Nuclear decay
27Al + He 》 39P

Explanation:

The unbalanced nuclear equation is

We can insert the subscripts, because these are the atomic numbers of the elements

That leaves only the superscript of He to be determined,

The main point to remember in balancing nuclear equations is that the sums of the superscripts must be the same on each side of the equation.  

Then

27 + x = 31, so x = 31 - 27 = 4

Then, your nuclear equation becomes

periodic trends is an atom that gets larger as the number of electronic shells increase, the radius of atoms increases as you go down a certain group in the periodic table of elements. in general, the size of an atom will decrease as you move from left to right of a certain period.

thats how its supposed to be