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Trends in the Periodic Table

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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Trends in the Periodic Table

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  1. Trends in the Periodic Table

  2. 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.

  3. 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.

  4. 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).

  5. 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.

  6. Going down a group, the ionization energy decreases. • Electrons are further out, so the nuclear charge is not felt as strongly. • Shielding effect contributes.

  7. 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.

  8. 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

  9. 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.

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