C2 The Periodic Table

C2.1 Development of the periodic table

• The periodic table of elements developed as chemists tried to classify the elements
• It arranges them in an order in which similar elements are grouped together
• The periodic table is so named because of the regularly repeating patterns in the properties of elements
• 1808 Dalton: Order of atomic weights
• 1864 Newlands: Ordered by mass, noticed properties of every eighth element was similar, didn't take into account undiscovered elements
• 1869 Mendeleev's periodic table left gaps for the unknown elements
• When newly discovered elements matched his predictions, his table was accepted by the scientific community
• 
• Caption: Mendeleev's periodic table

C2.2 Electronic structures and the periodic table

• The atomic number of an element determines its position in the periodic table
• The number of electrons in the outermost shell of an atom determines its chemical properties
• The group number in the periodic table equals the number of electrons in its outermost shell
• The atoms of metals tend to lose electrons whereas those of non metals tend to gain electrons
• The noble gases in group 0 are unreactive because of their very stable electron arrangements
• 
• Caption: The periodic table

C2.3 Group 1 - the alkali metals

• The elements in group 1 are called the alkali metals
• They have a silvery, shiny surface, becomes dull in air
• Easy to cut, low density, density increases going down
• Melting points and boiling points decrease going down the group
• All the metals react with water to produce hydrogen and an alkaline solution containing the metal hydroxide
• They form 1+ ions in reactions to make ionic compounds
• These are generally white and dissolve in water giving colourless solutions
• The reactivity of the alkali metals increases going down the group
• 
• Caption: Lithium, sodium and potassium reacting in water

C2.4 Group 7 - the halogens

• They all form 1- ions, halides, in their ionic compounds with metals
• They are all diatomic
• Melting points increase going down
• Poor conductors of heat and electricity
• They form covalent compound by sharing electrons with other non-metals
• A more reactive halogen can displace a less reactive halogen from a solution of one of its salts
• The reactivity of halogens decreases going down the group
• 
• Caption: The colours of the halogens

C2.5 Explaining Trends

• The electrostatic attraction between the electrons in the outermost shell and the nucleus depends on:
• the distance between the outermost electrons and the nucleus
• the number of occupied inner shells which provide a shielding effect
• the size of the positive charge in the nucleus
• The increased nuclear charge due to extra protons in the nucleus is outweighed by the other two factors
• it is easier for the larger atoms to lose electrons going down a group and harder for them to gain one
• 
• Caption: Electronic structure of the halogens (note the sizes)

C2.6 The Transition Elements

• Compared with the alkali metals, transition elements have much higher melting points and densities
• Stronger and harder
• Good conductors of heat and electricity
• Have high densities
• High melting points, except mercury
• Much less reactive
• Do not react vigorously with oxygen or water
• Can form ions with different charges in compounds that are often coloured
• Transition elements and their compounds are important industrial catalyst
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• Caption: The colours of aqueous transition metal ions