Atomic Structure

When we learned about the classification of materials in an earlier lesson, we learned that if you have a material that is a pure substance that cannot be separated by physical means and cannot be separated by chemical reactions, then you have an element. Samples of an element are made up of atoms. In this section, we'll start our look at the inner structure of the atom. We'll start with the subatomic particles, electrons, protons, and neutrons, and then look at the mass of an atom (the mass number), how the particles are arranged in an atom (its structure), and what determines an element's identity (the atomic number).

Electrons

All atoms (except hydrogen)* consist of 3 kinds of particles: protons, neutrons, and electrons. The properties of an atom – its mass, its charge, what element it is, what isotope of that element it is – are completely determined by how many of each kind of particle it has. The chart below lists the basic properties of these three particles.

*Most hydrogen atoms do not have any neutrons. A small percentage, however, have either one or two neutrons and the majority of those have only one.

The unit of mass you see in the chart, amu, is an “atomic mass unit.” We have discussed the amu a bit in prior lessons as the unit used for atomic masses. This chart should already be making it a bit more clear why the amu was chosen to be the size that it is.

Electrons are the smallest and the lightest of the three particles. Compared to the protons and neutrons they have practically no mass and so contribute essentially nothing to the mass of the atom. An electron actually weighs about 0.00054 amu. This is so much less than the mass of a proton or a neutron that the mass of the electrons is ignored in determining the mass of an atom.

Protons

Protons are 1,837 times more massive than electrons and contribute up to about half the mass of the atom. They also have a positive charge, the opposite of electrons. The attraction between these unlike charges and the repulsion between like charges is responsible for the chemical properties of atoms.

Because protons and electrons are the only charged particles in atoms, we find the net charge on an atom by comparing the number of protons and electrons. When they are equal, the atom is neutral. An atom with one more electron than protons would have a net charge of -1. An atom with one electron fewer than protons would have a net charge of +1. These charged atoms are called ions.

net charge = (# of protons) - (# of electrons)

Atoms are neutral by default (net charge = 0)

Ions have a net charge, either positive or negative.

Atoms become ions only by gaining or losing electrons. The charge on an anion {pronounced AN-ion}, or negative ion, is equal to the number of extra electrons it has gained. The charge on a cation {CAT-ion}, or positive ion, is equal to the number of electrons it has lost.

For example, an atom with 17 protons and 17 electrons would be neutral. If this atom gained one electron, it would become an ion, specifically an anion, and its net charge would be -1, as it would now have 18 electrons.

Neutrons

Mass Number

As has been mentioned, both protons and neutrons have masses equal to (almost) exactly one amu. Therefore, measured in amu’s, the mass of an atom is equal to the number of protons plus the number of neutrons it contains. This number is called the mass number of the atom.

The mass number (A) is the approximate mass of an atom in amu.

A = (# of protons) + (# of neutrons)

The mass number is very close to the actual mass of the atom in amu but it is not exactly equal to the mass for three reasons:

• The electrons, though very light, do weigh something.

• The mass of the proton and the neutron aren’t exactly 1 amu. The proton weighs 1.0073 amu and the neutron weighs 1.0087 amu. We will ignore these discrepancies, but you should be aware they exist.

• When protons and neutrons bind together in a nucleus, some of their mass is converted to the energy necessary to hold them together.

Atomic Number

What determines what element an atom is? The identity of an atom is determined solely by the number of protons in its nucleus, the atomic number (Z). All atoms of a given element, no matter how they may differ otherwise, have the same atomic number.

Z = # of protons

The atomic number is the most fundamental property of an element. A zinc atom, for example, can gain or lose electrons and it is still zinc. It can have more or fewer neutrons and still be zinc. But if it changes the number of protons it has, it is no longer zinc, it is a different element with different chemical properties.

Two atoms with the same atomic number are atoms of the same element, but they may still have different numbers of neutrons and therefore different masses. Such atoms are called isotopes. All elements have more than one isotope. Hydrogen, for example, has three, one with no neutrons in its nucleus, one with one neutron, and one with two neutrons.

Many of the isotopes of each element are unstable, that is, their nuclei spontaneously break apart and become other elements. Such isotopes are said to be radioactive. Ordinary matter, the matter we deal with every day, is made almost entirely of isotopes that are stable. There are common uses of radioactive isotopes, however. They are used in medicine to kill cancer cells and to help take pictures of the interior of the body, in home smoke detectors, and, of course, in nuclear power plants and nuclear weapons.

Structure of an Atom

Even though protons and neutrons are much larger than electrons, they occupy very little of the volume of an atom. They are bound tightly together in the center of the atom in a tiny ball called the nucleus. So the nucleus contains virtually all of the mass of the atom (from protons and neutrons) and all of its positive charge (from protons).

In the next several sections of this lesson, we’ll look at some of the experiments that led to our current ideas about the nature of atoms and the particles they are made of.