Atomic Structure and Periodic Table

• 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
• There are about 100 different elements
• Element


  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
• Mixture


  Two or more elements or compounds not chemically combined, chemical properties of each substance unchanged, can be separated by physical processes
• 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 nucleus is charged
• Bohr model


  Electrons orbit the nucleus at specific distances

  
• Proton


  Positive charge in the nucleus
• Neutron


  Neutral particle in the nucleus
• Electron


  Negative charge orbiting the nucleus

  
• Atomic number


  Number of protons in an atom of an element
• Mass number


  Sum of protons and neutrons in an atom
• Isotopes


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


  Average value taking account of the abundance of the isotopes of an element
• Electronic structure


  Representation of the electrons in an atom occupying the lowest available energy levels
• Periodic table


  Elements arranged in order of atomic number, with similar properties in columns (groups)
• Early periodic tables were incomplete and some elements were placed in inappropriate groups if the strict order of atomic weights was followed
• Mendeleev


  Left gaps for undiscovered elements and changed order based on atomic weights, elements with properties predicted by Mendeleev were later discovered
• Metals


  Elements that react to form positive ions
• Non-metals


  Elements that do not form positive ions
• Noble gases


  Group 0 elements, unreactive, have stable arrangements of electrons
• Alkali metals


  Group 1 elements, have characteristic properties due to single electron in outer shell
• Alkali metals reacting with oxygen

  1. Lithium forms lithium oxide

  2. Sodium forms sodium peroxide

  3. Potassium forms superoxides
• Alkali metals reacting with chlorine


  Form metal chlorides (MCl), highly exothermic reactions
• Alkali metals reacting with water

  Produce hydrogen gas and metal hydroxide
• In Group 1


  Reactivity of elements increases going down the group
• Halogens


  Group 7 elements, have similar reactions due to seven electrons in outer shell, non-metals
• In Group 7


  Further down the group, higher relative molecular mass, melting point and boiling point, reactivity decreases
• Chlorine reacting with metals

  Forms ionic metal chlorides, highly exothermic
• Bromine reacting with metals

  Forms metal bromides, less reactive than chlorine
• Iodine reacting with metals

  Forms metal iodides, least reactive among halogens
• Chlorine reacting with non-metals

  Forms covalent compounds, including with other halogens and hydrogen
• Bromine reacting with non-metals

  Forms covalent compounds, including with hydrogen and oxygen
• Iodine reacting with non-metals

  Forms covalent compounds, including with hydrogen and oxygen
• Transition elements


  Metals with similar properties, different from Group 1 elements, have higher melting points, greater densities, strength and hardness, less reactive with oxygen, water and halogens, many have ions with different charges, form coloured compounds, useful as catalysts
• Elements are grouped into periods (horizontal rows) and groups (vertical columns).