Chemistry different mass numbers are known as isotopes. For example, all atoms with 6 protons in their nuclei are atoms of the chemical element carbon, but atoms of carbon may have mass numbers of 12 or 13. The standard presentation of the chemical elements is in the periodic table, which orders elements by atomic number. The periodic table is arranged in groups, or columns, and periods, or rows. The periodic table is useful in identifying periodic trends. A "compound" is a pure chemical substance composed of more than one element. The properties of a compound bear little similarity to those of
Neutron number Neutron number The neutron number, symbol N, is the number of neutrons in a nuclide. Atomic number (proton number) plus neutron number equals mass number: Z+N=A. The difference between the neutron number and the atomic number is known as the neutron excess: D = N - Z = A - 2Z. Neutron number is rarely written explicitly in nuclide symbol notation, but appears as a subscript to the right of the element symbol. In order of increasing explicitness and decreasing frequency of usage: Nuclides that have the same neutron number but a different proton number are called isotones. This word