5 Why?Attraction experienced by an electron towards a particular nucleus depends on:Number of protons in nucleusAtomic radiusAmount of electron screeningWhy increased EN across period?REASON: Number of protons in the nucleus increases = increased electron attraction towards nucleusREASON: decreased atomic radius= increased electron attraction towards nucleusWhy decreased EN down a group?REASON: Atomic radius increases down the group= decreased electron attraction towards nucleusREASON: Increased electron shielding down the group = decreased electron attraction towards nucleus
6 ExceptionsGallium and germanium have higher EN than aluminum and siliconElements in the 6th period to the 12th period (transition metals) increase EN down the group.
7 Possible explanations Due to phenomenon called d-block contraction, the addition of full a d-orbital decreases the atomic radius to size close to 3rd period sizeReason why Ga is smaller than Al and thus more ENCertain transition metals have unusually small atomic radiibecause the d-electrons are not as effective in shielding outer electrons from the increasing positive nucleus
9 Electronegativity is a relative scale used to determine polarity and ionic character of bonds Not based on a measurable scalePauling scale commonly usedF = 4.0Cs= 0.7the difference in electronegativities more important than individual valuessmall difference between electronegativities of elements in a compound = low polarityLarge difference electronegativities = very polar
10 The B—F bond is more polar because > for the B—F than the B—Cl ExampleEx: What would have a more polar bond:B—F or B—Cl ?Solution:The B—F bond is more polar because > for the B—F than the B—Cl
11 Polarity and Ionic character As increases, the ionic character of the bond also increasesSmall yields a more covalent bondLarger yields a more ionic bond