Trends in the periodic table: 16 04/10/99 Trends in the periodic table: Ionization Energy Atomic Radius Electron Affinity Electronegativity

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

04/10/99 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. 04/10/99 Periodic Trends Sizes of atoms and ions. Ionization energy. Electronegativity Electron affinity.

Atomic Radius

Sizes of Atoms Bonding atomic radius tends to… 04/10/99 Sizes of Atoms Bonding 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 04/10/99 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?

04/10/99 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

Atomic Radius Li Be B + + + + + + + + + + + + As we add more protons, the electrons in the same shell have more attraction to the center This explains higher ionization energies 04/10/99 Atomic Radius Li Be B + + + + + + + + + + + +

04/10/99 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 number 04/10/99

Sizes of Ions Ionic size depends upon: Nuclear charge. 04/10/99 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. 04/10/99 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. 04/10/99 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.

04/10/99 Sizes of Ions Ions increase in size as you go down a column.

04/10/99 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

04/10/99 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.

04/10/99 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 04/10/99 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 04/10/99 Trends in First Ionization Energies Generally, as one goes across a row, it gets harder to remove an electron.

Ionization Losing an electron is called ionization 04/10/99 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 energy 04/10/99 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 number 04/10/99

Electron Affinity

Electron Affinity The opposite of Ionization 04/10/99 Electron Affinity The opposite of Ionization Electron Affinity is the addition of electrons to an atom

04/10/99 Electron Affinity Energy change accompanying addition of electron to gaseous atom: Cl + e−  Cl−

Trends in Electron Affinity 04/10/99 Trends in Electron Affinity In general, electron affinity becomes more exothermic as you go from left to right across a row.

04/10/99 Electron Affinity

04/10/99 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 04/10/99 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

04/10/99 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

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04/10/99 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?

04/10/99 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?