ATOMIC STRUCTURE AND THE HISTORY OF THE PERIODIC TABLE

• Compound


  A substance formed from two or more elements chemically combined in fixed proportions
• Compounds can only be separated into elements by chemical reactions
• Chemical reaction


  A process that involves the formation of one or more new substances, often with a detectable energy change
• Mixture

  Two or more elements or compounds not chemically combined together
• Separation techniques for mixtures


  • Filtration
  • Crystallisation
  • Simple distillation
  • Fractional distillation
  • Chromatography
• These physical processes do not involve chemical reactions and no new substances are made
• Separation and purification techniques for mixtures

  • Students should be able to describe, explain and give examples of the specified processes
  • Students should be able to suggest suitable separation and purification techniques for mixtures when given appropriate information
• New experimental evidence may lead to a scientific model being changed or replaced
• Plum pudding model of the atom

  The atom is a ball of positive charge with negative electrons embedded in it
• Nuclear model of the atom

  The mass of an atom is concentrated at the centre (nucleus) and the nucleus is charged
• Niels Bohr adapted the nuclear model by suggesting that electrons orbit the nucleus at specific distances
• 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.

  
• The experimental work of James Chadwick provided the evidence to show the existence of neutrons within the nucleus
• Students should be able to describe

  • Why the new evidence from the scattering experiment led to a change in the atomic model
  • The difference between the plum pudding model of the atom and the nuclear model of the atom
• In an atom, the number of electrons is equal to the number of protons in the nucleus. 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.
• Relative masses of subatomic particles

  • Protons
  • Neutrons
  • Electrons

  
• Isotopes


  Atoms of the same element with different numbers of neutrons
• 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.
• The relative atomic mass of an element is an average value that takes account of the abundance of the isotopes of the element.
• Students should be able to calculate the relative atomic mass of an element given the percentage abundance of its isotopes.
• Electronic structure


  The arrangement of electrons in an atom, represented by numbers or a diagram
• Students should be able to represent the electronic structures of the first twenty elements of the periodic table in both forms.
• Periodic table


  A table of elements arranged in order of atomic number and with similar properties in columns (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.
• Students should be able to

  • Explain how the position of an element in the periodic table is related to the arrangement of electrons in its atoms and hence to its atomic number
  • Predict possible reactions and probable reactivity of elements from their positions in the periodic table
• Before the discovery of protons, neutrons and electrons, scientists attempted to classify the elements by arranging them in order of their atomic weights.
• Mendeleev overcame some of the problems by leaving gaps for elements that he thought had not been discovered and in some places changed the order based on atomic weights.
• The noble gases have eight electrons in their outer shell, except for helium, which has only two electrons.
• The boiling points of the noble gases increase with increasing relative atomic mass (going down the group).
• Students should be able to

  • Explain how properties of the elements in Group 0 depend on the outer shell of electrons of the atoms
  • Predict properties from given trends down the group
• Alkali metals


  The elements in Group 1 of the periodic table, which have characteristic properties due to the single electron in their outer shell
• In Group 1, the reactivity of the elements increases going down the group.
• Students should be able to

  • Explain how properties of the elements in Group 1 depend on the outer shell of electrons of the atoms
  • Predict properties from given trends down the group
• Helium


  Element with only two electrons
• The boiling points of the noble gases

  Increase with increasing relative atomic mass (going down the group)
• Properties of the elements in Group 0

  • Depend on the outer shell of electrons of the atoms