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Chapter 2

Chapter 2 Notes

Biological Sciences
Course Code
Mark Fitzpatrick

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Chapter 2
It is electrons, however, that determine how atoms will interact in chemical reactions
About 98 percent of the mass of every living organism (bacterium, turnip, or human) is
composed of just six elements: carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur.
Sodium and potassium, for example, are essential for nerve function; calcium can act as a
biological signal; iodine is a component of a vital hormone; and magnesium and
molybdenum are essential to plants (magnesium as part of their chlorophyll pigment, and
molybdenum for incorporating nitrogen into biologically useful substances).
Hydrogen doesnt have a neutron unless its an isotope.
Most isotopes are stable. But some, called radioisotopes, are unstable and spontaneously
give off energy in the form of (alpha), (beta), or (gamma) radiation from the atomic
nucleus. Known as radioactive decay, this release of energy transforms the original atom.
These transformations can extend even to a change in the number of protons, so that the
original atom is now a different element.
radiation from 60Co (cobalt-60) is used in medical practice to kill cancer cells.
The behaviour of electrons explains how chemical reactions occur.
The region of space where the electron is found at least 90 percent of the time is the
electrons orbital.
Hydrogen bond: A weak electrostatic bond which arises from the attraction between the
slight positive charge on a hydrogen atom and a slight negative charge on a nearby oxygen
or nitrogen atom.
A hydrogen bond is weaker than most ionic bonds because it is formed by partial charges
(+ and ). It has about one-tenth (10 percent) the strength of a covalent bond between a 
hydrogen atom and an oxygen atom. However, where many hydrogen bonds form, they have
considerable strength and greatly influence the structure and properties of substances.
Hydrogen bonds also play important roles in determining and maintaining the three-
dimensional shapes of giant molecules such as DNA and proteins
Polar and polar tend to interact with each other while non polar and non polar tend to react
with each other.
Uncharged and nonpolar groups of atoms are hydrophobic. Hydrophobic interactions
between nonpolar substances are enhanced by van der Waals forces, which result when two
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