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C1: Atomic structure (1.5: History of the atom (The ancient Greeks were…
C1: Atomic structure
1.5: History of the atom
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In the early 1800's, Dalton linked his ideas to strong experimental evidence - Dalton suggested atoms were tiny, hard spheres - these atoms could not be divided or split
At the end of the 1800's, Thomson discovered a tiny negatively charged particle called the electron
Thomson proposed the 'plum pudding' model for the atom - the model suggested that negative electrons were embedded in a ball of positive charge - like the electrons as the bits of plum in a plum pudding
Ten years later, Geiger and Marsden were experimenting with alpha particles (dense positively charged particles) - they fired alpha particles at a very thin piece of gold foil
A few alpha particles were repelled showing that there must be a tiny spot of positive charge in the centre of the atom - Rutherford then proposed the nuclear model
In the nuclear model, electrons orbit around a nucleus (centre of the atom) - the nucleus contains positively charged protons
Bohr then revised the nuclear model - he suggested that the electrons were orbiting the nucleus in energy levels (or shells) - the electrons were a set distance from the nucleus - Bohr's theoretical calculation matched the experimental observations
In 1932, Chadwick provided the experimental evidence that showed the existence of uncharged particles called neutrons in the nucleus
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1.2: Chemical equations
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In chemical reactions, the atoms get rearranged - symbol equations show the numbers and types of atoms in the reactants and products
Atoms are neither created nor destroyed in a chemical reaction - the number of atoms remains the same before and after the reaction
When writing a symbol equation you should always balance the equation - a balanced symbol equation has the same number of each type of atom on both sides of the equation
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C1.7: Ions, atoms and isotopes
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If an atom gains electrons, it becomes a negative ion - it has an overall negative charge because it has more negative electrons than positive protons
If an atom loses electrons, it becomes a positive ion - it has an overall positive charge because it has less negative electrons than positive protons
You can't see atoms because they are so incredibly small - an atom is about a tenth of a billionth of a metre across (0.000,000,000,1m)
Atoms of the same element always have the same number of protons, but they can have different numbers of neutrons
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Isotopes of an element have different physical properties, but always have the same chemical properties
1.1: Atoms
There are about 100 different elements from which all substances are made - the periodic table is a list of the elements
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Atoms are represented by chemical symbols, e.g. Na for an atom of sodium
The elements in the periodic table are arranged in columns, called groups - the elements in a group usually have similar properties
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When elements react, their atoms join with atoms of other elements - compounds are formed when 2 or more elements combine together
1.3: Separating mixtures
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Mixtures are separated by a physical process - physical processes do not involve chemical reactions, so no new substances are made
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