The periodic table arranges the elements by atomic number. Patterns in the table allow their properties and reactions to be predicted.
There are over 100 known elements, and their physical and chemical properties vary enormously, from gases such as helium to metals such as iron, from unreactive gold to violently reactive sodium.
Those differences are not random. They follow a pattern, and finding that pattern is what the periodic table is for.
Mendeleev arranged the known elements in order of atomic weight, and placed elements with similar properties in the same column.
His insight was to leave gaps where no known element fitted, predicting that undiscovered elements would fill them, and to swap a few elements out of strict weight order to keep similar ones together. When the predicted elements were found with roughly the predicted properties, his table was accepted.
The modern table is arranged by atomic number. A horizontal row is a period; a vertical column is a group. For the main groups, elements in the same group behave similarly because they have the same number of electrons in their outer shell.
Metals are on the left and in the middle, non-metals on the right, with a rough staircase dividing them.
Metals are shiny, good conductors of heat and electricity, malleable and ductile, and usually have high melting points and high density.
Non-metals are usually dull, poor conductors, and brittle when solid, with lower melting points, and many are gases at room temperature. Graphite is an exception, being a non-metal that conducts electricity.
When an element burns in oxygen it forms an oxide, and whether that oxide is acidic or alkaline depends on which side of the table the element comes from.
Many metal oxides are basic: those that dissolve give alkaline solutions with a pH above 7. Many non-metal oxides are acidic: they dissolve to give solutions with a pH below 7, and sulfur dioxide from burning fuels therefore causes acid rain.
Because the table groups similar elements together, it lets you predict. Knowing that lithium and sodium react with water tells you potassium will too, and more vigorously, because reactivity increases down Group 1.
The same reasoning works across the table: an element's position tells you roughly how reactive it is, what charge its ions form, and which elements it will displace.
Reading the periodic table means reading symbols: one or two letters, first capital and second lower case. The table also helps with formulae: for many main-group elements the group tells you the number of outer electrons, and so the likely charge of the ion it forms.
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