Atom
Definition
Everyday Definition
An atom is the smallest unit that makes up matter.
Physical Definition
An atom is the basic unit of matter that cannot be separated chemically.
Explanation
An atom means the smallest unit of matter that can no longer be divided. Since physics is the discipline that investigates the motion of objects and the organizing principles of nature, it is only natural that physicists have studied atoms. Comparing the everyday definition and the physical definition above, one can see that their nuances differ. The physical definition emphasizes the word “chemically,” which implies that an atom can be split by other means. In fact, an atom is composed of a nucleus and electrons, and the nucleus in turn consists of protons and neutrons. Particles smaller than an atom in this way are called subatomic particles.
In other words, the statement “it can no longer be divided” is not an accurate expression, at least from the standpoint of modern physics. The name atom itself derives from the ancient Greek word for “that which cannot be divided any further,” atomos, but today we know that atoms are made of smaller particles, and that even the protons and neutrons among them are made of more fundamental particles such as quarks. Nevertheless, the reason an atom is still treated as the basic unit of matter is that the moment you split an atom, the substance loses its chemical properties. That is, an atom is the smallest unit that retains the chemical properties of matter.
The Structure of an Atom
An atom consists of a nucleus located at its center and electrons orbiting around it. The nucleus in turn is composed of positively (+) charged protons and uncharged neutrons.
| Constituent particle | Charge | Mass (approx.) |
|---|---|---|
| Proton | $+e$ | $1.673 \times 10^{-27}\ \mathrm{kg}$ |
| Neutron | $0$ | $1.675 \times 10^{-27}\ \mathrm{kg}$ |
| Electron | $-e$ | $9.109 \times 10^{-31}\ \mathrm{kg}$ |
Here, $e \approx 1.602 \times 10^{-19}\ \mathrm{C}$ is the elementary charge. An atom usually has an equal number of protons and electrons, so it is overall electrically neutral. If it loses or gains electrons so that the balance of charge is broken, it is called an ion.
In terms of mass, protons and neutrons are about $1800$ times heavier than electrons, so nearly all of an atom’s mass is concentrated in the nucleus. In terms of size, however, the opposite is true: while an atom’s diameter is about $10^{-10}\ \mathrm{m}$, the nucleus’s diameter is about $10^{-14}\ \mathrm{m}$, roughly ten thousand times smaller. In other words, most of an atom’s mass is concentrated in the nucleus, which occupies only a tiny fraction of the volume, and the remaining space is essentially empty space occupied by the electrons.
How electrons are distributed around the nucleus cannot be explained by classical orbital motion; instead, according to quantum mechanics, it is described probabilistically in the form of orbitals.
Distinguishing Element from Atom
Element and atom are often used interchangeably, but the two are distinct concepts.
- An element is a concept that refers to the kind of atom. The classified names such as hydrogen, oxygen, and carbon are precisely the elements.
- An atom refers to each individual particle corresponding to that element.
For example, “hydrogen” is an element, and each of the “two hydrogen atoms in a water molecule” is an atom. By analogy, an element corresponds to the species “Homo sapiens,” while an atom corresponds to each individual person.
What determines which element a given atom belongs to is the number of protons, which is called the atomic number $Z$. One proton means hydrogen, six means carbon, and so on. On the other hand, there also exist atoms that have the same number of protons but different numbers of neutrons; these are the same element but differ only in mass. Such atoms are called isotopes of one another.