GCSE C3 Chapter 1
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- Created on: 13-03-14 16:05
The early periodic table
- During the 19th century many new elements were being discovered and chemists were attempting to put these elements into a table of common groups
- The scientists did not know about the structures of atoms
- They tried to classify elements based on their atomic weights
- In 1863 Newlands presented his idea about his law of octaves
- He claimed that the properties of elements repeated themselves every 8th element
- He used this to group all 62 known elements at time into 7 groups based on atomic weights
- However after the element calcium the properties of elements did not match
- So scientists did not accept his ideas
- In 1869 Mendeleev used Newlands ideas to produce a better table of elements
- He did this by leaving gaps for the undiscovered elements
- He then used his table to predict the properties of undiscovered elements
- When these elements were discovered he was proved right
- So scientists more readily accepted his ideas
- The table Menedeleev produced forms the basis of the modern periodic table
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The modern periodic table
- Scientists found out about protons and neutrons at the start of the 20th century
- Soon after this they developed models for the arrangment of electrons in atoms
- In the periodic table the elements are arranged in groups according to their atomic number
- The elements are lined up in vertical collumns called groups
- Elements in the same group have similar properties because they have the same number of electrons in thier higheast energy level
- Within a group the reactivity depends on the electrons in the highest energy level
- Going down a group there are more occupied energy levels
- Therefore the forces of attraction between the nucleus and the electrons in the highest occupied energy level is weaker
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The alkali metals
- The elements in group one are called the alkali metals
- They react readily with water and oxygen so must be sotred carefully
- They react with halogens to produce salts that are white or colourless
- Sodium + chlorine = Sodium chloride
- 2Na(s) + Cl(g) = 2NaCl(s)
- They dissolve in water to produce white or colourless solutions which are alkaline
- They are soft solids at room temperature with low melting and boiling points
- They have low densities so lithium, sodium and pottasium float on water
- They react with water to produce hydrogen gas and a metal hydroxide which is an alkali
- Sodium + water = sodium hydroxide + hydrogen
- 2Na(s) + 2H2O(l) = 2NaOH(aq) + H2(g)
- The reactivity increases going down the group because:
- Going down the group more energy levels are occupied
- Meaning the highest energy levels are further away from the nucelus
- Therefore the forces of attraction between the nucleus and the electrons in the highest energy levels are weaker
- Therefore it is easier for the alkali metal to lose its electron in its highest energy level
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Transition elements
- The transition elements are found in the periodic table between groups 3 and 4
- They are all metals so sometimes called the transition metals
- They have higher boiling points (except for mercury)
- High densities
- Malleable and ductile
- React slowly if at all with oxygen and water at room temperature
- Strong
- Useful building materials often as alloys
- React with non-metallic elements to produce positive ions with varous charges
- Produce colourful compounds
- Transition metals and thier compounds are used as catalysts in many reactions
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The halogens
- The halogens are non-metallic elements in group 7 of the periodic table
- They can also form covalent bonds with other non-metallic elements to form molecules
- They exist as small molecules
- They have low melting and boiling points that increase going down the group
- At room temperature:
- Flourine is a pale yellow gas
- chlorine is a green gas
- Bromine is a brick-red liquid
- Iodine is a grey solid which easily vapourises into a violet gas
- Form charges with -1
- Their reactivity decreases going down the group because:
- Halogens need to gain electrons to become stable
- More occupied energy levels
- Highest occupied energy level is further away from the nucleus
- So forces of attraction between nucleus + electrons in highest energy level decreases
- Meaning it is harder for the atoms to gain electrons
- A more reactive halogen can to displace a less reactive halogen from an aq solution
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