Begin to add protons, neutrons, and electrons to the atom by dragging them to their correct position from the buckets at the bottom. Analyse how the mass number changes when protons are added. Then do this with neutrons and electrons. Use your knowledge of these particles to explain why the mass number changes in this way.

As a text-based AI, I can't physically drag items, but I can explain the process to you.

1. Protons and Neutrons: Both of these particles are located in the nucleus of an atom. When you add a proton, the atomic number increases by one because the atomic number is defined by the number of protons in an atom. The mass number also increases by one because the mass number is the sum of protons and neutrons. When you add a neutron, the atomic number stays the same because neutrons don't affect the atomic number. However, the mass number increases by one because you've added another particle to the nucleus.

2. Electrons: These particles exist in the electron cloud that surrounds the nucleus. Adding or removing electrons changes the charge of the atom but not the mass number. This is because electrons have a much smaller mass compared to protons and neutrons. Their mass is so small that it's almost negligible when calculating the mass number.

So, in summary, adding protons or neutrons increases the mass number because they contribute significantly to the mass of an atom. Adding electrons does not change the mass number because their mass is negligible.