5Why?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
6ExceptionsGallium and germanium have higher EN than aluminum and siliconElements in the 6th period to the 12th period (transition metals) increase EN down the group.
7Possible 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
9Electronegativity 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
10The 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
11Polarity and Ionic character As increases, the ionic character of the bond also increasesSmall yields a more covalent bondLarger yields a more ionic bond