Revision Guide: 4.1 Atomic Structure and the Periodic Table
           Atoms,ElementsandCompounds
           Atoms
                                                                              Compounds
           All substances are made of atoms. An atom is the
                                                                              Some elements combine through chemical
           smallest part of an element that can exist.
                                                                              reactions to form compounds.
           Atoms of each element are represented by a chemical
                                                                              Compounds are made from two or more
           symbol,egOrepresents an atom of oxygen, Na
                                                                              different elements (types of atoms) combined
           represents an atom of sodium.
                                                                              together in fixed proportions and can be
                                                                              represented by formulae using the symbols of
            Elements and the periodic table
                                                                              the atoms from which they were formed, e.g CO
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            Anelement is a substance with only one type of atom.
            Elements are listed in the periodic table. There are about 100
                                                                              Compoundshavedifferent propertiesfrom the
            different elements.
                                                                              elements they are made from.
            Elements can be classified as metal or non-metal depending
                                                                              Compounds canonly be separated into elements
            on their properties.
                                                                              by chemical reactions.
            The columns in the periodic table are called groups and
            contain similar elements.
            The rows in the periodic table are called periods. Elements
            show a gradual change in properties across a period.
            Mixtures
                                                                                 Separating Techniques
            A mixture consists of two or more elements or compounds not          Mixtures can be separated by physical processes
            chemically combined together.                                        such as filtration, crystallisation, simple distillation,
            The chemical properties of each substance in the mixture are         fractional distillation and chromatography. These
            unchanged.                                                           physical processes do not involve chemical
                                                                                 reactions.
                        thermometer        Simple Distillation                Crystallisation/ Evaporation
                                                                                                                    Evaporating
                                                                                                                    basin
                                            Leibig condenser
                             Water
                             out
                           Round           Water in
                           bottomed
                   Heat    flask                                                 Type of mixture separated: A soluble solid
                                                                                 and aliquid(E.g. salt and water)
                                                                              To separate a soluble solid from a (non-
               Typeof mixture separated:
                                                                              flammable) liquid we use evaporation. If we want
               soluble solid dissolved in a liquid (usually water) e.g.
                                                                              to create hydrated salt crystals then do not
               salty water
                                                                              evaporate all the water from the mixture
               explanation:
               liquid boils off and condenses in the condenser. The
                                                                              The evaporating basin is wide and shallow, which
               thermometer will read the boiling point of the pure
                                                                              gives the liquid a large surface area for quicker
               liquid.
                                                                              evaporation
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                        Fractional Distillation
                                                        Filtration
         Fractionating
         column               Liebig condenser
                                                                                Filter
                                                         residue                paper
                                                                            Filter
                                                                            funnel
                                                                            filtrate
                                                         Type of mixture separated:
                                                         insoluble solid suspended in a liquid
         Type of mixture separated:
                                                         (usually water) e.g. sand and water.
         Soluble liquids with different boiling points e.g. crude oil
                                                        Explanation:
         The fractionating column has a temperature gradient and
                                                        Theinsoluble solid (called residue) gets caught in
         is hotter at the bottom than at the top
                                                        the filter paper, because the particles are too big
                                                        to fit through the holes in the paper.
         Explanation: When a mixture of soluble liquids is heated all
                                                        The filtrate is the substance that comes through
         liquids are evaporated. The liquid with the lower boiling point,
                                                        the filter paper.
         however, forms the greatest percentage of vapour. As the
         vapourmoves up the fractionating column it becomes more
         rich with the component that has the lowest boiling point. This
         is due to the vapour mixture condensing and evaporating as it
         movesup thecolumn.
         A thermometer measures thetemperature of the fractions
         before they condense. The liquid with the lowest boiling point
         will be the first 'fraction' to collect.
           Seechapter 4.8for information about chromatography
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                History of Development of the Atom
             Before the discovery of the electron atoms were thought to be tiny spheres that could not be divided
                    Plum-puddingmodel
                    Thediscovery of the electron led to
                    the plum-pudding model of the
                                                                                                            electrons
                    atom. The plum-pudding model
                    suggested that the atom was a ball
                    of positive charge with negative
                    electrons embedded in it.
