Since bromine (Br) is a halogen, it has seven valence electrons. Within its structure it share's five of these with covalently bonded fluorine (F) atoms, leaving two electrons to form a lone pair at the top of the molecule. Since the lone pair is a region with associated with more negative charge since it is not balanced by any atom's positively charged nucleus, the molecule is polar.
From an electronegativity perspective, BrF5 is contains many polar covalent bonds since the difference between bromine's electronegativity (2.96) and fluorine's (3.98) is greater than .5 (even though both are halogens). In general, bromine's electronegativity is less than fluorine's because Br has more layers of electrons shielding the nucleus and decreasing its pull on average over its valence electrons. Fluorine does not have as many layers of internal electrons that could contribute to this shielding effect so it has a much larger electronegativity (in fact the highest of any element on the periodic table!).
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| BrF5 Structure Ball and Stick Model. Created with MolView. |
In fact, due to the presence of highly electronegative halogens, this molecule will spontaneously react with many organic substances and water (H2O). Per studies in the 1950s and 60s, BrF5 was found to be an extremely effective oxidizer (i.e. removing electron's from molecules) as a result of the presence of many halogens [1]. However, this led to many difficulties in handling and performing research experiments with this substance in a safe and effective manner.
[1] http://www.astronautix.com/b/brf5.html
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