Why are elements arranged as they are in the periodic table?

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Elements in the periodic table are arranged in order of increasing atomic number (which is just the number of protons found in the nucleus of that element). Starting with hydrogen (H) with only 1 proton and reading left to right and then down, the atomic number goes up one for each element until you reach the highest atomic number of 106.

The reason the periodic table is the shape that it is has to do with the electronic configurations of the elements. The periodic table is arranged so that all the elements in each period (row) have similar electronic configurations to the other elements in that period. That also often means that elements in the same group (column) show similar chemical reactivity. Electrons in atoms are arranged in different orbitals (named "s", "p", "d", and “f").

Elements in the same group all have the same number of electrons in each orbital. For instance, the alkali metals in the first column (Lithium, Sodium, Potassium, Rubidium, Caesium, and Francium) all have 1 electron in their outermost shell/layer (it turns out there are many shell, each one bigger than the previous one -- Lithium has 2 shells while bigger atoms like Caesium have 6 shells). What matters is that they all have a single electron in that outermost shell regardless of how many shells they have in total. This same trend is observed in each group of the table, excluding most of the transition metals.
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