The development of modern atomic theory revealed much about the inner structure of atoms. It was learned that an atom contains a very small nucleus composed of positively charged protons and uncharged neutrons, surrounded by a much larger volume of space containing negatively charged electrons. The nucleus contains the majority of an atom's mass because protons and neutrons are much heavier than electrons, whereas electrons occupy almost all of an atom's volume. The diameter of an atom is on the order of 10 m, whereas the diameter of the nucleus is roughly 10 mâ€about 100,000 times smaller. For a perspective about their relative sizes, consider this: If the nucleus were the size of a blueberry, the atom would be about the size of a football stadium.

Atoms and subatomic particles are extremely small, so we use appropriately small units of measure, such as the atomic mass unit (amu) and the fundamental unit of charge (e), to describe their properties. The amu was originally defined based on hydrogen, the lightest element, then later in terms of oxygen. Since 1961, it has been defined with regard to the most abundant isotope of carbon, atoms of which are assigned masses of exactly 12 amu. Therefore, one amu = 1.67 x 10^-24 g. The unified atomic mass unit (A) is an alternative unit that is equivalent to the amu.

The proton has a mass of 1.0073 amu and a charge of 1+, while a neutron is slightly heavier with a mass of 1.0087 amu and no charge. The electron has a charge of 1∠and is much lighter than the other particles with a mass of about 0.00055 amu.

A chemical symbol is an abbreviation that we use to indicate an element or an atom of an element. For example, the symbol for mercury is Hg. We use the same symbol to indicate one atom of mercury or to label a container of many atoms of the element mercury.

Traditionally, the discoverer of a new element names the element. However, until the name is recognized by the International Union of Pure and Applied Chemistry (IUPAC), the recommended name of the new element is based on the Latin word for its atomic number. For example, element 106 was called unnilhexium , element 107 was called unnilseptium , and element 108 was called unniloctium for several years. These elements are now named after scientists; for example, element 106 is now known as seaborgium in honor of Glenn Seaborg, a Nobel Prize winner who was active in the discovery of several heavy elements. Element 109 was named in honor of Lise Meitner, who discovered nuclear fission, a phenomenon that would have world-changing impacts; Meitner also contributed to the discovery of some major isotopes.

The symbol for a specific isotope of any element is written by placing the mass number as a superscript to the left of the element symbol . The atomic number is sometimes written as a subscript preceding the symbol, but since this number defines the element's identity, as does its symbol, it is often omitted. For example, magnesium exists as a mixture of three isotopes, each with an atomic number of 12 and with mass numbers of 24, 25, and 26, respectively. These isotopes can be identified as Mg, Mg, and Mg. These isotope symbols are read as "magnesium-24," "magnesium-25," and "magnesium-26." All magnesium atoms have 12 protons in their nucleus. They differ only because a magnesium atom has 12 neutrons in its nucleus, a magnesium atom has 13 neutrons, and a magnesium atom has 14 neutrons.