BR is a better nucleophile than CL because it is less electronegative than BR and more willing to give up electrons, which is what nucleophiles do. BR has a 2.96 electronegative number and CL has a 3.16 electronegative number.
As electronegativity increases, nucleophilicity decreases. If an atom has a pair of electrons to donate, then the ability of that substance to donate their extra pair is inversely proportional to how closely and firmly held the atoms are. Electronegativity is the key factor for assessing how closely and firmly held atoms are.
BR is the chemical symbol for Bromine. Its atomic number is 35 and it has an atomic mass of 79.904. It has a density of 3.1 and a melting point of - 7.2 degrees Celsius with a boiling point of 58.8 degrees Celsius. On the ionic radius it is a 0.195 and on the Vanderwaals radius it is a 0.165. It has ten isotopes and was discovered by Anthoine Balard in 1826. It is a nonmetallic element that can be a liquid at normal room temperature and is usually a red or brown color. Bromine is corrosive to human tissue and if inhaled, it will cause damage as it is a very toxic substance. That said, it is a naturally occurring element that is found in crystal rock throughout the Earth's environment.