Atomic structure

Traps in Atomic structure

• Electron arrangement errors
  For Z ≤ 20: first shell holds 2, second 8, third up to 8 before the fourth starts (K = 2,8,8,1; Ca = 2,8,8,2).
• Puts 8 electrons in the first shell
  Shell capacities for the first 20 elements: 2, 8, 8, then the rest. Fill in order and check the total equals the atomic number.
• Miscounts atoms, electrons or steps
  After working, check by counting back: total electrons = atomic number; atoms in the formula match the subscripts; number of half-lives = time ÷ half-life.
• Uses the wrong arithmetic operation
  Write the relationship in words before substituting (crystals = solubility at high T − solubility at low T; neutrons = A − Z; three doublings = 2³). Then check the size of the answer.
• Picks a related substance or process with a different role
  Learn reagents and processes as one-line 'name → job' cards and contrast neighbours in a table (process | product | catalyst | conditions). Match the job in the stem, not just the topic.
• Mixes up a pair of similar chemical terms
  Keep a list of confusable pairs with one defining feature each and a hook. Underline the key word in the stem and check it against the definition, not the sound of the name.
• Reverses a trend, direction or effect
  State the rule with its direction before looking at the options (down group 1 reactivity INCREASES; exothermic forward reaction: heating shifts LEFT). An option that is the mirror image of the rule is the trap.
• Over-generalises a rule beyond where it holds
  Every rule in chemistry has conditions. Before applying one, check the conditions in the stem (electrode type, concentration, moles of gas on each side, metal position).
• Takes one factor as the whole answer
  Ask whether the option is necessary or sufficient. If the stem involves two changes or two conditions, both must appear in the answer.
• Uses everyday or macroscopic thinking instead of particles
  Ask what the particles, ions or electrons are doing. Properties of a substance come from its particles and bonds, not from how it looks or feels.
• Puts a substance or reaction in the wrong class
  Classify from the defining feature: functional group for organic series; bond type and structure for properties; reaction with acid AND alkali for amphoteric. Check each option against the feature, not the name.
• Uses the atomic number instead of the relative atomic mass
  Atomic number counts protons; relative atomic mass gives the mass of a mole. Mass calculations always use Ar (or Mr).