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GCSE Chemistry 3:
Unit One: The Modern Periodic Table…read more

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C3 1.1: The Early Periodic Table
In the 1800s, new elements were being found every year.
John Dalton is credited for being the first person to try to order the elements in a certain way. He
ordered them by means of mass and published his findings in `A New System of Chemical Philosophy'.
In 1864, John Newlands built on Dalton's ideas and devised `the law of octaves', where he noticed that
when the elements were ordered by mass number, every eighth element had similar properties.
Newlands' theory only worked up to calcium; he thought all the elements had been found, but scientists
were still finding more. The theory just broke down from then.
In 1869, Dmitri Mendeleev solved the problem. He ordered the elements that had been discovered (50
at the time) by atomic mass. He discovered that there were periodic trends between elements.
Mendeleev left gaps for elements yet to be discovered, and even managed to successfully predict their
chemical and physical properties.…read more

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C3 1.2: The Modern Periodic Table
Organising elements in order of atomic mass arranged those of similar properties in vertical groups.
Argon is before potassium otherwise they would both end up in the same group, even though they are
completely opposite on the reactive scale.
In group one, reactivity INCREASES as you go down the column because the outer electron is attracted
less easily, so it is given away more easily.
In group seven, reactivity DECREASES as you go down the column because the nucleus attracts the
extra electrons with less power. The nucleus is further away from the outer shell as you go down the
column.…read more

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C3 1.3: Group One ­ The Alkali Metals
The alkali metals are so reactive that they need to be stored in oil to stop them reacting with oxygen in the air.
They have very low density. They are very soft (they can be cut with a knife). When first cut, the metals look shiny, but
this shine soon disappears because the metal has already reacted with oxygen, forming a dull oxide layer on top.
They are all very reactive because they all only have one electron in their outer shell.
The melting and boiling points of the alkali metals are rather low for metals, and they get lower as you go down the
When in a reaction, they form `1+' ions, which are generally white solids that dissolve colourlessly in water.
The reaction between water and an alkali metal produces hydrogen (hence the fizzing of sodium and lithium; and the
flames of potassium), and a metal hydroxide (hence the name `alkali' metals). The solution is colourless with a high
pH, so Universal Indicator would go purple.
The alkali metals also react vigorously with the halogens (group seven). One example is chlorine. The alkali metals
react with chlorine to form white chlorides that are soluble in water (colourless in water).…read more

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C3 1.4: The Transition Elements
The transition metals are found in the central block of the periodic table.
They are hard and strong. They have high melting points and boiling points (with the exception of mercury).
They are great conductors of electricity and energy. They have high densities.
Their chemical properties make them very unreactive. So if there is any corrosion of a transition metal, it
happens very slowly. So, the transition elements have fantastic physical and chemical properties, making
them extremely useful for structural use.
The transition elements can all form different ions. For example, iron can form Fe2+ or Fe3+.
The ions of one transition element can be different in colour. For example, Fe2+ is green in colour, whereas
Fe3+ is reddish-brown (rusty) in colour.
The transition elements are very useful as catalysts in chemical processes. For example, hydrogenation
involves nickel and the Haber process involves iron.…read more

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C3 1.5: Group Seven ­ The Halogens
The Halogens (Group Seven) have low melting and boiling points. They are very more conductors of energy
and electricity.
All of the halogens exist as covalently-bonded pairs, such as F----F, Cl----Cl, and Br----Br.
The halogens take part in both ionic bonding and covalent bonding.
The halogens react with metals by gaining just one electron, making them very reactive.
The more reactive halogens displace the less reactive halogens. For example, bromine would displace
iodine, but chlorine would displace both of them.
Fluorine would displace all the other halogens, but it cannot be done In aqueous solutions because the
reaction with water is far too violent.…read more


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