Ch2c Flashcards
How are exceptions to the trends of effective nuclear charge explained?
They are explained by changes in e- configuration (Egap, pairing & exchange energy).
What are four atomic parameters?
1) IE
2) EA
3) Electronegativity
3) atomic radii.
Define ionization energy.
It is the energy required to remove an e- from the highest occupied orbital of the ground state.
Is ionization an endothermic or exothermic process?
Endothermic process because energy is being put in rather than released.
How does effective nuclear charge explain the “stability” of noble gases?
When comparing the effective nuclear charge experienced by the outer most e-, e- of Li+ is held much more tightly by the nucleus than the first e- removed.
What is the general trend for ionization energy in the periodic table?
decreases down a group since e- lost comes from larger sub shell, and increases across a period due to shielding by other e-s.
Describe the IE’s of TMs.
They remain roughly constant.
Define Electron Affinity (EA).
It is the energy required to remove an electron from an anion, and the e- is assumed to come from the lowest occupied orbital of the ground state anion.
What is another term for electron affinity?
zeroth IE of the neutral atom.
Is electron affinity an endothermic or exothermic process?
It can be either.
What is the general trend for electron affinity in periodic table?
There is less energy needed to remove e- from an anion because of the additive shielding.
Increases across a period due to higher effective nuclear charge.
Do we look at the neutral atom or anion when assessing EA?
Anion.
Which group of element has the lowest EA and is the process exo/or endothermic?
Noble gases, exothermic.
Why do noble gases have negative EA’s?
It is an exothermic process (energy is being released), because the e- being removed comes from a higher energy level, so the regular noble has configuration e-s intensely shield this extra electron in its larger orbital.
What does LUMO and HOMO stand for?
LUMO: Lowest Unoccupied Molecular Orbital.
HOMO: Highest Occupied Molecular Orbital.
Define electronegativity.
Electronegativity is a measure of the tendency for one element of a bonded pair to attract the e-s associated with the bond itself.
What scale is used to estimate electronegativity of an element?
The Pauling Scale.
How is polarity of a bond measured?
The polarity of a bond is measured by comparing the electronegativities of the two bonded elements.
What does electronegativity discuss?
The properties and reactivities of molecules.
Name four proposed scales of electronegativity.
- Mulliken.
- Allen.
- Allred and Rochow.
- Pauling, most commonly used.
Define the Mulliken scale of electronegativity.
An atom will have a high electronegativity if it has a high ionization energy, or positive electron affinity.
How did Mulliken calculate electronegativity?
He used the average of ionization energy and electron affinity. (short: average of IE & EA)
Define the Allen scale of electronegativity.
Designed for main group elements, in which used configuration energy or average of ionization enthalpy data for s & p valence electron. (short: average E of valence shell e-s)
Define the Allred and Rochow scale of electronegativity.
Bonding e- is held by the effective nuclear charge it experiences, thus has a force resisting its removal. He assumed the electronegativity was proportional to this resisting force. (short: electrostatic attraction to nucleus, is proportional to Z*/r2)
Define the Pauling Scale of electronegativity.
It is not directly based on IE or EA, instead considers the difference in bond strength between a bonds real strength and calculated strength with the use of dissociation energies of (perfectly covalent bonds). Partially ionic bonds would have a HIGHER observed dissociation energy, therefore the difference will be proportional to the difference in electronegativities.
The bond energies of H-H & F-F are 436 & 158 kJ/mol. If the H-F were perfectly covalent (it is not) the bond energy would be predicted to be (436x158)^(1/2) =262. What is actually observed?
Experimental H-F bond is 566 kJ/mol, so the difference (566-262=304) 304 kJ/mol is proportional to the difference in electronegativities.
What is the Pauling Scale formula for electronegativity?
difference electronegativities= 0.102sqrt(difference in dissociation energy of perfectly covalent bonds)
General trend for electronegativity in the periodic table.
Increases across a period and decreases down a group.
Define metallic radius.
Half the distance between the nuclei of metal atoms in the solid state.
Define covalent radius.
Half of the internuclear distance between the nuclei of two single-bonded atoms of the same species (homonuclear) The values are given for different bond orders (single, double, triple).
What is the collective term of metallic and covalent radius?
Atomic radius.
Define atomic radius.
Half the diameter of a neutral atom.
Define Van des Waal’s radius.
Half the distance between two unbonded atoms. Limits the closest approach of two non-bonded atoms.
General trend for single bond of non-polar covalent radii in periodic table.
increases down a group as new shells are used, and decreases across a row due to partial shielding by electrons in the same shel (recall: partila just means as Z* increases by one sigma increase by 0.35).
Where are lanthanides inserted in the periodic table?
Into the 6th row d-block.
What is the “Lanthanide Contraction”?
The insertion of the Lanthanides results in transition metals smaller than expected and roughly the same size as 5th row transition metals. Thus the covalent radius trend is not observed due to the effect of 14 lanthanides. As a consequence 2nd and 3rd row of TMs are more similar in covalent radii than the 1st row of TMs.
Define ionic radius.
The radius of an atom’s ion in ionic crystals structure.
Ionic radius of anion vs. cation.
The bigger the positive charge on the cation the smaller the ionic radius, because the atom will begin to lose e- shells. The bigger the negative charge the bigger the ionic radius, because e-s are added to shells and nuclear effective charge decreases.
How are ion sizes explained by effective nuclear charge?
- Cations are smaller than parent atoms, because fewer e-s will decrease the shielding, thus e-s will be held more tightly by the nucleus. The removal of an e- reduces repulsion between the remaining e-s and so they a re held closer by the positive charge.
- Anions are bigger than parent atoms, because more e-s will increase shielding, thus outermost e-s feel less effective nuclear charge and held more loosely by the nucleus. Greater repulsion when more e-s are added.
Why does ionic radius increase going down a group?
Because higher principle valence shells are introduced to accommodate valence e-s, further from the nucleus.
Ion gets larger with increasing coordination number (T/F)
True.
Which radius is smaller for when a metal atom or a metal ion?
The ionic radius for the metal ion is smaller than its atomic radius.
