CK-12-Physics-Concepts - Intermediate

22.1. Isotopes http://www.ck12.org

Since the identification of the neutron, the model of the atomic nucleus has changed very little. According to this
model, a nucleus is considered to be an aggregate of protons and neutrons. A proton has a positive charge of 1. 6 ×
10 −^19 Cand a mass of 1. 673 × 10 −^27 kg. The neutron is electrically neutral and has a mass of 1. 675 × 10 −^27 kg.
These two constituents of a nucleus are referred to collectively asnucleons.

Although the hydrogen nucleus consists of a single proton, the nuclei of all other elements contain both neutrons
and protons. The different types of nuclei are referred to asnuclides. The number of protons in a nucleus is called
theatomic numberand is designated by the symbolZ. The total number of nucleons, neutrons and protons, is
designated by the symbolAand is called themass number. A nuclide with 7 protons and 8 neutrons thus hasZ= 7
andA=15. The number of neutrons,N, isN=A−Z. To specify a given nuclide, we need give onlyAandZ. These
can be shown in a complete nuclear symbol which takes the form

A

ZX

whereXis the chemical symbol for the element,Ais the mass number, andZis the atomic number. For example, a
nitrogen nucleus containing 7 protons and 8 neutrons would be^157 N. Since all nitrogen atoms have 7 protons in their
nucleus, sometimes the 7 is omitted and the symbol is written simply as^15 N. This same nuclide is also sometimes
written as nitrogen-15.

The identity of an atom is determined by the number of protons in the nucleus. All hydrogen atoms must have
exactly 1 proton in the nucleus. The number of neutrons in the nuclei of atoms of a particular element, however,
may vary. For example, a hydrogen atom will always have 1 proton, but may have zero, one, or two neutrons in the
nucleus - as long as it has 1 proton, it is a hydrogen atom.

Nuclei that have the same number of protons in the nucleus but a different number of neutrons are calledisotopes.
Hydrogen has three isotopes.

1

1 H

2

1 H

3

1 H

One isotope of hydrogen has 1 proton and 0 neutrons in the nucleus so it has a mass number of 1. Another isotope
of hydrogen has 1 proton and 1 neutron in the nucleus so it has a mass number of 2. The third isotope of hydrogen
has 1 proton and 2 neutrons in the nucleus so it has a mass number of 3. (The isotope of hydrogen with a mass
number of 2 is sometimes called deuterium and the isotope of hydrogen with a mass number of 3 is sometimes
called tritium, but they are all hydrogen.) Carbon has six isotopes; carbon-11, carbon-12, carbon-13, carbon-14,
carbon-15, and carbon-16. The isotopes of a given element are not all equally abundant. For example, 98.9% of all
naturally occurring carbon is carbon-12 and about 1.1% is carbon-13. The other isotopes of carbon are even less
abundant.

The periodic table is a complete listing of all the known elements.

Each element has its own square in the periodic table. The square contains the chemical symbol for the element, the
atomic number, and theatomic weight. The atomic weight of an element is a weighted average of its isotopes.