Correct answer is magnesium bromide. This is an ionic compound with metal forming a positive ion - K+ and halogen forming a negative ion - Br-. When group 7 element form ions they have a suffix -ide. Bromine is the element and when it forms a negative ion bromine is called bromide with the suffix. 
Magnesium gives 2 electrons and bromine can take only 1 electron, therefore 2 bromine atoms are needed. Therefore magnesium forms ionic bonds with 2 bromine atoms. 
The compound is called magnesium bromide
        
             
        
        
        
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I'm pretty sure its an electron. I hope this helps! (:
        
                    
             
        
        
        
The average surface temperature of Uranus (nearly being the coldest planet in the solar system) is -224<span>°C</span>