trends in the periodic table. atomic radius the best measure of atomic radius is the bond radius....
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Trends in the Periodic Table
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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.
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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.
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Going across a period, the atomic radius decreasesElectrons 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).
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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.
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Going down a group, the ionization energy decreases.Electrons are further out, so the nuclear
charge is not felt as strongly.Shielding effect contributes.
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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.
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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- E is a negative number with a high absolute value
e.g. F: E = -328.5 kJ/mole
Low electron affinity: doesn’t “want” to gain an electron
e.g. Noble gases: have positive values of E
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Going down a group, the electron affinity decresesShielding more than offsets the increase in
nuclear charge Going across a period, the electron affinity
increases (E 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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