How is fission different than alpha or beta decay?

In alpha decay a radioactive isotope releases a helium
nucleus with mass of 4 and charge of +2 and a new element, with a lower mass in
produced. Thus, U- 235 if it released an alpha particle would form element 88 (Radium)
with a mass of 231. In beta decay, a radioactive isotope releases an electron which hass
no mass and a charge of -1.  Thus, if Pu-244 decayed by release of a beta particle, a
new element of mass 244 and atomic numer 95 (Americium) would be
formed.

In fission, a uranium nucleus is split by a neutron
into two smaller elements along with the release of additional neutrons and energy.  The
neutrons released can split additional uranium nuclei resulting in a either a controlled
chain reaction (nuclear energy) or an uncontrolled chain reaction (atomic
bomb)