In Rutherford’s gold foil experiment, what conclusion did he draw about the position and characteristics of the atom’s positively charged part based on the few alpha particles that bounced back or were deflected at large angles?

What the rare big deflections are telling us

Most alpha particles went straight through the gold foil, but a very small number were deflected through large angles or even bounced back. Those rare events are the key evidence for where the positive charge is inside the atom.

Linking alpha particle behavior to where charge must be

Alpha particles are positively charged. A large deflection or bounce-back can only happen if an alpha particle comes very close to another strong positive charge, because like charges repel.

If the positive charge were spread out throughout the atom, then many alpha particles would feel a moderate repulsion and many would deflect noticeably. But that did not happen, so the positive charge cannot be spread out.

Rutherford’s conclusion about the positively charged part

Rutherford concluded that:

• The positive charge is concentrated in a tiny region at the center of the atom, called the nucleus.
• This nucleus is very small compared with the size of the atom, since only a few alpha particles ever come close enough to it to be strongly repelled.
• The nucleus is very dense and contains almost all the mass of the atom, because such a strong “push back” needs a concentrated, massive center.

What this implies about the rest of the atom

Since most alpha particles passed through without deflection, most of the atom is empty space. Electrons occupy the space around the nucleus, but they do not stop alpha particles the way a dense, positively charged nucleus can.