Atoms, Ions, and Isotopes

All of the tangible, physical stuff of the world is made up of matter. The smallest discrete unit of matter that retains unique chemical properties is the atom. Molecules are formed when atoms are joined by chemical bonds.

Atoms are extraordinarily small. A typical atom is far to small to be seen, even under the microscope. Each atom is made up of subatomic particles. The three key subatomic particles include protons, neutrons, and electrons. Each of these has certain identifiable characteristics. Protons and neutrons are both located at the center of the atom, its nucleus. Electrons are found outside the nucleus. Protons and neutrons each have an equal mass or weight, given as 1 atomic mass unit (AMU). Electrons are much smaller and lighter, with a mass of 1/1795 AMU. Since this is so small, the mass of electrons is considered to be negligible and is ignored. In addition, some of these particles have an electrical charge. Protons carry a positive charge, electrons have a negative charge, and neutrons are uncharged.

Complete the table below, using the information given in the above paragraph.

Characteristics of Subatomic Particles

Atoms can be classified according to the number and arrangement of these particles. Each unique type of atom belongs to a different element. There are over 100 known elements, of which 92 are found naturally.

Each element is given a name, and symbolized by one or two letters, either the initials or an abbreviation of the English or Latin name for the element. Six of those elements are Hydrogen (H), Carbon (C), Oxygen (O), Nitrogen (N), Sodium (Na), and Chlorine (Cl).

In each element, all of the atoms have a particular number of protons. This number, the atomic number, can be used to identify the element. Since all complete atoms are electrically neutral, the atomic number also predicts the number of electrons present in the atom. However, atoms may ionize, by gaining or losing electrons. Atoms whose protons and electrons are not numerically equal are called ions.

Atoms also have a predictable mass. The mass of an atom is given by the sum of the protons and neutrons in the atomic nucleus. This value is given as the atomic weight or atomic mass of the element.

This information (the element's name, its atomic number, and atomic mass) is given in the Periodic Table of Elements, a useful reference for chemists. You may also see the information given for a single element as shown below. The top number represents the atomic weight; the bottom number the atomic number, and the letter or letters provide the symbol for the element. This form is often used in "nuclear" chemistry.

Here are 6 common elements:

Use this information to complete the following table, giving the characteristics of atoms of each of the six elements.

Characteristics of Common Elements
elementatomic #atomic weight# protons# neutrons# electrons

The electrons of an atom are found outside the nucleus. They are said to be in orbitals. An orbital is a 3-dimensional space (which can be any of a variety of shapes - spherical, teardrop-shaped, doughnut-shaped, and dumb-bell-shaped orbitals are common) in which electrons may be found most of the time. Each orbital can hold only two electrons. Note that an orbital is not an orbit; the electrons are not orbiting the nucleus like planets orbit the sun.

The shape and position of the orbitals, and the location of the individual electrons, is affected by the negative charge on the electrons. Objects having the same charge tend to repel each other. Electrons are drawn toward the positively charged nucleus, but are repelled by other electrons.

Electrons are found at varying distances from the nucleus. Two electrons in the first orbital are closest to the nucleus. This is referred to as the first energy level. Four orbitals (up to 8 electrons) are found in the second energy level. An additional 8 electrons may be found in the third energy level, and so on. Each inner energy level must be filled before electrons will occupy the next outer level. Thus, the position of the electrons around the nucleus of an atom is determined in part by the number of electrons present.

Use the information from your table at the top of the page to draw the positions of the electrons in each of the atoms shown. In each diagram, the nucleus is represented by the central circle and the concentric rings around the circle represent the 3 electron energy levels. Remember that not all energy levels need to be occupied.

An atom is described as "stable" if the outermost occupied electron energy level is filled. That is, if the innermost level has 2 electrons, or if the second or third levels have 8 electrons. Helium, for example, has an atomic number of 2 and therefore has 2 electrons. Both of these electrons would be found in the first electron energy level, so that level would be filled. Helium is considered to be one of the inert gases. It is unlikely to enter chemical bonds with other atoms.

More commonly, the outermost electron level is not completely filled. Atoms in this condition are chemically active, and will tend to gain, lose, or share electrons in order to complete their outermost energy levels. When atoms share electrons, covalent bonds are formed between the atoms; a molecule is produced.


Ions are charged particles, produced when an atom gains or loses one or more electrons. Ionization is likely to occur when an atom has a partially occupied outer electron energy level. Ionization is especially likely if the complete atom has only 1 or 2 electrons in its outermost energy level, or if it is only 1 or 2 electrons away from completing the full occupation of the energy level.

For example, both hydrogen and sodium have only one electron in their outermost electron level. They are both likely to release the single electron in that level. This will make the number of electrons and the number of protons different. Since they have more protons than electrons, they now have a net positive charge and are considered to be positive ions. This is shown by placing a plus sign + next to the symbol for the element.

Chlorine, on the other hand, has 7 electrons in its outermost electron level. Chlorine is likely to "grab" an extra electron - assuming one is available - to become a negatively charged ion (symbolized by a negative sign − next to the symbol for the element.)

The diagram below shows the ionization of a hydrogen atom. In the space to the right of the diagram, show the ionization of sodium and chlorine.


Although every atom of one element must have the same number of protons, the number of neutrons may vary. Since neutrons have mass, but do not carry any charge, they will affect the weight of the atom, but will not have any affect on the chemical properties of the atom. Two atoms that have the same number of protons [and thus belong to the same element], but have different atomic masses are isotopes of each other.

There are many naturally occuring isotopes. Carbon, for example has two isotopes, 12C and the much less common 14C. Some isotopes are stable; others tend to break down over time, releasing subatomic particles, and sometimes changing into completely different elements.