Why is atomic weight uneven?

Answer 1

Why should it be even?

Answer: it shouldn't. There's no particular reason for it to be. Neutrons and protons have slightly different masses, and neither of them is exactly one Atomic Mass unit.

Also, most tables give the "natural abundance" mass, which is obtained by multiplying the mass of each individual isotope (which usually is pretty close to a whole number) by the relative abundance of that isotope and adding the results.

For example, chlorine is about 1/4 chlorine-37 and about 3/4 chlorine-35. The average is thus around 35.5 (35.453 is the usual figure).

Answer 2

Atomic weight is uneven because it accounts for the average mass of all naturally occurring isotopes of an element, taking into consideration their abundance. Since different isotopes have different masses and abundances, the atomic weight is a weighted average that may not always align with a whole number.