                   Nuclearmodel
                The results from the Rutherford and Marsden’s alpha scattering experiments
                led to the plum-pudding model being replaced by the nuclear model.
                                                                                                                                                electrons
                In this model the centre of the atom was called the nucleus.
                This experiment showed:
                •   that all the mass of the atom was in the nucleus.                                                                         nucleus
                •   that all the positive charge of the atom was in the nucleus
                                                                                                                      Nuclear model
                The electrons were thought to orbit the nucleus, like planets around the sun.
                  In the experiment most of the alpha particles directed at thin gold foil passed through showing that
                  that all the mass of the atom was in the nucleus.
                  Buta fewwere deflected or bounced back, suggesting the positive charge was concentrated at the
                  centre of each gold atom.
                                                                                                                               x
                 Bohr Model                                                                                                   xx
                 Neils Bohr adapted the nuclear model by suggesting that electrons orbit the                                   x
                 nucleus at specific distances.
                                                                                                                       x             x            electrons
                                                                                                                       x             x
                 The electrons are on energy levels or shells.                                                                x
                  The theoretical calculations of Bohr agreed with experimental observations.
                                                                                                                              x x            nucleus
                                                                                                                        Bohr Model
                    Later experiments led to the idea that the positive charge of any nucleus could be subdivided into a whole
                    number of smaller particles, each particle having the same amount of positive charge. The name proton was
                    given to these particles.
                    Chadwick
                    Theexperimental work of James Chadwick provided the evidence to show the existence of neutronswithin the
                    nucleus. This was about 20 years after the nucleus became an accepted scientific idea. This could help explain
                    the existence of isotopes
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               The Atom
                                                                        Particle     Relative Mass      Relative Charge
                Atoms have a small central nucleus made
                                                                         Proton             1                  +1
                up of protons and neutrons around which
                                                                        Neutron             1                  0
                there are electrons.
                                                                        Electron       Very small              -1
                In an atom, the number of electrons is
                                                                       Size of atom
                equal to the number of protons in the
                                                                       Atomsare very small, having a radius of about
                                                                                    -10
                nucleus. Atoms have no overall electrical
                                                                       0.1 nm (1x 10   m).
                charge.
                                                                       The radius of a nucleus is less than 1/10 000 of
                                                                                                     -14
                                                                       that of the atom (about 1 x 10  m).
                All atoms of a particular element have the
                same number of protons.
                                                                       The number of protons in an atom is called its atomic number
                Atomsof different elements have different
                                                                       (proton number). Atoms are arranged in the modern periodic
                numbers of protons.
                                                                       table in order of their atomic number (proton number).
                                                                         To work out the number of neutrons in an atom subtract the
               Most of the mass of an atom is in the nucleus
                                                                         atomic number from the mass number
                                                                         Example Beryllium : atomic number 4, mass number 9.
               The total number of protons and neutrons in
                                                                         It has 4 protons, 4 electrons
               an atom is called its mass number
                                                                         and9-4 =5 neutrons
                 An atom of Lithium (Li) can be represented as follows:
                            Mass Number                 7       Li            Atomic Symbol
                         Atomic Number                   3
                           Theatomic number,is the number of protons in the nucleus.
                           Themass number is the total number of protons and neutrons in the atom.
                                       Number of neutrons = A - Z
               Isotopes
               Atomsof the same element can have different numbers of neutrons; these atoms are called isotopes of that element.
               Isotopes of an element have the same chemical properties because they have the same electronic structure
             Calculating Relative Atomic Mass
              Therelative atomic mass of an element is an average value that takes account of
              the abundance of the isotopes of the element.
                                                                                                              35       37
              R.A.M =  (isotopic mass x % abundance)                   Example 1. Chlorine has two isotopes    Cl and  Cl. 75% of a
                                                                                             35              37
                                         100                            sample of chlorine is  Cl and 25% is   Cl.
                                                                        Calculate the relative atomic mass of chlorine.
                                                                        R.A.M = [(75 x 35) + (25 x 37)] /100
                                                                               = 35.5
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