Variation of Atomic Properties with Electronic Structure
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Atomic and Ionic Size
Even though its difficult to define the "size" of an atom or ion, they do behave in many ways as though they have characteristic sizes.
The size of atoms range from about 1.4 x 10-10 to 5.7 x 10-10 m in diameter.
Their radii, which is the way their size is usually spcified, range from about 7.0 x 10-11 to 2.9 x 10-10 m.
A million carbon atoms placed side by side would be less than 0.2 mm long.
To more easily express the size of atoms, a unit has been made, called the angstrom (Å).
1 Å = 1 x 10-10m
The angstrom is not an SI unit.
Instead, most current journals use picometers (10-12 m) or nanometers (10-9 m).
Basically, you should know the trends of that happen to the sizes of elements as you go across the periodic table.
Going from top to bottom within a group, the size increases because of larger orbitals as you go down the column.
Going from left to right within a period, the size decreases because the increasing positive nuclear charge felt is attracting the valence electrons inward.
Atoms gain or lose electrons to form ions, and when they do, significant size changes take place.
When electrons are added to an atom (creating a negative ion), the repulsions between them increase, and causes them to push away from each other, therefore making the size of the ion larger than the neutral atom.
When electrons are removed from an atom (creating a positive ion), the repulsions decrease, and the remaining electrons are drawn closer to the nucleus, therefore making the size of the ion smaller than the neutral atom.
Ionization Energy
Ionization energy (IE) is the energy needed to remove an electron from an isolated, gaseous atom or ion in its ground state.
In other words, it is a measure of how much energy is required to remove an electron, and thus a meaure of how tightly the atom is holding the electron.
In the periodic table, going from top to bottom within a group, the IE decreases because the electrons are farther away from the nucleus in the bottom elements.
Going from left to right within a period, the IE increases because the increasing nuclear charge is holding electrons more tightly.
It increases irregularly though because in some places, there is a change of the nature of the subshell (for example: from a 2s to a 2p).
It is also because the electrons that are in pairs inside orbitals are easier to remove than when they are alone because a pair of electrons repel each other.
The electron configuration of the noble gases are very stable because they have filled up their subshells.
The have a very high IE and it is extremely difficult to break an electron away, and therefore the noble gases do not react much.
Electron Affinity
Electron affinity (EA) is the potential energy change associated with the addition of an electron to a gaseous atom or ion in its ground state.
For nearly all elements, the addition of one electron to a neutral atom is exothermic, and the EA is a negative value.
This is because the incoming electron experiences an attraction to the nucleus as it approaches, causing the potential energy to be lowered.
But in an already negative ion, work must be done to force another electron in.
The trend for EA is the same as IE, with similar irregularities because those that have lose electrons easily will have little attraction for additional electrons.
