Chemistry

• Dalton's atomic theory:

  • Atoms are tiny particles made of elements
  • Atoms cannot be divided
  • All the atoms in an element are the same
  • Atoms of one element are different from those of other elements
• Thompson's discovery about electrons:

  • They have a negative charge
  • They can be deflected by a magnet and electric field
  • They have very small mass
• Plum pudding model explanation:

  • Atoms are made up of negative electrons moving around in a sea of positive charge
• Rutherford's proposals after the gold leaf experiment:

  • Most of the mass and positive charge of the atom are in the nucleus
  • Electrons orbit the nucleus
  • Most of the atom’s volume is the space between the nucleus and the electrons
  • Overall positive and negative charges must balance
• Current model of the atom explanation:

  • Protons and neutrons are found in the nucleus
  • Electrons orbit in shells
  • Nucleus is tiny compared to the total volume of the atom
  • Most of the atom’s mass is in the nucleus
  • Most of the atom is empty space between the nucleus and the electrons
• Charge of a proton: 1+
• Charge of an electron: 1-
• Particle with the same mass as a proton: Neutron
• Particles that make up most of an atom’s mass: Protons and neutrons
• Letter used to represent the atomic number of an atom: Z
• Atomic number tells about an element: Atomic number = number of protons in an atom
• Letter representing the mass number: A
• How to calculate mass number: Mass number = number of protons + number of neutrons
• How to calculate the number of neutrons: Number of neutrons = mass number - atomic number
• Isotope definition: Atoms of the same element with different number of neutrons
• Reason why different isotopes of the same element react in the same way:

  • Neutrons have no impact on the chemical reactivity
  • Reactions involve electrons, isotopes have the same number of electrons in the same arrangement
• Ions definition: Charged particles formed when an atom loses or gains electrons
• Charge of an ion when electrons are gained: Negative; positive charge when electrons are lost
• Unit used to measure atomic masses: Unified atomic mass unit, u
• Relative atomic mass definition: The weighted mean mass of an
• Relative atomic mass is the weighted mean mass of an atom of an element compared with one twelfth of the mass of an atom of carbon-12
• The unit of relative atomic mass is "no units"
• Relative isotopic mass is the mass of an atom of an isotope compared with one twelfth of the mass of an atom of carbon-12
• The relative isotopic mass is the same as the mass number
• Two assumptions made when calculating mass number:

  1. Contribution of the electron is neglected

  2. Mass of both proton and neutron is taken as 1.0 u
• To calculate the relative molecular mass and relative formula mass, add the relative atomic masses of each atom making up the molecule or the formula
• Uses of mass spectrometry:

  • Identify unknown compounds
  • Find relative abundance of each isotope of an element
  • Determine structural information
• A mass spectrometer works by:

  1. Converting the sample into positive ions

  2. Passing ions through the apparatus for separation based on mass to charge ratio

  3. Analyzing the data with a computer to produce a mass spectrum
• The group number indicates a vertical column in the periodic table
• Group number is related to the number of electrons in the outer shell
• Metals usually lose electrons
• The elements beryllium, boron, carbon, and silicon don't tend to form ions because it requires a lot of energy to transfer outer shell electrons
• Molecular ions are covalently bonded atoms that lose or gain electrons
• The charge of an ammonium ion is +1 (NH4+)
• The charge of a hydroxide ion is -1 (OH-)
• The charge of a nitrate ion is -1 (NO3-)
• The charge of a carbonate ion is -2 (CO32-)
• An empirical formula is the simplest whole number ratio of atoms of each element present in a compound
• To calculate the empirical formula:

  1. Divide the amount of each element by its molar mass

  2. Divide the answers by the smallest value obtained

  3. If there is a decimal, divide by a suitable number to make it a whole number
• The charge of a sulfate ion is -2 (SO42-)