C1 Atomic structure and the periodic table

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  • 1.1 Atomic structure and the periodic table
    • 1.1.3 Atoms
      • All substances are made of atoms. An atom is the smallest part of an element that can exist.
      • 1. Before the discovery of the electron, atoms were thought to be tiny spheres that could not be divided. 
        • 2. The plum pudding model suggested that the atom is a ball of positive charge with negative electrons.
          • 3. The results from the alpha particle scattering experiment led to the discovery that the mass of an atom was concentrated at the centre (nucleus) and that the nucleus was charged.
            • 4. Niels Bohr adapted the nuclear model by suggesting that electrons orbit the nucleus at specific distances.
              • 5. Later experiments led to the idea that the positive charge of any nucleus could be subdivided into a whole number of smaller particles. The name proton was given to these particles.
    • 1.1.1 Compounds 1.1.2  Mixtures
      • Compounds are formed when two or more elements go through a chemical reaction to form one or more new substances, and often involve a detectable energy change. 
        • Compounds can only be separated into elements by chemical reactions.
      • A mixture consists of two or more elements or compounds not chemically combined together. The chemical properties of each substance in the mixture are unchanged. 
        • Mixtures can be separated by physical processes such as filtration, crystallisation, simple distillation, fractional distillation and chromatography. These physical processes do not involve chemical reactions and no new substances 
    • 1.1.4 Subatomic particles
      • Protons: +1 Neutrons: 0 Electrons: -1
        • Protons: 1 Neutrons: 1 Electrons: 1/1836
      • In an atom, the number of electrons is equal to the number of protons in the nucleus. Atoms have no overall electrical charge.
      • The number of protons in an atom of an element is its atomic number. All atoms of a particular element have the same number of protons. 
        • The relative atomic mass of an element is an average value that takes account of the abundance of the isotopes of the element.
      • The sum of the protons and neutrons in an atom is its mass number. Atoms of the same element 
    • 1.1.5 The periodic table
      • The elements in the periodic table are arranged in order of atomic number and so that elements with similar properties are in the same group.
      • The elements in Group 0 of the periodic table are called the noble gases. They are unreactive and do not easily form molecules because they have a full outer shell.
      • Elements that react to form positive ions are metals. Elements that do not form positive ions are non-metals.
      • 1. The early periodic tables were incomplete and some elements were placed in inappropriate groups if the strict order of atomic weights was followed.
        • 2. Mendeleev overcame some of the problems by leaving gaps for elements that he thought had not been discovered and in some places the order based on atomic weights 
          • 3. Elements with properties predicted by Mendeleev were discovered and filled the gaps. Knowledge of isotopes made it possible to explain why the order based on atomic weights was not always correct.


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