bbc

• Atom


  The smallest part of an element that can exist
• Atoms


  • Consist of a nucleus containing protons and neutrons, surrounded by electrons in shells
  • The number of subatomic particles in an atom can be calculated from the atom's atomic number and mass number
• Atomic number


  The number of protons in the nucleus of an atom
• Mass number


  The total number of protons and neutrons in the nucleus of an atom
• Isotopes


  Atoms of the same element with the same number of protons but different numbers of neutrons
• Relative atomic mass


  The average mass of the atoms of an element compared to the mass of a carbon-12 atom
• Ideas about atoms have changed over time as scientists developed new atomic models and gathered new experimental evidence
• John Dalton


  Published his ideas about atoms in 1803, thinking that all matter was made of tiny particles called atoms which he imagined as tiny spheres that could not be divided
• J.J. Thomson


  Carried out experiments and discovered the electron, leading him to suggest the plum pudding model of the atom
• Plum pudding model


  An early model of the atom in which an atom is a ball of positive charge with negative electrons embedded in it
• Ernest Rutherford's experiment


  Fired positively charged alpha particles at thin gold foil, with most going straight through but a few being scattered in different directions
• Nuclear model


  The scientific idea that an atom has electrons surrounding a nucleus that contains protons and neutrons
• In the nuclear model, the mass of an atom is concentrated at its centre in the nucleus, and the nucleus is positively charged
• Mendeleev's periodic table


  An early periodic table of the elements arranged in order of increasing atomic weights, taking into account the properties of the elements and their compounds
• Mendeleev's periodic table


  • Had gaps for elements not known at the time
  • Showed elements with similar chemical properties lined up in groups
  • Some pairs of elements next to each other were in the wrong order based on their atomic weights
• Mendeleev's predictions using gaps

  1. Looked at properties of elements next to gaps

  2. Predicted properties of undiscovered elements that would fit the gaps

  3. Example: Predicted 'eka-silicon' which turned out to be germanium with similar properties
• Iodine has a lower atomic weight than tellurium


  But iodine has similar chemical properties to chlorine and bromine, so Mendeleev swapped the positions of iodine and tellurium to align iodine with the halogen group
• The modern periodic table arranges elements in order of atomic number in periods and groups
• Electronic structure


  Models how electrons are arranged in atoms
• The periodic table is part of the Chemistry (Single Science) subject
• Before discovering protons, neutrons and electrons, scientists tried to classify elements by arranging them in order of their atomic weights
• Early periodic tables were incomplete, since many elements were unknown, and some elements were placed in groups with elements that were not similar to them
• Dmitri Mendeleev was a Russian chemist who published his first periodic table of the elements in 1869
• Mendeleev's periodic table arranged the elements in order of increasing atomic weights and took into account the properties of the elements and their compounds
• Mendeleev's periodic table


  An early periodic table created by Mendeleev
• Modern periodic table


  Elements are in order of atomic number in periods and groups
• Electronic structure


  Models how electrons are arranged in atoms
• Metals


  • Shiny element that is a good conductor of electricity and heat, and which forms basic oxides
  • In their chemical reactions, metal atoms lose electrons to form positive ions
• Non-metals


  • Element that is a poor conductor of electricity and heat, and which forms acidic oxides
• When metal atoms react

  They lose electrons to form positive ions
• Forming positive ions


  • When magnesium burns in air, each atom loses two electrons to form a Mg2+ ion
  • When sodium reacts with chlorine, each sodium atom loses one electron to form a Na+ ion
• Elements that do not form positive ions in their chemical reaction are non-metals
• Most metal oxides are basic, while most non-metal oxides are acidic
• Differences in physical properties between metals and non-metals (when solid)

  • Metals have high melting and boiling points, are good conductors of electricity and heat, are shiny, have high density, are malleable and ductile
  • Non-metals have low melting and boiling points, are poor conductors of electricity and heat, are dull, have low density, are brittle
• Elements in the same group of the periodic table

  • Show trends in physical properties, such as boiling point
  • Have the same number of electrons in their outer shell
  • Are similar in their chemical properties
• Group 7


  The group in the periodic table that starts with fluorine and ends with astatine
• Halogen


  • An element placed in group 7 of the periodic table
  • The name 'halogen' means 'salt-producing' because halogens produce a range of salts when they react with metals
• Displace


  • Take the place of another substance in a chemical reaction
  • A metal can displace a less reactive metal from its oxide, removing oxide ions from the less reactive metal and becoming an oxide itself
• Chlorine is more reactive than iodine


  A solution of chlorine can displace iodine from potassium iodide solution
• Displacement reaction of bromine solution with sodium iodide solution

  Br2(aq) + 2NaI(aq) → 2NaBr(aq) + I2(aq)