4.2 - The Modern Periodic Table

Ordered by increasing atomic number

• The elements in the periodic table are placed in order of increasing atomic number (the number of protons in an atom).
• It's read left to right along each row, then down to the start of the next row (hydrogen is 1, helium is 2, lithium is 3, and so on).
• Atomic number is also called proton number.

Why the order matters

• Arranging the elements by atomic number causes similar chemical properties to recur at regular intervals.
• Properties that repeat regularly like this are described as periodic.
• The table is laid out so that elements with similar properties line up beneath one another in columns.

Groups, periods and the named groups

Groups - The vertical columns

• The vertical columns are called groups.
• Elements in the same group have similar chemical properties because they have the same number of electrons in their outer shell.
• Groups are numbered from Group 1 to Group 7, and the far-right column is Group 0.

Periods - The horizontal rows

• The horizontal rows are called periods, numbered from period 1 at the top downwards.
• A new row begins each time the pattern of properties repeats.
• Period 1 contains only two elements: hydrogen and helium.

Named groups and blocks

• Group 1 - The alkali metals (reactive metals)
• Group 2 - The alkaline earth metals (reactive metals)
• Group 7 - The halogens (reactive non-metals)
• Group 0 - The noble gases (unreactive non-metals)
• Transition metals - The large block of metals located between Group 2 and Group 3

States of the elements at room temperature

At room temperature and pressure:

• Most elements are solids.
• Only two elements are liquids - Bromine (a non-metal) and mercury (a metal).
• Eleven elements are gases - Hydrogen, nitrogen, oxygen, fluorine, chlorine, and the six noble gases (helium, neon, argon, krypton, xenon, radon).
• The gases sit at the top and towards the right-hand side of the periodic table.