atomic structure and the periodic table

• Mendeleev


  Predicted the properties of elements that hadn't been discovered yet
• Mendeleev


  Changed the order of some elements to group them with elements with similar properties
• Elements with properties predicted by Mendeleev were discovered and filled the gaps
• This helped to confirm Mendeleev's ordering by properties and refute ordering by atomic weights
• Modern Periodic Table


  Elements are ordered by increasing atomic number
• This means there are repeating patterns in the properties of the elements
• Elements


  • Similar properties are in vertical columns called Groups
  • Horizontal rows are called Periods
  • Metals are found on the Left side
  • Non-metals are found on the right
• Explain why atoms are always neutral overall?


  They have the same number of positively charged protons and negatively charged electrons so the charges cancel out.
• Explain why the nucleus is always positively charged

  It contains only protons and neutrons. Protons have a positive charge and neutrons have no charge (they are neutral)
• Explain why the reactivity of the alkali metals increases as you go down the group.

  As you go down group 1:

  > The number of shells increase which means that the atoms are bigger.

  > The distance between the positive nucleus and the outer electron is greater which means that the attraction between the positive nucleus and the outer electron is weaker.

  > This means that it is easier to lose the outer shell electron and it is more reactive.
• Explain the trend in the reactivity of the halogens


  As you go down group 7, the number of shells increase which means that the atom size increase. This means that the distance between the positive nucleus and the incoming negative electron increases and so the attraction between the two is weaker. This means that the electron is gained less easily so the halogen is less reactive.
• Dalton discovered that all matter is made up of tiny particles called atoms in the 1800s. His models were indivisible solid spheres
• Thompson discovered electrons in 1897 and created the plum pudding model
• Bohr discovered shells in 1913 and updated the nuclear model.
• Rutherford completed the alpha particle scattering experiment in 1911 and discovered a positive nucleus that holds most of the atom’s mass. He also found that atoms are mostly empty space.
• Chadwick discovered neutrons are also in the nucleus in 1932. The nuclear model was updated again.
• The group 1 alkali metals, compared to other metals like the transition metals, are softer and have lower densities and melting points
• When group 1 metals react they lose the outer shell electron to form a 1+ ion
• An ionic compound is formed when the group 1 metals react and transfer their outer shell electron to the non-metal.
• The group 7 halogens are gases, very reactive, have a low density and are thermal and electrical insulators.
• The group 7 elements are all made of diatomic molecules (molecules containing 2 atoms)
• When group 7 halogens react with non-metals, they share an electron to form a covalent bond, forming a covalent compound.
• Transition metals have a high density, form different ions and are useful as catalysts. They have high melting points, are strong and hard, and not very reactive.
• Isotopes are different forms of an element that have the same number of protons (and electrons) but different numbers of neutrons.
• The first shell can only hold 2 electrons, the second shell 8, the third shell 8, and the fourth 18.
• Number of protons = Atomic number

  Number of neutrons = Mass number - atomic number

  Number of electrons = Atomic number
• Relative atomic mass


  Total mass of all atoms / total number of atoms
• Molecule


  Two or more atoms that are chemically bonded together
• Atoms of elements with incomplete outer shell of electrons are reactive
• Metal atoms lose electrons to get a full outer shell, which form positive ions
• Non-metal atoms gain electrons to get a full outer shell, which form negative ions
• Noble gases are the group 0 elements
• Noble gases are unreactive because they have full outer shells.