Chemistry

• Atoms are the smallest part of an element that can exist
• Examples of atoms


  • Lithium atom
  • Helium atom
• 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 discovered in the nucleus of the atom
• Atoms are always neutral, the number of electrons is always equal to 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), mass number (protons + neutrons), then calculate number of neutrons, draw nucleus with protons and neutrons, 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) for each isotope, add together and divide by 100
• Isotopes


  Same number of protons but different number of neutrons, have same chemical properties but different physical properties
• Calculating relative atomic mass with given abundance of Isotopes

  Percentage abundance times relative atomic mass of one isotope + percentage abundance times relative atomic mass of another isotope, divide by 100
• Relative atomic mass of chlorine is 35.5
• Early periodic table


  • Arranged by atomic weight, every 8th element had similar properties, some elements not in appropriate places
• Mendeleev's periodic table


  Followed atomic weights but left gaps for undiscovered elements, changed positions to match groups more closely
• Knowledge of isotopes explained why order based on atomic weight was not always correct
• Modern periodic table


  • Arranged in order of atomic number, elements with similar properties in groups, rows are called periods
• Metals


  React to form positive ions
• Differences between metals and non-metals

  • Metals are hard, shiny, high density, good conductors of electricity and heat, malleable

  Non-metals are dull, low density, poor conductors of electricity and heat, brittle
• Group 0 (noble gases)


  • Unreactive, have full outermost shell, boiling points increase down group
• Group 1 (alkali metals)


  • Have 1 electron in outermost shell, react with oxygen to form metal oxides, react with chlorine to form metal chlorides, react violently with water to form metal hydroxides and hydrogen gas