What is formed when two or more elements are bonded together?

a compound

Explanation:

a compound consists of two or more elements combined

A compound is a substance formed when two or more chemical elements are chemically bonded together. In mixture, the substance present are not chemically bonded together. Example 1: pure water is compound made from two elements hydrogen.

considering the question as per the attachment

here we can see that the radon nuclide has 222 as the atomic mass number and 86 as the atomic number.

when it undergoes nuclear fission it gives polonium which has atomic mass number as 218 and atomic number will be 84 along with a helium nucleus which has 4 atomic mass number and 2 as its atomic number.

222 = 218 + 4 (atomic mass number)

86 = ? + 2; therefore ? = 86-2 = 84.

the atomic mass number of polonium will be 84.