Ask A Scientist Chemistry Archive

name         Marianne P.
status       educator
age          40s

Question -   Do elements that have more electrons take up more space
than elements that have less electrons?
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Dear Marianne,

No. F has the smallest covalent radius of all of the atoms, but
F has more electrons than Li, Be, B, C, N, or O.
The size is determined not only by the increasing number of electrons (which
tends to increase the size) but the increasing charge on the nucleus (which
tends to decrease the size as it increases the electron-nucleus
attraction) and also on increases in the principal quantum number of the
outermost electrons (which tends to increase the size of the atom because
large-n orbitals are farther away from the nucleus than small-n orbitals
with the same subshell number l).

The increase in Z is the most important effect as you go from left to
right, so the atoms get smaller.  The increase in the principal
quantum number n is the more important as you go from top to bottom,
so the atoms get bigger.

best regards,
Prof. Topper
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Yes, the larger the atomic number (and hence more electrons) the more
space an atom occupies. Although the term "atomic radius" is used, it has no
precise definition, so your use of the term "take up more space" is really
appropriate. Roughly speaking, an approximate radius of an atom is:
R(approx.) ~ (n^2/Z)* R(bohr) where n is the principle quantum number of the
valence shell, Z is the atomic number and R(bohr) is 5.29 nanometers. The
text "Atoms and Molecules" by Karplus and Porter, among others, gives a
detailed discussion regarding your inquiry.

Vince Calder
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Not necessarily.  If you go from top to bottom in one column of the periodic
table, the atomic radii of the elements in the lower rows (more electrons)
will be larger.  However, it is not so simple going from left to right.  For
instance, in the lanthanide row (the row from cerium to lutetium), the
atomic radii actually get smaller from left to right.

How can this happen?  It is a matter of positive and negative charges.  An
atom's size is dictated by how far away from the nucleus its outermost
electrons are.  This is determined by two factors: how much energy the
electron has (the more energy it has, the faster it moves, and the farther
away from the nucleus it swings), and how much charge from the nucleus it
feels (the greater the nuclear charge, the stronger the force pulling the
electron toward the nucleus, so the closer it will stay).  Since atoms have
the same number of protons (positive charges in the nucleus) and electrons,
more electrons means more protons, and more positive charge in the nucleus.
How much of that positive charge an electron feels depends on how many other
electrons get between it and the nucleus, "screening" the charge.  As much
as they can, electrons stay away from each other as they swarm around the
nucleus.  If one proton and one electron are added to an atom, the other
electrons will usually feel an increased attraction to the nucleus - the
additional electron does not quite cancel out all the influence of the
additional proton.

Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
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