Question

What type of reaction is most likely to occur when barium reacts with fluorine? combustion synthesis decomposition single replacement?

Answer 1

Answer is: synthesis. 
Chemical reaction: Ba + F₂ → BaF₂.
Synthesis is type of reaction where two or more compounds (in this reaction barium and fluorine) react to form one product (in this reaction BaF₂).
BaF₂ - barium fluoride, salt, white cubic crystals, soluble in methanol and ethanol.

Answer 2

The  type  of  reaction  is  synthesis    since  it   involve   the   transfer    of  the  electron  between  fluorine  and    barium.  Barium  has  two  valence  electrons  which  it  need  to  lose  to  become  stable  while  chlorine   require  only   one   electron  to  be  stable.  Barium  therefore   transfer   the   two  electron  to  the  two  fluorine  atoms
Ba(s)  +  F2(g) --> Baf2(s)