CHAPTER 2 - ATOMS & MOLECULES.

In this chapter you will learn about:
Symbols and formulas;
The Dalton's Atomic Theory;
Atomic structure and isotopes;
Relative Atomic Masses and average atomic masses;
Moles and the Avogadro's number;
Molecular and Molar Masses from formulas.

ELEMENTS - the fundamental constituent of matter;
There are 115 elements known to date and counting, but only 88 are naturally occurring; the rest are synthesized in the particle-accelerators. Eight of those natural elements account for about 98% of the entire elemental masses found in the Earth's crust. They are O, Si, Al, Fe, Ca, Na, K, and Mg. Oxygen alone accounts for nearly one-half of the total mass of crustal elements. It is also the most abundant element in the human body.

Chemical Symbols and Formulas
Each element is represented by a chemical symbol. Most symbols are taken from the first letter of the common English names of the elements. Others are derived from the Latin's names of the respective elements. For example,

Ag = silver (Argentum); Na = sodium (Natrium) Au = gold (Aurium); Pb = lead (Plumbum);
Cu = copper (Cuprum); Sb = antimony (stibium);
Fe = iron (Ferrum); Sn = tin (stannum);
Hg = mercury (Hydragyrum); W = tungsten (wolfrum) K = potassium (Kalium);

Memorize the NAMES & CHEMICAL SYMBOLS of the following elements:
1. All elements from H (Z = 1) through Kr (Z = 36);
2. The alkali metals (Group IA): Li, Na, K, Rb, and Cs;
3. The alkaline earth metals (Group IIA): Be, Mg, Ca, Sr, Ba, and Ra;
4. The halogens (Group VIIA): F, Cl, Br, and I;
5. The noble gas elements (Group VIIIA): He, Ne, Ar, Kr, Xe, and Rn
6. The following metals: Ag, Cd, Sn, Sb, Pd, Pt, Au, Hg, Pb, and U.

WHAT ARE ELEMENTS MADE OF?
Most elements occur in a variety of compounds; few exist free in nature.
For examples, oxygen occurs in compounds such as H2O, SiO2 (quartz and sand), oxides of metals (like Fe2O3), calcium carbonate (CaCO3), in various forms for silicates in soils and rocks, and in complex biological molecules. Only a small fraction of oxygen occurs as free element in the atmosphere.

Most metals occur in minerals, either as oxides, sulfides, or carbonate. For example, iron occurs in iron ores such as Fe2O3 and Fe3O4. Zinc and lead are found in zinc blend (ZnS) and galena (PbS), respectively, and calcium and magesium in dolomite (MgCO3 & CaCO3). The only metals found in free elemental forms are silver, gold and platinum. These are the noble (unreactive) metals.

The noble gases occur as single atoms (He, Ne, Ar, Kr, Xe, and Rn);
Elemental halogens occur as gases (F2 & Cl2), a volatile liquid (Br2), and a solid (I2); they are composed of diatomic molecules. None of them occurs in free elemental form in nature.

Other elemental gases composed of diatomic molecules are hydrogen, nitrogen, and oxygen (H2, N2, and O2).

Molecules are groups of atoms bonded together that exist and act as single entities.
Two or more elements may combine chemically to form compounds.
1. During reactions, masses are conserved, and this property of matter is summarized in the law of the conservation of mass.
2. It was also found that, a given compound always contains the same composition (by mass) of the elements, regardless of the origin of that compound. This is summarized in the law of constant composition; for examples, water always contains 11.1% H and 88.9% O, by mass, and carbon dioxide always contains 27.3% C and 72.7% O, by mass.

John Dalton (1766 - 1844) offered the following explanations, which is known as Dalton's Atomic Theory.

1. Elements are made up of atoms.
2. Atoms of a given elements are identical physically and chemically;
3. Atoms of different elements are physically and chemically different from one another;
4. Atoms of different elements may be combined (chemically) in simple ratios to form compounds. (A given compound always contains the same relative number and type of its atoms.)
5. Atoms of two different elements may