Atomic sizeAtomic size

Patterns in Atomic SizePatterns in Atomic Size

• Valence electrons are involved in reactions and determine size (usually outer energy level –e)

• Two factors affect size: nucleus and the balance between attractions & repulsions in the atom.

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• Atomic size refers to the distance the valence electrons are from nucleus (half the distance between nuclei of identical atoms bonded together)

• It can affect the properties of atoms & elements

Decreasing Atomic Size Across a PeriodDecreasing Atomic Size Across a Period• As the attraction between the + nucleus and the -

valence electrons , the atomic size .• From left to right, size decreases because there is an

increase in nuclear charge and Effective Nuclear Charge (# protons – # core electrons)

• Each valence electron is pulled by the full ENC

Li (ENC = 1)

+++

Be (ENC = 2)

++++

B (ENC = 3)

+++++

Sizes of ionsSizes of ions• Ions are atoms that have either gained or lost

electrons (so that the # of electrons is not equal to the # of protons)

• The size of an atom can change dramatically if it becomes an ion (as more –e are added to s and

p sublevel, -e are pulled closer to nucleus) Example:Na+ is smaller than Na because it has lost its 3s electron. Its valence shell is now 2s22p6 (it has a smaller value of n)

• Changing n values is one explanation for the size of ions. The other is …

Sizes of ions: electron repulsionSizes of ions: electron repulsion

9 +

• When an atom becomes a – ion (adds an electron to its valence shell) the repulsion between valence electrons increases without changing ENC

Sizes of ions: electron repulsionSizes of ions: electron repulsion• Valence electrons push each other away

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• Thus, F– is larger than F

• Exceptions: Al and Ga (Ga has 10 d –e on inner

level so –e cloud shrinks because of higher nuclear charge)

Sizes of ions: electron repulsionSizes of ions: electron repulsion• Example: Sorting from largest to smallest:

Mg, Mg+, Mg2+

• Mg is largest.• Mg+ has lost one electron. There is less

repulsion between valence electrons (actually none-one valence electron). Less repulsion means the valence electron can move closer to the nucleus so the atom/ion becomes smaller

• Mg2+ is the smallest. It has lost both of its 3s electrons. The n value of valence electrons drops from

3 down to 2, making the ion smaller

IonizationIonization• Ionization is any process that results in formation of an

ion or group of bonded atoms that has an over all = or – charge

• Ionization energy is the energy required to remove 1e- from a gaseous neutral atom ( first –e removed uses1st ionization energy)

Be

++++

B

+++++

Ionization Energy (IE)Ionization Energy (IE)• If n is small and ENC is large, electrons will be difficult

to remove so the IE will be high• 2nd IEs are higher than the first because you are

removing a - charge (electron) from an increasingly + atom/ion –units kJ/ mol

• 3rd, 4th 5th etc IEs make a huge jump after the electrons in the outer shell are lost (it is not difficult for Mg to lose 12th and 11th electron, but very difficult for it to lose it’s 10th electron)

• Periodic trends refer to 1st IE only• Nonmetals have higher IE than metals• As ions assume noble gas configuration it is more

difficult to remove -e

Electron Affinity (EA)Electron Affinity (EA)

• Electron affinity - energy change that occurs when –e is acquired by neutral gaseous atom

X(g) + e– X–(g) = energy (-)

• Some atoms are forced to gain –e

X(g) + energy (+) X-

Atoms forced to gain –e are unstable and will lose –e spontaneously

EA more

• Trend - EA is the same as for IE (Halogens gain –e easiest, large

• For an atom with a high IE -difficult to remove an electron (due to a small size or high ENC) – but easy to add a new one

• Noble gases do not follow the trend in EA (a filled valence shell makes it energetically unfavorable to add an electron)

• As –e are added to p block, electron affinity becomes more negative because of increasing electrical charge

EA more

• Exceptions are Groups 14 and 15- adding –e to C give half filled p sublevel- harder to for –e to pair in N because of this

• Trends are not as regular as IE usually –e is added with greater difficulty down group

Trends in Size, IE, and EA Trends in Size, IE, and EA • IE, and EA are the opposite of atomic radius

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Size

Ionization energy

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Electron Affinity

Energy ( exothermic or endothermic)Energy ( exothermic or endothermic)• Energy can be described according to whether

we are gaining or losing energy• Endothermic: requires energy (given a + sign)

E.g. lifting a book, removing an electron• Exothermic: gives off energy (given a – sign)

E.g. dropping a book.• IE is positive (it takes energy to remove an e–)• 1st EA is negative (energy is given off – i.e. it is

energetically favorable to add an electron)• After 1st EA, energy may be required to add

electrons to an increasingly negative atom/ion• Example: EA of –200 is greater than –100