Atomic Structure and the Periodic Table

• Atoms are the smallest part of an element that can exist
• Atomic symbols

  Capital O is oxygen, capital L with lowercase i is lithium, capital H with lowercase e is helium
• There are about 100 different elements each with their own separate symbols
• Elements

  Can chemically react to make compounds, sometimes energy is released from or taken into the reaction
• Compounds

  Two or more elements chemically combined in fixed proportions
• Examples of compounds

  • Carbon dioxide
  • Water
• Chemical reaction: sodium reacts with chlorine

  Sodium + chlorine → sodium chloride
• Formula equations should be balanced, the number of each atom on each side of the arrow should be the same
• Mixtures

  Two or more elements or compounds that are not chemically combined together, each substance in the mixture keeps its chemical properties, and mixtures can be separated without a chemical reaction
• Methods for separating mixtures

  • Filtration
  • Crystallization
  • Simple distillation
  • Fractional distillation
  • Chromatography
• Filtration


  Separates an insoluble solid from a liquid
• Crystallization


  Separates a soluble solid from a solvent
• Simple distillation


  Separates two different liquids based on their different boiling points
• Fractional distillation


  Can separate many liquids at the same time
• Chromatography


  Separates inks or pigments from a mixture
• Before the discovery of electrons, atoms were thought to be tiny spheres that could not be divided
• Plum pudding model of atoms

  Electrons embedded in a ball of positive charge
• Alpha particle scattering experiment

  Most alpha particles pass straight through a thin gold foil, some are deflected, and a small proportion are reflected straight back
• The alpha particle scattering experiment led to the nuclear model of the atom
• Nuclear model of the atom

  • Positively charged nucleus with electrons orbiting it, mostly empty space
• Niels Bohr adapted the nuclear model to suggest electrons orbit the nucleus at specific distances
• Protons and neutrons were later discovered in the nucleus
• Atoms are always neutral, the number of electrons equals the number of protons
• Subatomic particles

  • Protons: positive charge, mass 1
  • Neutrons: neutral charge, mass 1
  • Electrons: negative charge, very small mass
• Drawing an atom from periodic table information

  Find atomic number (protons), same number of electrons, find number of neutrons by subtracting atomic number from mass number, draw protons and neutrons in nucleus, draw electrons in shells
• Electronic structure

  Electrons fill the lowest or innermost available shells in an atom
• Isotopes

  Atoms of the same element with the same number of protons but different number of neutrons
• Calculating relative atomic mass

  Use formula: (percentage abundance x relative atomic mass) + (percentage abundance x relative atomic mass) / 100
• The same number of protons but different number of neutrons they have the same chemical properties but they have different physical properties
• Calculating relative atomic mass with given abundance of Isotopes

  1. Percentage abundance times relative atomic mass of one isotope

  2. Percentage abundance times relative atomic mass of another isotope

  3. Add those two together

  4. Divide by 100
• Relative atomic mass of chlorine is 35.5
• Atomic weight


  Elements were arranged by atomic weight before electrons, protons and neutrons were discovered
• Every eighth element had similar properties
• Some elements were not in the appropriate places if the order of atomic weights was strictly followed
• Dmitri Mendeleev


  His periodic table followed atomic weights but left gaps where he thought new elements would be found
• Knowledge of Isotopes explained why the order based on atomic weight was not always correct
• Periodic table

  • Elements are arranged in order of atomic number
  • Elements with similar properties are in groups
  • Rows are called periods
• Transition metals

  Elements in the middle block of the periodic table from group 2 to group 3
• Electronic structure


  Number of electrons in the outermost shell of an atom
• Elements in the same group have the same number of electrons in their outermost shell