1.8: Subatomic Particles - Protons, Neutrons, And Electrons

The Number of Protons Define the Nature of the Elements

The number of protons in the nucleus of an atom is itsatomic number (\(Z\)). This is the defining trait of an element: Its value determines the identity of the atom. For example, any atom that contains six protons is the element carbon and has the atomic number 6, regardless of how many neutrons or electrons it may have. A neutral atom must contain the same number of positive and negative charges, so the number of protons equals the number of electrons. Therefore, the atomic number also indicates the number of electrons in an atom. The total number of protons and neutrons in an atom is called its mass number (\(A\))). The number of neutrons is therefore the difference between the mass number and the atomic number:

\[\begin{align*} \ce{atomic\: number\:(Z)\: &= \:number\: of\: protons\\ mass\: number\:(A)\: &= \:number\: of\: protons + number\: of\: neutrons\\ A-Z\: &= \:number\: of\: neutrons} \end{align*}\]

The identity of an element is defined by \(Z\), the number of protons in the nucleus of an atom of the element, which is different for each element. The known elements are arranged in order of increasing Z in the periodic table (Figure \(\PageIndex{2}\)). The rationale for the peculiar format of the periodic table is explained later. Each element is assigned a unique one-, two-, or three-letter symbol. The names of the elements are listed in the periodic table, along with their symbols, atomic numbers, and atomic masses. The chemistry of each element is determined by its number of protons and electrons.

The symbols for several common elements and their atoms are listed in Table \(\PageIndex{2}\). Some symbols are derived from the common name of the element; others are abbreviations of the name in another language. Symbols have one or two letters, for example, H for hydrogen and \(\ce{Cl}\) for chlorine. To avoid confusion with other notations, only the first letter of a symbol is capitalized. For example, \(\ce{Co}\) is the symbol for the element cobalt, but \(\ce{CO}\) is the notation for the compound carbon monoxide, which contains atoms of the elements carbon (\(\ce{C}\)) and oxygen (\(\ce{O}\)). All known elements and their symbols are in the periodic table.

Table \(\PageIndex{2}\): Some Common Elements and Their Symbols

Element	Symbol	Element	Symbol
aluminum	Al	iron	Fe (from ferrum)
bromine	Br	lead	Pb (from plumbum)
calcium	Ca	magnesium	Mg
carbon	C	mercury	Hg (from hydrargyrum)
chlorine	Cl	nitrogen	N
chromium	Cr	oxygen	O
cobalt	Co	potassium	K (from kalium)
copper	Cu (from cuprum)	silicon	Si
fluorine	F	silver	Ag (from argentum)
gold	Au (from aurum)	sodium	Na (from natrium)
helium	He	sulfur	S
hydrogen	H	tin	Sn (from stannum)
iodine	I	zinc	Zn

Traditionally, the discoverer (or discoverers) 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(s) for its atomic number. For example, element 106 was called unnilhexium (Unh), element 107 was called unnilseptium (Uns), and element 108 was called unniloctium (Uno) for several years. These elements are now named after scientists or locations; for example, element 106 is now known as seaborgium (Sg) in honor of Glenn Seaborg, a Nobel Prize winner who was active in the discovery of several heavy elements