2.41. Rutherford’s Atomic Model http://www.ck12.org
FIGURE 2.78
The Nucleus Takes Center Stage
Rutherford made the same inferences. He concluded that all of the positive charge and virtually all of the mass of
an atom are concentrated in one tiny area and the rest of the atom is mostly empty space. Rutherford called the
area of concentrated positive charge the nucleus. He predicted—and soon discovered—that the nucleus contains
positively charged particles, which he named protons. Rutherford also predicted the existence of neutral nuclear
particles called neutrons, but he failed to find them. However, his student James Chadwick discovered them several
years later. You can learn how at this URL: http://www.light-science.com/chadwick.html.
The Planetary Model
Rutherford’s discoveries meant that Thomson’s plum pudding model was incorrect. Positive charge is not spread
evenly throughout an atom. Instead, it is all concentrated in the tiny nucleus. The rest of the atom is empty space
except for the electrons scattered through it. In Rutherford’s model of the atom, which is shown in theFigure2.79,
the electrons move around the massive nucleus like planets orbiting the sun. That’s why his model is called the
planetary model. Rutherford didn’t know exactly where or how electrons orbit the nucleus. That research would be
undertaken by later scientists, beginning with Niels Bohr in 1913. New and improved atomic models would also
be developed. Nonetheless, Rutherford’s model is still often used to represent the atom. You can see an animated
version of the model at this URL: http://www.clickandlearn.org/gr9_sci/atoms/modelsoftheatom.html.
Summary
- Ernest Rutherford discovered the nucleus of the atom in 1910. He sent a beam of alpha particles toward gold
foil and observed the way the particles were deflected by the gold atoms. From his results, he concluded that
all of the positive charge and virtually all of the mass of an atom are concentrated in one tiny area, called the
nucleus, and the rest of the atom is mostly empty space. - In Rutherford’s planetary model of the atom, the electrons move through empty space around the tiny positive
nucleus like planets orbiting the sun.