CHEMISTRY - a quick review and a study guide
 
 

ELEMENT

(1) a substance that can't be decomposed into other substances.

(2) a substance where all of the atoms have the same number of protons (atomic number) although the number of neutrons may differ (the atomic weights may differ). The different atomic weight variants of an element are called the isotopes of that element. (for example c₁₂, c₁₃, and c₁₄ are all isotopes of carbon, all have 6 protons but each has a different number of neutrons).

• Protons and neutrons are found in the nucleus of the atom while the electrons are located in various orbitals or shells which surround the nucleus. These orbitals have specific numbers of electrons which they can accomodate. (how do the shells differ in their electron capacity?)

• If an atom is neutral then it has an equal number of protons and electrons. Neutral atoms are especially stable if their orbitals are filled to capacity. This is the case with the noble gases.

•  However the atoms of most elements have incompletely filled orbitals and these atoms form bonds to achieve stability.
• click to see larger image

BONDS
 

• Ions and ionic bonds

Electrons are lost or gained in the unfilled shell. The atom becomes charged -- either positive (cation) or negative (anion).

click image for larger view

Charged atoms (or molecules) of opposite charge are attracted to each other and can form reversable, ionic bonds with each other.

Ionic compounds dissolved in water conduct electricity very well. Dissolved ions are therefore called electrolytes.

• COVALENT BOND
•  
• In covalent bonds, pairs of electrons are shared between atoms in order to fill their outer shells. click me

• Hydrogen bonds

Covalently bonded atoms may be electrically neutral however the electrons may not be evenly distributed across the molecule.

Such molecules will have oppositely charged areas which can be attracted to other charged molecules. A good example of this occurs between water molecules where such electrostatic bonds are called hydrogen bonds

.

• Hydrophobic bonds

This type of non-covalent bond describes the interaction of non-polar, hydrophobic molecules when they are put into water. Hydrophobic bonds are very important in the formation of membranes and in enzyme-substrate binding.

 

THE MOLECULES OF LIFE

·  Water

Hydrogen bonds between water molecules contributes to water's high boiling point.

Water molecules actually repel each other at freezing; this means that frozen water is less dense than liquid water.

Also, the hydrogen bonds can absorb and hold on to energy; this makes water a good temperature buffer (ie.: water changes temperature slowly).

The polar nature of water molecules contributes to water's excellent solvent properties.

Water can ionize to form hydrogen (+) ions and hydroxyl (-) ions. The concentration of these ions determines the ph of water or other solutions.