
Reaction with water
React with water to form hydrogen gas and the metal hydroxide which dissolves in water to give an alkaline solution.
2 Na(s) + 2 H2O(l) → 2 NaOH(aq) + H2(g)
Alkali elements are more reactive going down the group as the outer electron (further from the nucleus) is less strongly attracted to the positive nucleus and hence more easily lost.
Group 7 (halogens) elements
Properties of halogens
React with metals (gaining an electron in the outer shell) to form ionic compounds which have high melting points due to the strong attraction between the metal cations (positive) and the halide anions (-1).
React with non-metals (sharing one electron in the outer shell) to form covalent molecular compounds which have low melting points due to the weak intermolecular forces of attraction.
Trend in reactivity
Halogens become less reactive going down the group as the outer electrons (further from the nucleus) are less strongly attracted to the positive nucleus and hence additional electrons are less easily gained.
More reactive halogens will displace non-metals of lower reactivity from their salts
Cl2(aq) + 2KBr(aq) → Br2(aq) + 2 KCl(aq)
Reactivity of transition metals
Uses of transition metals