Isotopes

A particular atom with a given number of protons in its nucleus may exist in several forms that differ from one another in their number of neutrons. The atomic number of these forms is thus the same, but their atomic mass is different. These different forms are called isotopes. All of the isotopic forms of a given

Table 2.1 Atoms Commonly Present in Organic Molecules

Atomic

Atomic

Number of

Atom

Symbol

Number

Mass

Shell 1

Shell 2

Shell 3

Chemical Bonds

Hydrogen

H

1

1

1

0

0

1

Carbon

C

6

12

2

4

0

4

Nitrogen

N

7

14

2

5

0

3

Oxygen

O

8

16

2

6

0

2

Sulfur

S

16

32

2

8

6

2

Chemical Composition of the Body 25

Hydrogen

1 proton 1 electron

Hydrogen

1 proton 1 electron

Carbon

6 protons 6 neutrons 6 electrons

Carbon

6 protons 6 neutrons 6 electrons

Proton O Neutron O Electron c

■ Figure 2.1 Diagrams of the hydrogen and carbon atoms. The electron shells on the left are represented by shaded spheres indicating probable positions of the electrons. The shells on the right are represented by concentric circles.

atom are included in the term chemical element. The element hydrogen, for example, has three isotopes. The most common of these has a nucleus consisting of only one proton. Another isotope of hydrogen (called deuterium) has one proton and one neutron in the nucleus, whereas the third isotope (tritium) has one proton and two neutrons. Tritium is a radioactive isotope that is commonly used in physiological research and in many clinical laboratory procedures.

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