The periodic table

• Elements are arranged in order of atomic number and so that elements with similar properties are in columns, known as groups
• Elements in the same periodic group have the same amount of electrons in their outer shell, which gives them similar chemical properties
• John Newlands ordered his table in order of atomic weight and realized similar properties occurred every eighth element - 'law of octaves' but broke down after calcium
• Dmitri Mendeleev ordered his table in order of atomic mass, left gaps for undiscovered elements, and changed the order based on atomic weights in some places
• The table is called a periodic table because similar properties occur at regular intervals
• Elements with similar properties are found in the same column (groups)
• Elements with properties predicted by Mendeleev were discovered and filled the gaps
• Knowledge of isotopes explained why the order based on atomic weights was not always correct
• When electrons, protons, and neutrons were discovered, elements were ordered by atomic number and placed in appropriate groups
• Metals react to form positive ions and are found to the left and towards the bottom of the periodic table
• Non-metals do not form positive ions and are found towards the right and top of the periodic table
• Alkali metals have characteristic properties due to the single electron in their outer shell
• Metals in group one react vigorously with water to create an alkaline solution and hydrogen
• Alkali metals react with oxygen to create an oxide and with chlorine to form a white precipitate
• Reactivity of alkali metals increases going down the group
• Noble gases have 8 electrons in their outer shell (except helium, which has 2) and are unreactive due to a stable arrangement of electrons
• Boiling points of noble gases increase with increasing relative atomic mass going down the group
• Halogens have seven electrons in their outer shell and react with metals to form ionic compounds and with nonmetals to form covalent compounds
• Reactivity decreases down the group for halogens due to the increase in relative molecular mass, melting point, and boiling point
• A more reactive halogen can displace a less reactive one in an aqueous solution of its salt