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Periodic Trends

>> Parts of this equation/concept include:

A -	Ionization Energy
B -	Electron Affinity
C -	Electronegativity >> Ionic versus Covalent Bonding

Ionization energy is the energy required to remove an electron from a gaseous atom. The higher the energy, the more difficult it is to remove the electron. The trend is for ionization energy to be higher for atoms further to the right on the periodic table, with the noble gases having the highest ionization energy. Ionization energy also increases from bottom to top on the periodic table.

>> Example 1

Rank the following atoms from highest to lowest ionization energy.

Ca, F, Ne, Ge

Solution:

The element whose position is the highest and furthest right in the periodic table is neon. Therefore it has the highest ionization energy. The one whose position is closest to the bottom and furthest left is platinum. Therefore it has the lowest ionization energy. Fluorine is very near neon; it should be next highest. Germanium is between fluorine and calcium and so is its ionization energy.

highest Ne > F > Ca > Pt lowest

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Electron affinity is the energy involved when an electron is added to a gaseous atom. Negative values of energy mean that energy was released during the process. Atoms with negative values of electron affinity have a very strong attraction for electrons. Positive values of electron affinity have very little attraction for electrons.

Electron affinity values become more negative for atoms further to the right on the periodic table up until the noble gases. Electron affinity values for noble gases are positive. Electron affinity values become more negative (or less positive) for atoms higher on the periodic table.

>> Example 2

Rank the following atoms for electron affinity.

Ca, F, Ne, Ge

Solution:

The atom that is highest and furthest right is neon, but that is a noble gas, which is perfectly happy with its electron configuration, so it is less likely to gain an electron. Fluorine is right next to neon and one electron short of a noble gas configuration, so it should have a very high affinity for electrons. Neon has no affinity for electrons.

highest electronegativity N > Cl > P > As > Ge lowest electronegativity

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Electronegativity is the tendency of an atom to draw electrons to itself when in a covalent bond. Consequently, the trends are the same as for electron affinity. The atoms with the highest electronegativity are fluorine, then oxygen, then nitrogen. It is also important to know that the electronegativity of hydrogen is slightly less than that of carbon.

>> Example 3

Rank the following from highest electronegativity to the lowest electronegativity.

N, P, Ge, As, Cl

Solution:

Nitrogen is on the list of the top three, so it is the highest. Chlorine is in the rightmost column (not counting noble gases) and still fairly high up, so it is next. Organizing the rest of the elements based on how close they are to fluorine, the ranking is

most negative F < Ge < Ca < Ne most positive

>> Ionic versus Covalent Bonding

Ionic bonds form from the attraction of ions to each other. The ions form when atoms very different in electronegativity actually gain and lose electrons. A bond is considered ionic if the difference in electronegativity between the bonding atoms is greater than 2.5. This is commonly the case between metals and nonmetals. If the electronegativity difference is zero, the atoms form a nonpolar covalent bond. This will only occur between atoms of the same element. Bonds where the electronegativity difference is between zero and 2.5 are polar covalent bonds.

>> Example 4

When bonds form between the following atoms, will the bond be ionic, polar, covalent, or nonpolar covalent?

1. Ca and F
2. H and F
3. F and F
4. O and F

Solution:

1. a metal and a nonmetal,  = 3.0, therefore an ionic bond
2. two nonmetals,  = 1.9, therefore a polar covalent bond
3. the same atom,  = 0, therefore a nonpolar covalent bond
4. two nonmetals,  = 0.5, therefore a polar covalent bond

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