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Chemistry revision ((John Dalton thought that all matter was made of tiny…
Chemistry revision
John Dalton thought that all matter was made of tiny particles called atoms, which he imagined as tiny solid balls. Dalton’s model included these ideas:
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J.J. Thomson discovered the electron.
Atoms are neutral overall,
so in Thomson’s ‘plum pudding model’:
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In 1909 Ernest Rutherford designed an experiment to test the plum pudding model. In the experiment, positively charged alpha particles were fired at thin gold foil. Most alpha particles went straight through the foil. But a few were scattered in different directions.
This evidence led Rutherford to suggest a new model
for the atom,
called the nuclear model. In the nuclear model:
the mass of an atom is concentrated at its centre, the nucleus
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Isotopes of an element have the same number of protons,
but different numbers
of neutrons in their nuclei. Isotopes of an element have:
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In 1869 Russian chemist Dimitri Mendeleev started the development of the periodic table, arranging chemical elements by atomic mass. He predicted the discovery of other elements, and left spaces open in his periodic table for them
An English scientist called John Newlands put forward his Law of Octaves in 1864. He arranged all the elements known at the time into a table in order of relative atomic mass.
When he did this, he found a pattern among the early elements. The pattern showed that each element was similar to the element eight places ahead of it.
For example, starting at Li (lithium), Be (beryllium) is the second element, B (boron) is the third and Na (sodium) is the eighth element. He then put the similar elements into vertical columns, known as groups.
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Sub-atomic Particles
Protons- Found in the nucleus, they have a positive charge and are represented by the atomic number. They have a mass of 1.
Neutrons- Also found in the nucleus, they have a neutral charge and are represented with the protons in the mass number. They have amass of 1.
Electrons- They are found in shells and are also represented by the atomic number. They have a negative charge and a mass of 0.
Niels Bohr improved Rutherford’s model. Using mathematical ideas, he showed that electrons occupy shells or energy levels around the nucleus.
The Group 1 elements are called the alkali metals. They are placed in the vertical column on the left-hand side of the periodic table. All the Group 1 elements are very reactive. They must be stored under oil to keep air and water away from them. Group 1 elements form alkaline solutions when they react with water, which is why they are called alkali metals. More reactive as they go down, melting/boiling points get lower going down.
The Group 7 elements are called the halogens. They are placed in the vertical column, second from the right, in the periodic table. Chlorine, bromine and iodine are the three common Group 7 elements. Group 7 elements form salts when they react with metals. Melting/boiling points get higher going down, less reactive going down
Group 0 contains non-metal elements placed in the vertical column on the far right of the periodic table. The elements in group 0 are called the noble gases. They exist as single atoms. The noble gases all have low boiling points. Boiling points increase going down. Unreactive.
Positive and negative ions form when a metal reacts with a non-metal, by transferring electrons. The oppositely charged ions are strongly attracted to each other, forming ionic bonds.
