Atomic Weight

Atoms have very little mass. A single hydrogen atom, for example, has a mass of only 1.67 X 10~24 g. The atomic weight scale indicates an atom's mass relative to the mass of other atoms. This scale is based upon assigning the carbon atom a mass of 12. On this scale,

TABLE 2-1 Essential Chemical Elements in the Body

Element

Symbol

MAJOR ELEMENTS:

99.3% OF TOTAL ATOMS

Hydrogen

H (63%)

Oxygen

O (26%)

Carbon

C (9%)

Nitrogen

N (1%)

MINERAL ELEMENTS: 0.7% OF TOTAL ATOMS

Calcium

Ca

Phosphorus

P

Potassium

K (Latin kalium)

Sulfur

S

Sodium

Na (Latin natrium)

Chlorine

CI

Magnesium

Mg

TRACE ELEMENTS:

LESS THAN 0.01% OF

TOTAL ATOMS

Iron

Fe (Latin ferrum)

Iodine

I

Copper

Cu (Latin cuprum)

Zinc

Zn

Manganese

Mn

Cobalt

Co

Chromium

Cr

Selenium

Se

Molybdenum

Mo

Fluorine

F

Tin

Sn (Latin stannum)

Silicon

Si

Vanadium

V

TABLE 2-2 Characteristics of Major Subatomic Particles

Mass Relative

to Electron

Electric

Location

Particle

Mass

Charge

in Atom

Proton

1836

+ 1

Nucleus

Neutron

1839

0

Nucleus

Electron

1

-1

Orbiting the

nucleus

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Chemical Composition of the Body CHAPTER TWO

Chemical Composition of the Body CHAPTER TWO

a hydrogen atom has an atomic weight of approximately 1, indicating that it has one-twelfth the mass of a carbon atom; a magnesium atom, with an atomic weight of 24, has twice the mass of a carbon atom.

Since the atomic weight scale is a ratio of atomic masses, it has no units. The unit of atomic mass is known as a dalton. One dalton (d) equals one-twelfth the mass of a carbon atom. Thus, carbon has an atomic weight of 12, and a carbon atom has an atomic mass of 12 daltons.

Although the number of neutrons in the nucleus of an atom is often equal to the number of protons, many chemical elements can exist in multiple forms, called isotopes, which differ in the number of neutrons they contain. For example, the most abundant form of the carbon atom, 12C, contains 6 protons and 6 neutrons, and thus has an atomic number of 6. Protons and neutrons are approximately equal in mass; therefore, 12C has an atomic weight of 12. The radioactive carbon isotope 14C contains 6 protons and 8 neutrons, giving it an atomic number of 6 but an atomic weight of 14.

One gram atomic mass of a chemical element is the amount of the element in grams that is equal to the numerical value of its atomic weight. Thus, 12 g of carbon (assuming it is all 12C) is 1 gram atomic mass of carbon. One gram atomic mass of any element contains the same number of atoms. For example, 1 g of hydrogen contains 6 X 1023 atoms, and 12 g of carbon, whose atoms have 12 times the mass of a hydrogen atom, also has 6 X 1023 atoms.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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