Chemistry paper 2

• The Dalton model of an atom has changed over time due to the discovery of subatomic particles
• Recall the relative charge and relative mass of protons, neutrons, and electrons:

  • Proton: +1 charge, mass of 1
  • Neutron: 0 charge, mass of 1
  • Electron: -1 charge, mass of 1/1836
• Explain why atoms contain an equal number of protons and electrons:

  • Atoms are neutral, with protons having a +1 charge and electrons having a -1 charge
  • The amount of protons equals the amount of electrons, canceling out the charges
• Describe the nucleus of an atom as very small compared to the overall size of the atom
• Recall that most of the mass of an atom is concentrated in the nucleus
• Recall the meaning of the term mass number of an atom:

  • Mass (nucleon) Number = number of protons + neutrons
• Describe atoms of a given element as having the same number of protons in the nucleus, and that this number is unique to that element
• Describe isotopes as different atoms of the same element containing the same number of protons but different numbers of neutrons in their nuclei
• Calculate the numbers of protons, neutrons, and electrons in atoms given the atomic number and mass number:

  • Atomic (proton) Number = number of protons (equal to number of electrons if it's an atom)
  • Calculate number of neutrons by doing mass number ÷ atomic number
• Explain how the existence of isotopes results in relative atomic masses of some elements not being whole numbers:

  • Isotopes have the same number of protons but different numbers of neutrons, leading to different atomic masses
  • Relative atomic mass is calculated using the abundance of different isotopes, resulting in non-whole numbers
• Calculate the relative atomic mass of an element from the relative masses and abundances of its isotopes:

  • Example: Calculate the R.A.M. of chlorine in a sample with isotopes chlorine-35 and chlorine-37 in specific proportions
  • R.A.M. = (mass of isotope-A x % of isotope-A) + (mass of isotope-B x % of isotope-B) / 100
  • Calculate the R.A.M. of chlorine in the sample
• Three ideas in John Dalton’s theory about the atom:

  • Atoms cannot be created, divided or destroyed.
  • Atoms of the same element are exactly the same and atoms of different elements are different.
  • Atoms join with other atoms to make new substances
• Discovery that caused the original Dalton model of an atom to change: The discovery of subatomic particles
• How JJ Thomson discovered the electron:

  • Thomson experimented with a cathode ray tube.
  • The beam moved towards the positively charged plate, indicating the particles had a negative charge
• Atomic model proposed by JJ Thomson: Plum pudding model with negatively charged electrons scattered through a positively charged material
• Discovery from Ernest Rutherford's gold foil experiment:

  • Most particles passed straight through, suggesting atoms were mostly empty space.
  • Few particles were deflected or bounced back, indicating a tiny, dense, positively charged nucleus
• Rutherford’s new model of the atom:

  • Mass concentrated in the central nucleus.
  • Mostly empty space.
  • Electrons travel in random paths around the nucleus
• Structure of an atom:

  • Small central nucleus made up of protons and neutrons.
  • Electrons orbit (move around) the nucleus in shells
• Radius of the nucleus: 1x10-14 m, which is 1/10000 of the atomic radius
• Relative masses of protons, neutrons, and electrons:

  • Proton: 1
  • Neutron: 1
  • Electron: 1/1836
• Relative charges of protons, neutrons, and electrons:

  • Proton: +1
  • Neutron: 0
  • Electron: -1
• Reason why atoms contain equal numbers of protons and electrons: Atoms are stable with no overall charge, so the number of protons and electrons must balance
• Mass of an atom is concentrated in the nucleus
• Atomic number of an atom represents the number of protons
• Mass number of an atom is the number of protons and neutrons found in the nucleus
• Atoms of the same element have the same number of protons in the nucleus, which is unique to that element
• Isotope definition: Atoms with the same number of protons but different number of neutrons
• Boron with atomic number 5 and mass number 11 has:

  • 5 protons
  • 5 electrons
  • 6 neutrons
• Reason why relative atomic mass is not always a whole number: Different isotopes of the same element have different mass numbers, so the relative atomic mass is an average of these isotopes
• Values required to calculate the relative atomic mass of chlorine: Mass numbers and relative abundances of its isotopes
• Two values required to calculate the relative atomic mass of chlorine:

  • Mass numbers and relative abundances of all the isotopes of chlorine
• How Mendeleev arranged elements in his periodic table:

  • Elements arranged with increasing atomic masses
  • Elements with similar properties put into groups due to periodic trends in chemical properties
  • Switched the position of some elements
  • Gaps left for undiscovered elements
• How Mendeleev predicted the properties of new elements:

  • Left gaps in his periodic table
  • Used the properties of elements next to these gaps to predict the properties of undiscovered elements
• Why Mendeleev's table lacked accuracy in ordering elements:

  • Isotopes were poorly understood at the time
  • Protons and neutrons had not yet been discovered
• How elements are arranged in the modern periodic table:

  • In order of increasing atomic number
• Fill in the blank: 'Elements in the same group (column) have similar _______ ______':

  • Chemical properties
• Why elements in the same column have similar chemical properties:

  • Same number of outer shell electrons
  • Number of outer shell electrons determines how an atom reacts
• What the period (row) number tells you about all the elements in that period:

  • Elements in the same period have the same number of electron shells
  • e.g. all elements in period 4 have 4 shells of electrons
• What the group (column) number tells you about all the elements in that group:

  • All elements in the same group have the same number of outer electrons
  • e.g. all elements in group 2 have 2 electrons in their outer shell
• On which side of the periodic table metals are positioned:

  • Left-hand side