Most of the mass of an atom is its nucleus. Each proton and neutron has a mass nearly 2000 times that of the mass of an electron. Electrons that surround the nucleus determine an atom's diameter. So electrons affect an atom's size and nucleons its mass. Nuclear fission is the breaking apart of atomic nuclei into almost equal sized pieces.
Nuclear fusion is the welding together of atomic nuclei.
They are opposite processes. Nuclear fission has been powering nuclear plants for the second half of the 20th century. As this test question is being written, nuclear fusion is not yet practical, although it may be the fuel of choice in the 21st century. You should have two paired arrows facing in opposite directions within the first shell and four paired electrons and two unpaired electrons in the second shell. Distinguish between electron affinity and electronegativity.