Atomic Structure

In the fifth century b.c.e., the Greek philosopher Dem-ocritus reasoned that we can cut matter such as a gold nugget into smaller and smaller pieces, but there must ultimately be particles so small that nothing could cut them. He called these imaginary particles atoms1 ("indivisible"). Atoms were only a philosophical concept until 1803, when English chemist John Dalton began to develop an atomic theory based on experimental evidence. In 1913, Danish physicist Niels Bohr proposed a model of atomic structure similar to planets orbiting the sun (figs. 2.1 and 2.2). Although this planetary model is too simple to account for many of the properties of atoms, it remains useful for elementary purposes.

Saladin: Anatomy & I 2. The Chemistry of Life I Text I I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

Table 2.1 Elements of the Human Body

Percentage of

Name

Symbol

Body Weight

Major Elements (total 98.5%)

Oxygen

O

65.0

Carbon

C

18.0

Hydrogen

H

10.0

Nitrogen

N

3.0

Calcium

Ca

1.5

Phosphorus

P

1.0

Lesser Elements (total 0.8%)

Sulfur

S

0.25

Potassium

K

0.20

Sodium

Na

0.15

Chlorine

Cl

0.15

Magnesium

Mg

0.05

Iron

Fe

0.006

Trace Elements (total 0.7%)

Chromium Cr

Molybdenum

Mo

Cobalt Co

Selenium

Se

Copper Cu

Silicon

Si

Fluorine F

Tin

Sn

Iodine I

Vanadium

V

Manganese Mn

Zinc

Zn

Chapter 2 The Chemistry of Life 57

At the center of an atom is the nucleus, composed of protons and neutrons. Protons (p+) have a single positive charge and neutrons (n0) have no charge. Each proton or neutron weighs approximately 1 atomic mass unit (amu), defined as one-twelfth the mass of an atom of carbon-12. The atomic mass of an element is approximately equal to its total number of protons and neutrons.

Around the nucleus are one or more concentric clouds of electrons (e_), tiny particles with a single negative charge and very low mass. It takes 1,836 electrons to equal 1 amu, so for most purposes we can disregard their mass. A person who weighs 64 kg (140 lb) contains less than 24 g (1 oz) of electrons. This hardly means that we can ignore electrons, however. They determine the chemical properties of an atom, thereby governing what molecules can exist and what chemical reactions can occur. The number of electrons equals the number of protons, so their charges cancel each other and an atom is electrically neutral.

Electrons swarm about the nucleus in concentric regions called electron shells (energy levels). The more energy an electron has, the farther away from the nucleus its orbit lies. Each shell holds a limited number of electrons (see fig. 2.1). The one closest to the nucleus holds a maximum of 2 electrons, the second one holds a maximum of 8, and the third holds a maximum of 18. The outermost shell never holds more than 8 electrons, but a shell can acquire more electrons after another one, farther out, begins to fill. Thus, the third shell will hold 18 electrons only in atoms with four or more shells. The elements known to date have up to seven electron shells, but those ordinarily involved in human physiology do not exceed four.

The electrons of the outermost shell, called valence electrons, determine the chemical bonding properties of

First energy level

First energy level

Carbon (C) 6p+, 6n0 Atomic number = 6 Atomic mass = 12

Second energy level

Second energy level

Nitrogen (N) 7p+, 7n0 Atomic number = 7 Atomic mass = 14

Third energy level

Third energy level

Atomic Number

Sodium (Na) 11p+, 12n0 Atomic number = 11 Atomic mass = 23

Fourth energy level

Fourth energy level

Potassium Atom Model

Potassium (K) 19p+, 20n0 Atomic number = 19 Atomic mass = 39

Carbon (C) 6p+, 6n0 Atomic number = 6 Atomic mass = 12

Nitrogen (N) 7p+, 7n0 Atomic number = 7 Atomic mass = 14

Sodium (Na) 11p+, 12n0 Atomic number = 11 Atomic mass = 23

Potassium (K) 19p+, 20n0 Atomic number = 19 Atomic mass = 39

Figure 2.1 Bohr Planetary Models of Four Representative Elements. Note the filling of electron shells as atomic number increases. Protons are represented as p+ and neutrons are represented as n0

Will potassium have more tendency to give up an electron or to take one away from another atom?

Saladin: Anatomy & I 2. The Chemistry of Life I Text I I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

58 Part One Organization of the Body an atom. An atom tends to bond with other atoms that will fill its outer shell and produce a stable number of valence electrons. A hydrogen atom, with only one electron shell and one electron (fig. 2.2), tends to react with other atoms that provide another electron and fill this shell with a stable number of two electrons. All other atoms react in ways that produce eight electrons in the valence shell. This tendency is called the octet rule (rule of eights).

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