II. Chemical bonds and molecules.

Terms

A. Some common elements, their symbols, atomic numbers, and atomic masses are:

Table of common elements

Element	Symbol	Atomic number	Atomic mass
Hydrogen	H	1	1.008
Carbon	C	6	12.011
Nitrogen	N	7	14.007
Oxygen	O	8	15.999
Sodium	Na	11	22.990
Phosphorus	P	15	30.974
Sulfur	S	16	32.064
Chlorine	Cl	17	35.453
Potassium	K	19	39.102
Calcium	Ca	20	40.08
Iron	Fe	26	55.847

B. All atoms are constructed similarly.

1. There is a nucleus with protons (particles with a positive electrical charge) and neutrons (neutral particles). [Exception: H nucleus has no neutrons.]

2. Shells of electrons (particles with negative charge) surround the nucleus. Neutrons and protons are ca. 1800 times heavier than electrons.

3. There are several concentric electron shells, each with its characteristic maximum number of electrons.

a. The first three shells accommodate 2, 8, and 8(18) electrons respectively.

b. Electrons fill the inner shells first.

4. In its elemental (unreacted) state, an atom has equal numbers of protons and electrons.

5. The atomic number of an element is the number of protons in the nucleus. The atomic number determines the chemical nature of the element, and all atoms with the same atomic number behave alike.

6. The atomic mass of an element is (approximately) the sum of the masses of protons, neutrons, and electrons present. Since electrons are so small compared to protons and neutrons, the atomic mass is essentially n + p.

C. Atoms can gain or lose electrons to become more stable. Such a charged atom is an ion.
1. The number gained or lost is that required to produce an outer shell that is filled with electrons. E.g. Na⁰ – 1e —> Na⁺; Cl⁰ + 1e —> Cl¯

2. An atom that gains electrons becomes negatively charged and is an anion; an atom that loses electrons becomes positively charged and is a cation.

3. Atoms do not ordinarily gain or lose protons. That would change them into a different element.

D. The number of neutrons in the nucleus can vary over a narrow range for an element. The different forms have identical chemical properties but slightly different masses. They are isotopes. Some are stable, but others are unstable (radioactive isotopes). The different isotopes of an element are indicated by placing a superscript before the symbol, where the superscript is the sum of the protons and neutrons. E.g. ¹H, ²H, and ³H are the three isotopes of hydrogen, the first being by far the most common.

E. The number of electrons in the outer shell of an element largely determines how it behaves chemically. Atoms with the same number of electrons show many similarities. This enables the construction of a periodic table of the elements.

A. Ionic bonds result from the attraction of positively and negatively charged ions.
1. The partners in an ionic bond can exchange easily; eg Na⁺ Cl¯  Ca²⁺ 2Cl¯.

2. Many molecules can exist in unionized and ionized forms; eg
HOH and H⁺OH¯.

B. A covalent bond forms when two atoms share a pair of electrons. eg H· + H· —> H:H

1. Covalent bonds are stable, i.e. the partners do not shift around easily.

2. An atom can form as many covalent bonds as it has unpaired electrons to share. The number of bonds that can be formed is the valence. The following are important to learn:

H has one; O and S, two; N, three; C, four; P, five.

3. Examples of molecules formed with covalent bonds:

water (H₂O); ethanol (C₂H₅OH); methane (CH₄); butane (C₄H₁₀); methanol (CH₃OH); acetic acid (CH₃COOH); ethane (C₂H₆); ammonia (NH₃).

4. Note that some pairs of atoms can form double bonds (= two covalent bonds between a single pair of atoms): C=C, C=O, etc. Although single bonds can rotate freely around the axis of the bond, double bonds do not allow such rotation.

5. There is no limit to the number of atoms that can occur in a molecule; some (eg DNA) have hundreds of millions.

6. Molecules that contain covalently bound carbon are called organic molecules. All other molecules are inorganic.

7. Certain groupings of atoms (functional groups) appear in many different organic molecules:

hydroxyl group = –O–H

carboxyl group = –COOH

amino group = –NH₂

C. Hydrogen bonds are formed when the following structures occur in the required spatial relationship:

–OH + O ––> –OH····O=

=NH + O ––> =NH····O=

–OH + N ––> –OH····N=

1. Hydrogen bonds are very weak but are very important in biological systems.

2. Water is a liquid rather than a gas at room temperature because of hydrogen bonds.

3. DNA replication and gene function depend on hydrogen bonds as critical parts of the action.

D. The sum of all the atomic masses of the atoms in a molecule is the molecular weight.

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