C2: Periodic Table
Development of the Periodic Table
John Dalton first arranged the elements in the order of their atomic weight
John Newlands noticed that every 8th element seemed similar. Today we now know these as noble gases
Dmitri Mendeleev arranged the 50 elements that had been found in his time into the order of their atomic weights and so that a periodic pattern in their properties could be seen. He then used his table to predict the properties of undiscovered elements.
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 outside shell of an atom determines its chemical properties
The group number equals the number of electrons on the outside shell
Metals - Able to conduct electricity, malleable and ductile when solid, lose electrons to form positive ions.
Non-metals - Electrical insulators, brittle when solid, gains electrons to form negative ions
Noble gases - They Have a full outer shell which makes them very stable and mostly unreactive. The boiling point gets higher going down group 0
The Alkali Metals
All the alkali metals are reactive. They must be stored in oil to stop them from reacting with the oxygen in the air
The reactivity increases as you go down because the pull from the positive proton decreases making it easier to lose an electron (the electrostatic attraction is weaker)
Compared to other metals they have a very low density so they can float on water. They are also soft enough to be cut with a knife and have a silvery, shiny surface when first cut
They have one electron on their outside shell which makes them reactive because they only need to lose one electron to become stable
The form 1+ ions and boil at relatively low temperatures
The Halogens
When you add them to water they fizz and produce hydrogen gas. This reaction produces an alkali metal hydroxide Potassium reacts so vigorously that the hydrogen produced ignites.
They are toxic non-metals with low melting and boiling points that increase the further down the group
They are poor conductors of heat and electricity and exist as diatomic molecules
They have seven electrons on the outside shell which makes it harder to gain electrons because the positive pull from the positive nucleus is weaker making it harder to attract an electron
The gain one electron to form a 1- ion
A more reactive halogen will displace a less reactive halogen from solutions of its salts
Explaining Trends
Larger atoms lose electrons easier going down a group
Larger atoms gain electron less easy going down a group
The electrostatic attraction depends on: the distance between the outer electrons and the nucleus, the number of shells, the size of the positive charge
This is because the inner shells 'shield' the outer electrons from the positive charge of the nucleus.
The effect of increased nuclear charge is outweighed by the effect of distance and shielding
The Transition Elements
They are hard and strong, good conductors of electricity and thermal energy. They have high densities and high melting points (except mercury)
They are less reactive than the alkali metals
Transition metals are very good structural materials
Transition elements can form more than 1 ion. These ions can be different colours
Transition metals are also used as catalysts