C1 - Atomic Structure

• The periodic table provides chemists with a structured organisation of the known chemical elements from which they can make sense of their physical and chemical properties
• The historical development of the periodic table and models of atomic structure provide good examples of how scientific ideas and explanations develop over time as new evidence emerges
• The arrangement of elements in the modern periodic table can be explained in terms of atomic structure which provides evidence for the model of a nuclear atom with electrons in energy levels
• Atom


  The smallest part of an element that can exist
• Chemical symbol


  Represents an atom of an element, e.g. O for oxygen, Na for sodium
• Number of different elements


  • About 100
• Elements are shown in the periodic table
• Compound


  Formed from elements by chemical reactions, contains two or more elements chemically combined in fixed proportions, can be represented by formulae
• Chemical reaction

  Involves the formation of one or more new substances, often involves a detectable energy change
• Compounds can only be separated into elements by chemical reactions
• Word equation


  Represents a chemical reaction using words
• Symbolic equation


  Represents a chemical reaction using symbols and formulae
• Mixture


  Two or more elements or compounds not chemically combined, chemical properties of each substance unchanged
• Methods to separate mixtures

  • Filtration
  • Crystallisation
  • Simple distillation
  • Fractional distillation
  • Chromatography
• Physical processes to separate mixtures do not involve chemical reactions and no new substances are made
• New experimental evidence may lead to a scientific model being changed or replaced
• Plum pudding model


  Atom is a ball of positive charge with negative electrons embedded in it
• Nuclear model


  Mass of atom concentrated at the centre (nucleus) and the nucleus is charged
• Bohr model


  Electrons orbit the nucleus at specific distances, theoretical calculations agreed with experimental observations
• Proton


  Particle with the same amount of positive charge, making up the nucleus
• Neutron


  Particle within the nucleus, discovered by James Chadwick
• The scattering experiment is likely Ernest Rutherford's gold foil experiment in 1909
• Plum pudding model

  • Proposed by J.J. Thomson in 1904
  • Describes the atom as a uniform, positively charged sphere with negatively charged electrons embedded within it
• Nuclear model

  • Proposed by Ernest Rutherford in 1911
  • Describes the atom as consisting of a small, dense, positively charged nucleus at the center, surrounded by negatively charged electrons orbiting around the nucleus
• The key difference between the two models is the depiction of the distribution of charge within the atom and the location of the majority of the atom's mass
• In an atom, the number of electrons is equal to the number of protons in the nucleus, so atoms have no overall electrical charge
• Atomic number


  The number of protons in an atom of an element
• Atoms of different elements have different numbers of protons
• Atoms are very small, having a radius of about 0.1 nm (1 x 10-10 m)
• The radius of a nucleus is less than 1/10 000 of that of the atom (about 1 x 10-14 m)
• Almost all of the mass of an atom is in the nucleus
• The sum of the protons and neutrons in an atom is its mass number
• Isotopes


  Atoms of the same element with the same amount of protons but different amounts of neutrons

  Therefore atoms with the same atomic number but a different atomic mass.
• Atoms can be represented with their atomic number and mass number
• Students should be able to calculate the numbers of protons, neutrons and electrons in an atom or ion, given its atomic number and mass number
• Relative atomic mass


  An average value that takes account of the abundance of the isotopes of the element
• Electronic structure

  The arrangement of electrons in an atom, can be represented by numbers or diagrams
• The elements in the periodic table are arranged in order of atomic (proton) number and so that elements with similar properties are in columns, known as groups
• Elements in the same group in the periodic table have the same number of electrons in their outer shell (outer electrons) and this gives them similar chemical properties
• Valence

  Is the amount of electrons in the outermost shell of the atom.