The periodic table · GCSE Chemistry

How the table is arranged

Elements are arranged in order of atomic (proton) number.
Elements with similar properties are in columns called groups. Rows are called periods.
It is called a periodic table because similar properties occur at regular intervals.
Elements in the same group have the same number of electrons in their outer shell, so they have similar chemical properties.

Before protons, neutrons and electrons were known, scientists arranged elements in order of atomic weight.
Early tables were incomplete, and some elements were put in the wrong groups if the order of atomic weight was followed strictly.
Dmitri Mendeleev left gaps for elements he thought had not yet been discovered, and in some places changed the order based on atomic weights so elements with similar properties lined up.
Elements with the properties Mendeleev predicted were later discovered and filled the gaps.
Knowing about isotopes explains why ordering by atomic weight was not always correct.

Metals are elements that react to form positive ions. Most elements are metals; they are found to the left and towards the bottom of the table.
Non-metals do not form positive ions. They are found towards the right and top.
Metals are usually shiny, good conductors of heat and electricity, malleable and have high melting points. Non-metals are usually dull, poor conductors and brittle when solid.

Periodic table: A table of the elements arranged in order of atomic number, with similar elements in columns.
Group: A vertical column of elements with the same number of outer electrons.
Period: A horizontal row of elements.
Dmitri Mendeleev: The chemist who arranged the elements, leaving gaps for undiscovered ones.
Atomic weight: An old name for relative atomic mass, used to order early periodic tables.
Metal: An element that reacts to form positive ions.
Non-metal: An element that does not form positive ions.
Malleable: Able to be hammered into shape without breaking.