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Trends in the periodic Trends in the periodic table:table:
Ionization Energy Ionization Energy Atomic RadiusAtomic Radius
Electron AffinityElectron AffinityElectronegativityElectronegativity
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Objectives Periodic Trends
• Be able to interpret trends that occur on the periodic table
• Be able to predict relative sizes of atoms
• Be able to predict more electronegative atom
• Be able to predict relative ionization energy
Introductory
• There is a special stability associated with having a full outer shell (8 valence electrons) – Half full shells and a full s offer some stability
• Atoms give up and lose electrons in order to gain this special stability (Ionic Ch 4)
• Atoms also share electrons to gain this outer shell (Covalent Ch 4)
Periodic Trends
– Sizes of atoms and ions.– Ionization energy.– Electronegativity– Electron affinity.
Atomic Radius
Sizes of AtomsBonding atomic radius tends to… …decrease from left to
right across a row
…increase from top to bottom of a column
Why? Explain both trends
Atomic Radius
• What gives atoms their size is the electrons
• The largest atoms are in the bottom left of the periodic table
• Why not the right since there are more electrons?
• Short answer: more protons– But why? There are more electrons too?
Atomic Radius
• Long answer: As you more from left to right across the periodic table, you stay at the same principal energy level.
• The electrons are added to an existing orbital, but now there are more protons to pull them in closer
• When you go from a Noble gas to the next row, then you have a big jump
As we add more protons, the electrons in the same shell have more attraction to the center
This explains higher ionization energies
Li
+++
Be
++++
B
+++++
Atomic Radius
Atomic Radius
– This also means that:
• The 1s shell of Hydrogen (H) is much larger than the 1s shell of Cesium (Cs)
Atomic radius vs. atomic numberAtomic radius vs. atomic number
Ca
K
H He
Li
B
Be
C N O FNe
NaMg
Al Si P S Cl
Ar
0
50
100
150
200
250
0 2 4 6 8 10 12 14 16 18 20
Element
Ato
mic
Rad
ius
(pm
)
Sizes of Ions• Ionic size depends
upon:– Nuclear charge.
– Number of electrons.
– Orbitals in which electrons reside.
Sizes of Ions• Cations are smaller
than their parent atoms.– The bigger the
positive charge, the smaller you are
– The outermost electron is removed and repulsions are reduced.
Sizes of Ions• Anions are larger
than their parent atoms.– The larger the
negative charge, the larger you are
– Electrons are added and repulsions are increased.
Sizes of Ions
• Ions increase in size as you go down a column.
Sizes of Ions
• In an isoelectronic series, ions have the same number of electrons.– In an isoelectronic series, the most positive charge is
the smallest
Ionization Energy
Ionization Energy
• Amount of energy required to remove an electron from the ground state of a gaseous atom or ion.– First ionization energy is that energy required
to remove first electron.– Second ionization energy is that energy
required to remove second electron, etc.
Ionization Energy
• It requires more energy to remove each successive electron.
• When all valence electrons have been removed, the ionization energy takes a “quantum leap”.
Trends in First Ionization Energies
• As one goes down a column, less energy is required to remove the first electron.– For atoms in the same
group, Zeff is essentially the same, but the valence electrons are farther from the nucleus.
Trends in First Ionization Energies
• Generally, as one goes across a row, it gets harder to remove an electron.
IonizationIonization• Losing an electron is called ionization• This loss can be caused by
– The presence of another element that has a tendency to gain electrons
– Energy absorbed by the electron causing it to jump out of orbit (High energy photon needed)
• An ion is an atom that has either a net positive or net negative charge
• Q: what would the charge be on an atom that lost an electron? Gained two electrons?
• A: +1 (because your losing a -ve electron)• A: -2 (because you gain 2 -ve electrons)
Ionization energyIonization energy• Ionization energy is the energy required to
remove one outer electron from an atom• This value is dependent on many factors. • General trends you find about ionization
energy are– Atoms in the top right of the periodic table have
the largest ionization energies– As you move down a column the energy
required becomes less as the electron is farther away from the nucleus
– As you move right across a row the ionization energy becomes larger (atomic radius smaller)
Ionization energy vs. atomic numberIonization energy vs. atomic number
Ca
K
H
He
Li
B
Be C
N O
F
Ne
Na
MgAl
SiP S
Cl
Ar
0
500
1000
1500
2000
2500
0 2 4 6 8 10 12 14 16 18 20
Element
Ioni
zatio
n en
ergy
(kJ
/mol
)
Electron Affinity
Electron Affinity
• The opposite of Ionization
• Electron Affinity is the addition of electrons to an atom
Electron Affinity
Energy change accompanying addition of electron to gaseous atom:
Cl + e− Cl−
Trends in Electron Affinity
In general, electron affinity becomes more exothermic as you go from left to right across a row.
Electron Affinity
Electron Affinity
• Take note of the high Electron Affinity associated with the Halogens (F, Cl, Br, I)
• Noble Gases as well as some other groups hover at zero due to factors such as– Pairing up of electrons (Nitrogens group)– New principal energy level (Noble gases)– Addition of orbital and/or new type of orbital
(Alkaline Earth Metals)
Electronegativity
Electronegativity
• Summed up as: How bad an atom wants an electron
• Closely associated with electron affinity– If the atom releases energy by gaining an
electron, it will have a high electronegativity
• A high electronegative corresponds with high desire to grab another electron
Electronegativity
• Linus Pauling is the one who did extensive work with this trend. Also made a simple scale of electronegativity
• He used the 2nd row of the periodic table, Li through F
• He assigned Lithium 1.0 and Flourine 4.0 – +0.5 as you move across
• Used Flourine as standard to set other #’s
Practice Questions
• What atom has the highest ionization energy: F, He, Ar, Cs
• What atom has the largest neutral radius: F, He, Cs, Ba
• What atom has the highest electronegativity: O, S, N, P
• Which group has no electronegativity?
Practice Questions
• What atom has a larger radius: Na with a +1 charge, Mg with a +2 charge?
• What atom has a larger radius: N with a -3 charge, O with a -2 charge, or F with a -1 charge?
• What do you think happens when a neutral F and neutral Li come in contact?
Practice Questions
• Which column has the largest electronegative value?
• Which columns have no electron affinity?
