The periodic table and its development

The periodic table and its development

1. The early periodic table

   • Newlands' law of octaves
     • Spotted a pattern when arranging elements in order of atomic mass
     • Noticed properties of elements repeat every eighth element
       • 'Law of Octaves'
     • Inconsistencies in his method meant his ideas weren't accepted at the time
   • Mendeleev's achievement
     • Same idea as Newland but noticed undiscovered elements which he left gaps shown by * in the table
       • Allowed him to organise table in a repeating pattern
     • Elements with similar properties occur in groups at regular intervals - periods
   • Changing the order
     • Following the discovery of protons, neutrons and electrons, it became clear that elements should be placed in order of atomic number not mass
     • Mendeleev thought the atomic masses of some elements had been measured inaccurately but he'd placed the elements in order of atomic number without realising
     • Mendeleev's table was accepted because of his remarkable predictions turning out to be correct

2. The modern periodic table

   • Structure
     • Elements are arranged by proton number
       • Making elements naturally fall into groups with similar properties
   • Electronic Structure and the periodic table
     • Atomic number describes the number of protons in the atom
       • Therefore also the number of electrons and their arrangement around the nucleus
     • Pattern is directly linked to the way in which electrons are arranged around the atoms of each element
     • Period number tells you how many energy levels containing electrons the element has
     • Group number tells you how many electrons an element has in its outer energy level
     • Periodic table predicts the chemical properties of elements
   • Metals
     • Found in the middle and on the left of the periodic table
     • Groups 1 and 2 contain very reactive but quote soft metals with low melting points
     • The transition metals block contains 'everyday' metals such as iron and copper
       • Not as reactive as group 1 and 2 but are harder, stronger and have higher melting points
   • Non-metals
     • Found to the right of the table
     • Groups 6 and 7 contain reactive non-metals
       • Gases at the top but solids further down
     • Group 0 contains the Noble gases, highly un-reactive metals
   • Groups 3,4 and 5
     • Elements at the top are non-metals
       • More metallic as you move down
     • Boundary between non-metals and metals zig-zags down the right
     • Elements on the boundary show intermediate properties between non-metals and metals
       • semi-conductors e.g. silicon

3. Group 1 - the alkali metals

   • Physical properties
     • All soft and conduct heat and electricity
     • Freshly cut is shiny but soon tarnishes in the air
     • etals have low density and relatively low melting and boiling points
   • Typical chemical reactions
     • React with water and oxygen in the air (which is why they tarnish)
       • Put in oil to stop this from happening
     • All have one outer electron
       • When they react, they lose this electron and form 1+ions
     • When they react with non-metals, they form ionic compounds
       • These compounds are white and dissolve in water to form colourless solutions
     • When reacted with water, they form a metal hydroxide and hydrogen
       • The metal hydroxides dissolve in water, making an alkaline solution.
   • Trends in chemical properties
     • Reactions get more vigorous as you go down
       • Because alkali metals react by losing their lone outer electron
     • Atoms get bigger as you go down the group as more energy levels are added
       • Means lone outer electron is further away from positive nucleus increasingly screened by the inner electron shells
         • Reduces attraction from the nucleus to the outer electron and makes the electron easier to lose
         • The bigger the atom, the more reactive the metal

4. Transition metals

   • 'Everyday' metals
     • Include most of the metals used for Construction, Machinery, Vehicles, Wiring etc.
     • Conduct heat and electricity
     • Compared with group 1 metals, the transition metals:
       • Higher melting points
       • Higher densities
       • Stronger and harder
       • Much less reactive
   • Special properties
     • Catlysts
     • Compounds containing transition metals are usually coloured
       • Some statues and roofs are made of copper
       • Many gemstones are coloured by transition metal compounds
       • Used in some paint pigments and ceramic glazes

5. Group 7 - Halogens

   • Coloured non-metals
     • Non-metals with coloured vapours
     • All made of molecules
     • They don't conduct heat or electricity
     • Low melting and boiling points
       • Forces between molecules are weak
   • Chemical Properties
     • All react with metals to form ionic compounds containing halide ions
     • Electrons are transferred from the metal atoms to the halogen atoms, forming halide ions with a 1- electric charge
     • Halogens react with non-metals to form molecular compounds containing covalent bonds
       • Electrons are shared between atoms
   • Reactivity
     • ess reactive as you go down the gorup
     • When reacted the atoms gain an electron
       • Atoms get bigger down the gorup
         • Electron gained is further from the nucleus
           • Attraction of the nucleus for the electron being gained is weaker and so the electron is harder to gain
     • A more reactive halogen will displace a less reactive halogen from a compound in a displacement reaction

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