# 1.7: The Structure of The Atom

Learning Objectives

• Outline milestones in the discovery of the nucleus
• Summarize and interpret the results of the Rutherford's gold-foil scatter experiments

## Thompson's Non-nuclear Model

Once scientists concluded that all matter contains negatively charged electrons, it became clear that atoms, which are electrically neutral, must also contain positive charges to balance the negative ones. Thomson proposed that the electrons were embedded in a uniform sphere that contained both the positive charge and most of the mass of the atom, much like raisins in plum pudding or chocolate chips in a cookie (Figure $$\PageIndex{1}$$). A competing model had been proposed in 1903 by Hantaro Nagaoka, who postulated a Saturn-like atom, consisting of a positively charged sphere surrounded by a halo of electrons.

## Rutherford's Nuclear Model

The next major development in understanding the atom came from Ernest Rutherford (1871 to 1937), a physicist from New Zealand who largely spent his scientific career in Canada and England. He performed a series of experiments using a beam of high-speed, positively charged alpha particles (α particles) that were produced by the radioactive decay of radium; α particles consist of two protons and two neutrons (you will learn more about radioactive decay in the chapter on nuclear chemistry). Rutherford and his colleagues Hans Geiger (later famous for the Geiger counter) and Ernest Marsden aimed a beam of $$α$$ particles, the source of which was embedded in a lead block to absorb most of the radiation, at a very thin piece of gold foil and examined the resultant scattering of the $$α$$ particles using a luminescent screen that glowed briefly where hit by an $$α$$ particle.

This experiment showed unambiguously that Thomson’s model of the atom (Figure $$\PageIndex{1}$$) was incorrect. Rutherford aimed a stream of α particles at a very thin gold foil target (Figure $$\PageIndex{2a}$$) and examined how the $$α$$ particles were scattered by the foil. Gold was chosen because it could be easily hammered into extremely thin sheets, minimizing the number of atoms in the target. If Thomson’s model of the atom were correct, the positively-charged α particles should crash through the uniformly distributed mass of the gold target like cannonballs through the side of a wooden house. They might be moving a little slower when they emerged, but they should pass essentially straight through the target (Figure $$\PageIndex{2b}$$). To Rutherford’s amazement, a small fraction of the α particles were deflected at large angles, and some were reflected directly back at the source (Figure $$\PageIndex{2c}$$). According to Rutherford, “It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.”

Rutherford’s results were not consistent with a model in which the mass and positive charge are distributed uniformly throughout the volume of an atom. Instead, they strongly suggested that both the mass and positive charge are concentrated in a tiny fraction of the volume of an atom, which Rutherford called the nucleus. It made sense that a small fraction of the α particles collided with the dense, positively charged nuclei in either a glancing fashion, resulting in large deflections, or almost head-on, causing them to be reflected straight back at the source.

## Intranuclear Structure

Rutherford and other scientists decided that a positively charged nuclear consists of ‘positive electrons’ to balance the charge of the surrounding electrons; this term proton was formally assigned to this particle by 1920. However, Rutherford could not explain why repulsions between the protons in the nuclei that contained more than one positive charge did not cause the nucleus to disintegrate, he reasoned that electrostatic repulsions between negatively charged electrons would cause the electrons to be uniformly distributed throughout the atom’s volume. Today it is known that strong nuclear forces, which are much stronger than electrostatic interactions, hold the nucleus together.

The historical development of the different models of the atom’s structure is summarized in Figure $$\PageIndex{3}$$. Rutherford established that the nucleus of the hydrogen atom was a positively charged particle, for which he coined the name proton in 1920. He also suggested that the nuclei of elements other than hydrogen must contain electrically neutral particles with approximately the same mass as the proton. The neutron, however, was not discovered until 1932, when James Chadwick (1891–1974, a student of Rutherford; Nobel Prize in Physics, 1935) discovered it. As a result of Rutherford’s work, it became clear that an $$α$$ particle contains two protons and neutrons, and is therefore the nucleus of a helium atom.

## Summary

Atoms are the ultimate building blocks of all matter. The modern atomic theory establishes the concepts of atoms and how they compose matter. Atoms, the smallest particles of an element that exhibit the properties of that element, consist of negatively charged electrons around a central nucleus composed of more massive positively charged protons and electrically neutral neutrons. Radioactivity is the emission of energetic particles and rays (radiation) by some substances.