9/16/2014
ALEKS Alex H. Yuan - 09/16/2014 9:55:09 PM EDT - Copyright © 2014 UC Regents and ALEKS Corporation
Alex H. Yuan - 09/16/2014 9:55:09 PM EDT
General Chemistry (First Semester) / Chem 110 FA 2014 – CH110 FA14 S25-36 (Dr.
Barber)
1.
Understanding periodic trends in atomic size
Re-order each list in the table below, if necessary, so that the atoms or ions in it are
listed in order of decreasing size.
atoms or ions
atoms or ions in order
of decreasing size
___ ___ ___
___ ___ ___
___ ___ ___
You can solve this problem by using four important facts about the size of atoms and ions:
Atoms or ions get smaller as you go from left to right along a period of the
Periodic Table.
For example, as you go across Period 2 the radius of the neutral atom shrinks from
pm in lithium to
pm in neon:
https://secure.aleks.com/alekscgi/x/Isl.exe/1o_u-IgNsIkr7j8P3jH-lJiYsPgPQIfQokR4hWMNIe-wKsFh3XosSrzjTjhrAPSQBKRp_4E6FEkL7dKArX7hGO…
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9/16/2014
ALEKS Alex H. Yuan - 09/16/2014 9:55:09 PM EDT - Copyright © 2014 UC Regents and ALEKS Corporation
Li
Be
B
C
N
O
F
Ne
Drag the slider to see how the size of the atom shrinks as you go across the
Periodic Table.
Atoms shrink as we go across a period because the effective nuclear charge
increases, and a stronger nuclear charge pulls the electrons closer to the nucleus.
For example, as we go across Period 2 the effective nuclear charge increases from
about
in lithium to about
in neon. That means the attractive force on the
outer electrons in a neon atom is about eight times stronger than the attractive force
on the outer electrons in a lithium atom. That's why the electrons in neon atoms orbit
much closer to the nucleus.
Atoms or ions get bigger as you go down along a group of the Periodic Table.
For example, as we go down Group 8A the radius of the neutral atom increases from
pm in helium to
pm in radon:
https://secure.aleks.com/alekscgi/x/Isl.exe/1o_u-IgNsIkr7j8P3jH-lJiYsPgPQIfQokR4hWMNIe-wKsFh3XosSrzjTjhrAPSQBKRp_4E6FEkL7dKArX7hGO…
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9/16/2014
ALEKS Alex H. Yuan - 09/16/2014 9:55:09 PM EDT - Copyright © 2014 UC Regents and ALEKS Corporation
He
Ne
Ar
Kr
Xe
Drag the slider to see how the size of the atom grows as you go down the Periodic
Table.
Atoms grow as we go down a group because each step down a group means another
shell of electrons around the atom. For example, as we go down Group 8A the
number of electron shells around the atom increases from in helium to in radon.
All those extra layers of electrons are why radon atoms are bigger than helium atoms.
Anions are larger than a neutral atom of the same element.
For example, look at this table:
neutral atom
symbol
radius
(pm)
anion
symbol
radius
(pm)
Adding extra electrons to an atom to make it an anion makes it bigger. Sometimes
that's because the new electrons must start another shell. But even if the new
electrons don't start a new shell, extra electrons "crowding in" among the rest
increases the general repulsion among the electrons orbiting the nucleus. This
pushes all the electrons apart from each other and makes the atom bigger.
Cations are smaller than a neutral atom of the same element.
For example, look at this table:
https://secure.aleks.com/alekscgi/x/Isl.exe/1o_u-IgNsIkr7j8P3jH-lJiYsPgPQIfQokR4hWMNIe-wKsFh3XosSrzjTjhrAPSQBKRp_4E6FEkL7dKArX7hGO…
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9/16/2014
ALEKS Alex H. Yuan - 09/16/2014 9:55:09 PM EDT - Copyright © 2014 UC Regents and ALEKS Corporation
neutral atom
symbol
radius
(pm)
cation
symbol
radius
(pm)
Removing electrons from an atom to make it a cation makes it smaller. Sometimes
that's because the lost electrons are the last in a shell, and that means the atom now
has one less layer of electrons. But even if the lost electrons aren't the last in a shell,
fewer electrons reduces the general repulsion among the electrons orbiting the
nucleus. That allows all the electrons to come closer to the nucleus without
"crowding" each other, and makes the atom smaller.
Using these four facts, you can order the atoms in the table by decreasing size.
Here is the completed table:
atoms or ions
atoms or ions in order
of decreasing size
Copyright © 2014 UC Regents and ALEKS Corporation
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