Atomic Structure

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Chapter: Anatomy and Physiology for Health Professionals: Levels of Organization : Chemical Basics of Life

Atoms are composed of subatomic particles and each atom consists of protons, neutrons, and electrons.

Atomic Structure

Atomic Structure

Atoms are composed of subatomic particles and each atom consists of protons, neutrons, and electrons. Protons and neutrons are similar in size and mass; however, protons bear a positive electrical charge, whereas neutrons are electrically neutral (uncharged). Electrons bear a negative electrical charge. An atom’s mass is determined mostly by the number of protons and neutrons in its nucleus. The nucleus contains approximately the entire mass (99.9%) of the atom. The mass of a larger object, such as the human body, is the sum of the masses of all its atoms. FIGURE 2-2 shows the components of an atom and its nucleus.


Electrons orbit an atom’s nucleus at high speed, forming a spherical electron cloud. Atoms normally contain equal numbers of protons and electrons. The number of protons in an atom is known as its atomic number. Thus, hydrogen (H), the simplestatom, has one proton, giving it the atomic ­number 1, whereas magnesium, with 12 protons, has the atomic number 12.

The atomic weight of an element’s atom equals the number of protons and neutrons in its nucleus. For example, oxygen has eight protons and eight neutrons, so its atomic weight is 16. An isotope is defined as when an element’s atoms have nuclei containing the same number of protons but different number of neu-trons. Isotopes may or may not be radioactive. Radio-activity is the emission of energetic particles known as radiation, which occurs because of instability of the atomic nuclei.

The nuclei of certain isotopes (radioisotopes) spontaneously emit subatomic particles or radiation in measurable amounts. The process of emitting radi-ation is called radioactive decay. Strong radioactive isotopes are dangerous because their emissions can destroy molecules, cells, and living tissue. For diag-nostic procedures, weak radioactive isotopes are used to diagnose structural and functional characteristics of internal organs. Radiation is basically identified as one of three common forms: alpha (α), beta (β), or gamma (γ). Gamma radiation is the most penetrating type and is similar to X-ray radiation.

Health professionals and researchers use radioac-tive isotopes for clinical applications because they are easily detected and measured. All isotopes of a certain element have the same atomic number. For example, two types of iodine, 125-iodine and 131-iodine, can substitute for 126-iodine in chemical reactions. Iodine may be used in diagnostic procedures involving the thyroid gland to detect thyroid cancer.


1. Define the term “atom” and explain its structure.

2. Differentiate between atomic weight and atomic number.

3. Describe the locations of electrons, protons, and neutrons.

4. Define the terms “atomic number” and “radioisotopes.”

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