Isotopes

What is an Isotope?

Isotopes are various forms of an element that have the same number of protons but a different number of neutrons. Some elements, such as carbon, potassium, and uranium, have multiple naturally-occurring isotopes. Isotopes are defined first by their element and then by the sum of the protons and neutrons present.

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, as shown in the figure below. 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 instance:

• Carbon-12 (or ¹²C) contains six protons, six neutrons, and six electrons; therefore, it has a mass number of 12 amu (six protons and six neutrons).
• Carbon-14 (or ¹⁴C) contains six protons, eight neutrons, and six electrons; its atomic mass is 14 amu (six protons and eight neutrons).

Note that isotope symbols are read as “element, mass number.” For instance, in the case of magnesium, ²⁴Mg is read as “magnesium 24,” and can be written as “magnesium-24” or “Mg-24.” All magnesium atoms have 12 protons in their nucleus. They differ only because a ²⁴Mg atom has 12 neutrons in its nucleus, a ²⁵Mg atom has 13 neutrons, and a ²⁶Mg has 14 neutrons.

Properties of Isotopes

While the mass of individual isotopes is different, their physical and chemical properties remain mostly unchanged.

Isotopes do differ in their stability. Carbon-12 (¹²C) is the most abundant of the carbon isotopes, accounting for 98.89% of carbon on Earth. Carbon-14 (¹⁴C) is unstable and only occurs in trace amounts. Unstable isotopes most commonly emit alpha particles (He²⁺) and electrons. Neutrons, protons, and positrons can also be emitted and electrons can be captured to attain a more stable atomic configuration (lower level of potential energy ) through a process called radioactive decay. The new atoms created may be in a high energy state and emit gamma rays which lowers the energy but alone does not change the atom into another isotope. These atoms are called radioactive isotopes or radioisotopes.

Information about the naturally occurring isotopes of elements with atomic numbers 1 through 10 is given in the table below. Note that in addition to standard names and symbols, the isotopes of hydrogen are often referred to using common names and accompanying symbols. Hydrogen-2, symbolized ²H, is also called deuterium and sometimes symbolized D. Hydrogen-3, symbolized ³H, is also called tritium and sometimes symbolized T.

Element	Symbol	Atomic Number	Number of Protons	Number of Neutrons	Mass (amu)	% Natural Abundance
hydrogen	1 1H (protium)	1	1	0	1.0078	99.989
	2 1H (deuterium)	1	1	1	2.0141	0.0115
	3 1H (tritium)	1	1	2	3.01605	— (trace)
helium	3 2He	2	2	1	3.01603	0.00013
	4 2He	2	2	2	4.0026	100
lithium	6 3Li	3	3	3	6.0151	7.59
	7 3Li	3	3	4	7.0160	92.41
beryllium	9 4Be	4	4	5	9.0122	100
boron	10 5 B	5	5	5	10.0129	19.9
	11 5 B	5	5	6	11.0093	80.1
carbon	12 6 C	6	6	6	12.0000	98.89
	13 6 C	6	6	7	13.0034	1.11
	14 6 C	6	6	8	14.0032	— (trace)
nitrogen	14 7 N	7	7	7	14.0031	99.63
	15 7 N	7	7	8	15.0001	0.37
oxygen	16 8 O	8	8	8	15.9949	99.757
	17 8 O	8	8	9	16.9991	0.038
	18 8 O	8	8	10	17.9992	0.205
fluorine	19 9 F	9	9	10	18.9984	100
neon	20 10Ne	10	10	10	19.9924	90.48
	21 10Ne	10	10	11	20.9938	0.27
	22 10Ne	10	10	12	21.9914	9.25

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