atoms, elements, compounds

• Atoms:

  • ATOMS are incredibly SMALL, and make up EVERYTHING in the universe.
  • They are the SMALLEST part of an ELEMENT that can exist. 
  • They are composed of THREE even smaller particles known as SUB-ATOMIC PARTICLES: PROTONS, NEUTRONS, and ELECTRONS.
• Inside the atoms:

  • At the center of an atom lies the NUCLEUS, containing PROTONS and NEUTRONS.
  • PROTONS have a positive charge (+1) and a relative mass of 1.
  • NEUTRONS have no charge (0) and a relative mass of 1.
  • ELECTRONS orbit around the nucleus in electron shells. They are negatively charged (−1) and are VERY SMALL, with virtually no mass when compared to protons and neutrons.
• Why are atoms neutral and have no overall charge?

  
  

  They have the SAME NUMBER OF PROTONS AND ELECTRONS, which CANCELS OUT THEIR CHARGES.
• Atomic number and mass number:


  • The ATOMIC NUMBER indicates how many protons are in an atom's nucleus.
  • The MASS NUMBER is protons + neutrons.
  • To find the number of NEUTRONS, SUBTRACT the atomic number from the mass number.
• Elements:

  • An ELEMENT is a substance made up of ONE TYPE OF ATOM.
  • There are over 100 different discovered ELEMENTS which can all be found in the PERIODIC TABLE.
  • All the atoms in an element contain the SAME NUMBER of PROTONS in their nucleus.
  • When atoms have different numbers of PROTONS, they are classified as DIFFERENT elements.
  • This means the ATOMIC NUMBER determines the ELEMENT of an atom. For example, Carbon atoms ALWAYS have 12 protons (has an atomic number of 12).
• Why are atoms neutral and have no overall charge?


  They have the SAME NUMBER of PROTONS and ELECTRONS, which CANCELS OUT THEIR CHARGES.
• Isotopes:

  • All atoms of a particular ELEMENT have the same ATOMIC NUMBER but may have different numbers of NEUTRONS.
  • ISOTOPES are different forms of the same element, having the SAME NUMBER OF PROTONS (same atomic number) but DIFFERENT NUMBERS OF NEUTRONS (different mass numbers).
• Compounds:

  • COMPOUNDS are substances that form when two or more elements react and their atoms combine.
  • These elements join in FIXED PROPORTIONS and are held together by CHEMICAL BONDS.
  • In compounds, only the ELECTRONS are involved in bond formation while the nuclei remain unchanged.
  • The PROPERTIES of a compound are usually quite different from the properties of the original elements.
• Relative atomic mass formula:

  R.A.M= (isotope 1)mass x abundance + (isotope 2) mass x abundance/100

  
• ABUNDANCE:


  The ABUNDANCE of an element is HOW COMMON a particular isotope is.
• ELECTRON STRUCTURE:

  • FIRST shell can hold up to 2 ELECTRONS.
  • SECOND and THIRD shells can each hold up to 8 ELECTRONS.
• ELECTRON STRUCTURE:

  • ELECTRONS are organised around the nucleus in SHELLS or energy levels, which determine the CHEMICAL PROPERTIES of an atom.
  • These shells are filled from the LOWEST to the HIGHEST energy levels, with the INNERMOST shells being filled FIRST.