Chap 2: Atoms, elements and compounds

• Element


  A pure substance made up of only one type of atom, the simplest form of matter
• Chemical symbol


  Represents an element, e.g. O for oxygen, H for hydrogen
• There are about 118 known elements, each with unique properties
• Elements cannot be broken down into simpler substances
• Elements


  • Oxygen molecule (O2)
  • Carbon
• Compound


  A substance formed when two or more different elements chemically combine, with distinct and unique properties
• Chemical formula


  Represents a compound, e.g. H2O for water, CO2 for carbon dioxide
• Compounds


  • Water (H2O)
  • Carbon dioxide (CO2)
• Mixture


  A combination of two or more substances (elements or compounds) that are physically mixed together but not chemically bonded
• Mixture


  • Salt water
• Atom


  The smallest indivisible unit of an element
• Molecule


  A group of atoms bonded together
• Difference between molecule and compound

  A molecule can be made of atoms of the same or different elements, while a compound is a specific type of molecule consisting of atoms of different elements chemically bonded together
• Differences between elements, compounds, and mixtures

  • Elements are made of only one type of atom
  • Compounds are made of two or more different types of atoms
  • Mixtures contain two or more elements or compounds that are not chemically bonded
• Nucleus


  The central core of an atom, containing protons and neutrons
• Proton


  Positively charged particle found in the nucleus
• Neutron


  Electrically neutral particle found in the nucleus
• Electron


  Negatively charged particle that orbits the nucleus in specific energy levels or shells
• Electron shells


  The energy levels where electrons orbit the nucleus
• Subatomic particles


  • Protons have a relative charge of +1 and mass of ~1 amu
  • Neutrons have a relative charge of 0 and mass of ~1 amu
  • Electrons have a relative charge of -1 and mass of ~1/1836 amu
• Atomic number (Z)


  The number of protons in the nucleus of an atom, which identifies the element
• Mass number (A)

  The total number of protons and neutrons in the nucleus of an atom
• The number of protons equals the number of electrons in a neutral atom
• The atomic number helps identify the element's location on the periodic table
• Relative atomic mass

  The average mass of all the isotopes of an element, often rounded to the nearest whole number
• The mass number is equal to the number of protons plus the number of neutrons
• Finding the number of neutrons in an atom

  Subtract the number of protons (atomic number) from the mass number
• Electron configuration

  A way of describing how electrons are arranged in an atom
• Filling electron shells

  1. Electrons fill up shells or energy levels from the innermost to the outermost

  2. Inner shells fill first before moving to the next shell
• Electron shell capacity

  • First shell can hold 2 electrons
  • Second shell can hold 8 electrons
  • Third shell can hold 8 electrons
• Electron configuration notation

  Writing the number of electrons in each shell separated by commas
• Atoms becoming ions

  • Atoms want a stable full outer shell of electrons
  • This can be done by giving away or taking in electrons
• Noble gases

  • Have full outer shells
  • Do not react with other elements
• Group number

  Equal to the number of outer shell electrons
• Period number

  Equal to the number of occupied electron shells
• Isotopes


  Different atoms of the same element with the same number of protons but different number of neutrons
• Ions


  Atoms with a net electric charge due to the gain or loss of electrons
• Isotopes of the same element have the same chemical properties because they have the same electronic configuration
• Relative atomic mass


  The average mass of all the isotopes of an element compared to 1/12 the mass of a carbon-12 atom
• Calculating relative atomic mass


  Percentage of isotope 1 x mass number of isotope 1 + percentage of isotope 2 x mass number of isotope 2 / 100