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Trends in the Periodic Table. Atomic radius. The best measure of atomic radius is the bond radius. Measure the distance between the nuclei of 2 atoms bonded together and divide by two. Going down a group, the atomic radius increases.

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atomic radius
Atomic radius
  • The best measure of atomic radius is the bond radius. Measure the distance between the nuclei of 2 atoms bonded together and divide by two.
slide3
Going down a group, the atomic radius increases.
    • Larger atoms have more electrons farther away from the nucleus.
    • The inner electrons shield the outer electrons from the full effect of the positive charge of the nucleus.
slide4
Going across a period, the atomic radius decreases
    • Electrons are being added to the same principal energy level.
    • For every added electron, a proton is also being added to the nucleus, increasing the charge, pulling the electrons tighter in.
    • This change is not as noticeable with heavier elements (inner electrons shield).
ionization energy
Ionization Energy
  • The energy required to remove an electron from an atom in the gas phase
  • There is a series of ionization energies for each electron removed. These energies get higher for each subsequent electron.
  • The trends given are for the first electron removed.
slide7
Going down a group, the ionization energy decreases.
    • Electrons are further out, so the nuclear charge is not felt as strongly.
    • Shielding effect contributes.
slide8
Going across a period, the ionization energy increases.
    • For every added electron, a proton is also being added to the nucleus, increasing the charge.
    • The same principal energy level is being filled, so the shielding effect is a constant.
  • There are some exceptions to this trend, normally in cases of full or half-full energy sublevels.
electron affinity
Electron Affinity
  • Measures how much an atom “wants” to gain electrons
  • Is the change in energy associated with gaining an electron
    • High electron affinity: really “wants” to gain an electron- rE is a negative number with a high absolute value
        • e.g. F: rE = -328.5 kJ/mole
    • Low electron affinity: doesn’t “want” to gain an electron
        • e.g. Noble gases: have positive values of rE
slide11
Going down a group, the electron affinity decreses
    • Shielding more than offsets the increase in nuclear charge
  • Going across a period, the electron affinity increases (rE becomes more negative).
    • Shielding remains constant, the nuclear charge increases
      • EXCEPTION: The noble gases have the lowest electron affinities of all.