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4.6 The Structure of the Atom

The Structure of the Atom


English physicist J J Thomson. Click here for original source URL.

The ancient Greeks thought of atoms as indivisible particles of matter, like tiny beads. Even Dalton imagined that atoms were little hard spheres. However, around 1900, Joseph John Thomson discovered particles called electrons. An electron has a negative electrical charge and is far lighter than the lightest atom known. Thomson proved that the atom has structure and that atoms are not truly fundamental building blocks of matter. The stage was set for Ernest Rutherford.

Ernest Rutherford's Gold Foil Experiment. Click here for original source URL.

Working at the University of Cambridge, New Zealand physicist Ernest Rutherford created an experiment to penetrate the atom. His results showed that atoms have structure and are not tiny hard spheres. Rutherford used tiny projectiles called alpha particles for his experiment. Alpha particles are emitted at high speed by some radioactive materials. They have a positive electric charge and they are much more massive than the electron. They have about the mass of an atom of a light element like helium. Rutherford bombarded the gold foil with alpha particles and saw three things. Most of the alpha particles passed straight through the foil as if it was not there. A small fraction of the alpha particles passed through but were deflected by a small angle. And an even smaller fraction recoiled completely to reverse their direction of motion. Rutherford used these results to deduce the invisible structure of the atom.

Rutherford's first result told him that atoms are not tiny hard spheres. Since the great majority of the alpha particles passed through the foil, the atom must be mostly made of empty space. The small deflections of some alpha particles told Rutherford that the center of the atom had a positive electrical charge. This is the atomic nucleus. He figured this out because the deflections were just the right size to be explained by positively charged alpha particles flying in and being repelled by a stationary positively charged nucleus. The deflections were caused by the well-known electrical force. The third result — a few alpha particles bouncing back the way they came — told Rutherford that almost the entire mass of the atom was concentrated in a tiny nucleus at the center.

Diagram of an idealized?Lithium?atom, primarily useful to illustrate the?nucleus?of an atom. This sort of design is scientifically inaccurate in many important respects, but serves as a powerful?mandala?of the nuclear age. Click here for original source URL.

After Rutherford's work, a familiar picture of the atom emerged. The small, dense atomic nucleus is surrounded by orbiting electrons. This picture of the atom as a miniature solar system has become a symbol of our modern scientific age. Yet the analogy with gravitational orbits is flawed. Rutherford had no explanation for why the electrons should stay in their orbits. In fact, a simple consideration of the electrical force shows that atoms made of positively charged nuclei and negatively charged electrons should not be stable. We do not need quantum theory to summarize the main points of atomic structure. All atoms consist of the following: each atom has a positively charged nucleus at its center, and a swarm of much less massive, negatively charged electrons circling around the nucleus.

The scales of atomic structure are truly tiny. Hydrogen is the smallest atom with a diameter of about 5.3 × 10-11 meters. About ten million hydrogen atoms would fit across the head of a pin! The nucleus of a hydrogen atom consists of a single positively charged particle called a proton. Heavier atoms have a second type of nuclear particle called a neutron. The neutron is similar in mass to the proton but electrically neutral. The proton is about 10,000 times smaller that the atom itself. The mass of a hydrogen atom is 1.7 × 10-27 kilograms. Even the smallest mote of dust contains trillions and trillions of atoms. Since the electron is nearly 2000 times less massive than the proton, it is a good approximation to say that the nucleus carries most of the mass of every atom. However, the tiny electron has an equal but opposite electrical charge to the more massive proton. These charges cancel exactly and normal matter is electrically neutral.

The number of protons in the nucleus defines the element. An atom of hydrogen, which is the simplest element, has one proton in its nucleus and a single orbiting electron. An atom of helium has two protons and two neutrons in its nucleus, and two orbiting electrons. Each different element has a different number of protons in the nucleus. The periodic table is just a sequence of atoms with increasing number of protons. The number of protons and electrons are always equal. There is no simple rule for the number of neutrons, but in any case neutrons affect the weight but not the chemical properties of an element.

The electrons act as a shield. Under ordinary, everyday conditions the electrons swarming around the nucleus prevent the nucleus of an atom from making contact with the nucleus of any other atom. If two atoms bump together, only the electron swarms interact. If the nucleus of an atom were represented by a tennis ball on the 50-yard line of a football stadium, the electrons would be tiny particles whirling around at the outskirts of the stadium. The neighboring atoms in a solid would be other tennis balls several hundred yards away, each with their own swarm of electrons. This analogy holds even for a dense material like lead or gold. The solidity of everyday objects is an illusion due to the electrical force within atoms. The emptiness of normal matter is one of the amazing consequences of atomic theory!

Atomic theory states that elements are fundamental and that one element cannot be changed into another by chemical means. What stops us from just adding or subtracting a proton from the nucleus to make a different element? It turns out a special force within the nucleus acts to keep it tightly bound. And the electrical repulsion between two positively charged nuclei stops them from ever merging. Atomic nuclei can merge or be ripped apart under conditions of extreme temperature and pressure. But in the everyday world, an element never changes. You can think of the atomic nucleus as a little fortress: no particle can leave and none can enter.

What about more complicated forms of matter? Unlike atomic nuclei, electrons are gregarious. They travel readily from one atom to another. Molecules form when atoms share their electron structures. If we consider elements to be building blocks of different size and mass, then molecules are groups of atoms joined by their electrons. Some molecules have as few as two or three atoms: water (H2O), carbon dioxide (CO2), nitrogen (N2). Organic molecules have can have hundreds of atoms. If we follow the building block analogy, there are about 110 different elements or different sized building blocks. About 90 of these occur in nature, the rest have been made fleetingly in the lab. On the other hand, the building blocks can be combined in an enormous number of ways. There are thousands of naturally occurring molecules, and tens of thousands that have been created in the lab.

Chemical reactions occur when atoms or molecules give up or receive electrons. Remember that the central nuclei never meet or collide in a chemical reaction. Elements with quite different mass can have similar chemical behavior if they have similar numbers of electrons in their outer regions. If you look at the periodic table, you will see that elements in the same vertical column often have very similar chemical properties. Chemical reactions are going on around us all the time.

This discussion of particles so small that they are invisible begs an obvious question. Are atoms real? Or are they just a theoretical idea that scientists find useful to describe nature? There is plenty of indirect evidence for tiny units of matter and in the past few years we have direct evidence too. We can use electrons to make extremely fine images of matter. We can also use electric fields to sense the positions of individual atoms. A new discipline called nanotechnology is emerging, which will enable engineers to fabricate devices from single atoms and molecules. We should have no doubt — atoms are real