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.