In short
The modern periodic table arranges the elements in order of increasing atomic number, in 18 groups (columns) and 7 periods (rows), so that elements with similar properties fall in the same group.
Mendeleev arranged elements by atomic mass. Moseley showed that atomic number is the better basis, and the modern periodic law says that the properties of elements are a periodic function of their atomic numbers. Elements in a group have the same number of outer electrons and similar chemistry. Moving along a period, the properties change gradually from metallic to non-metallic.
Sodium () and potassium () are in group 1. Each has one outer electron, and both react vigorously with water to make an alkaline solution and hydrogen.
Also searched as: modern periodic table, periodic table, ptable, p table, pt periodic table.
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