Biology 120 Notes (Part 2) Continuing the Discussion of Atoms and Other Molecules

Isotopes

• Isotopes deal with neutrons in the atom.

• They differ in the number of neutrons.

• Some isotopes spontaneously decay and release radiation.

• Electrons are attracted to the nucleus; it takes energy to hold electrons in place.

• Electrons determine chemical behavior of atoms.

Valence Electrons

• The outermost energy shell is called the valence shell.

• The octet rule is a rule describing how a valence shell can only hold eight electrons (unless the atomic number is two or less.)

Molecules

• Molecules are atoms held together by chemical bonds.

Chemical Bonds

• Molecular bonds contain energy.

• Valence shells determine the bonding activity.

• There are three types of covalent bonds which are called: ionic, covalent, and hydrogen.

Ionic Bonds

• An ion is a charged atom.

• An ionic bind forms when electrons are transferred from one atom to another.

• Salts are sold substances that usually sperate and exist as individual ions in water.

Formation of Sodium Chloride

• The first ring holds only two electrons

• After the second ring and as more rings are created, they will hold eight electrons each.

Covalent Bonds

• Covalent bonds are a type of bond that describes how electrons are shared.

• Covalent bonds can either be polar or nonpolar.

Nonpolar and Polar Bonds

• Electrons are not always shared equally.

• Atoms may have different electronegativities.

• Electronegatives describe the strength with which atoms pull electrons toward themselves.

• In general, when moving up to the right on the periodic table, the higher the electronegativity.

Hydrogen Bonds

• A hydrogen bond is a weak attraction between a slightly positive hydrogen atom and a slightly negative tom in another molecule.