Chemistry, Chapter 5, Section 3:

Electron Configurations &

Periodic Properties

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Atomic Radius• Defined as one-half the distance between

the nuclei of identical atoms that are bonded together (pm: picometers)

• The period trend is decreasing from left to right– Electron number increases and they get

pulled closer to the nucleus, decreasing the average size

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Atomic Radius• The group trend is increasing down a group

– As electrons occupy sublevels in higher energy levels they are located farther from the nucleus and the atom’s size increases

• Cation: an atom that has lost one or more electrons (positive ion)– Leads to a decrease in atomic radius

• Anion: an atom that has gained one or more electrons (negative ion)– Leads to an increase in atomic radius

Valence Electrons• Valence electrons are the electrons

available to be lost, gained or shared in the formation of chemical compounds

• For the main group elements, the valence electrons are in the outermost s an p sublevels

• Groups 1 and 2 have 1 and 2 valence electrons, respectively

• Group 13-18 have 3 through 8 electrons (group number minus 10)– Also referred to as groups IIIA through VIIIA

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Ionization Energy, IE• Defined as the energy required (kJ/mol) to

remove one electron from a neutral atom

• The period trend is increasing from left to right

• The group trend is decreasing down a group

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Ionization Energy, IE• The increase across a period is caused by

increasing nuclear charge– A higher charge more strongly attracts

electrons in the same energy level

• The decrease down a group is due to the higher energy levels– Outer energy level electrons are found farther

from the nucleus, thus more easily removed

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Ionization Energy, IE• Why do alkali metals have a low IE?

– They only have one electron in their outer energy level, which is easy to remove; therefore they are highly reactive

• Why do noble gases have a high IE?– They have their outer energy levels filled with

electrons, which are difficult to remove; therefore they are unreactive

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Removing Electrons from Cations• With sufficient energy, electrons can be

removed from cations– Referred to as the 2nd ionization energy (IE2) and

3rd ionization energy (IE3), etc.

• Notice with each successive IE, the number increases– Fewer electrons remain to shield electrons from

the attractive nuclear charge

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Electron Affinity• Defined as the energy change that occurs

when a neutral atom acquires an electron– Most atoms release energy (negative kJ/mol)

when this happens

– The more negative the value, the greater the electron affinity

• The period trend is increasing across

• The group trend is decreasing down a group

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Electronegativity• Defined as a measure of the ability of an

atom in a chemical compound to attract electrons– Electrons hold atoms together in chemical

bonding; some atoms draw electrons to themselves more than others

• The period trend is increasing across

• The group trend is decreasing down

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Electronegativity• Measured in Pauling units

• Fluorine (most electronegative) is assigned a value of 4.0; values of other elements is calculated in relation to this value

• Some noble gases do (rarely) form compounds; when they do their electronegativity is high, like the halogens

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Webelements.com Webelements.com

Atomic and Ionic Radii