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Basics of Chemistry

by Ron Kurtus (revised 25 May 2005)

Chemistry is a science of substances and their properties. It is concerned with how and why various materials combine or separate to form different substances.

Atoms, molecules and compounds are the "stuff" of Chemistry. The outer electron orbits or shells are what primarily determine the chemical characteristics of a material.

Questions you may have include:

This lesson will answer those questions.

Science of substances

Chemistry is the science that deals with the different kinds of matter, their properties and uses, the changes in which matter undergoes, and the conditions that influence these changes. In other words, it deals with the structure and composition of complex and simple substances.

Types of Chemistry

There are different subsets within the subject. Analytical Chemistry is concerned with identification of the kinds of matter and the quantity of each that compose complex substances. Organic Chemistry is the chemistry of carbon compounds, especially those within living matter. There are several other, less popular Chemistry subdivisions.

Studies reactions

Under certain conditions—such as the addition of heat—different materials will chemically react, forming new substances. Sometimes heat is a byproduct of a reaction. Chemistry studies these reactions, as well as predicts new reactions.

The stuff of Chemistry

The atom is the basic chemical unit.


An element is a material or substance made up of one type of atom. For example, iron is an element consisting of iron (Fe) atoms.

All the elements occurring in nature have been named and are given shorthand symbols to represent them. Examples of elements and their symbols are:

Some symbols represent the name, while others stand for the Latin version of the name.

There are 92 natural elements. Number 92 is Uranium. There are also several that have been created artificially, but they disintegrate rapidly.

(See Chemical Elements for a complete listing.)


A molecule is the chemical combination of two or more atoms. They can be of the same element such as the oxygen molecule (O2) or different elements as in the water molecule (H2O).


A compound is a molecule that is made up of at least two different elements. The water molecule is a compound.


Each element has unique physical and chemical properties. Likewise, each compound has unique physical and chemical properties that are typically much different than the elements that make up the compound. A good example is when the poisonous green chlorine gas is combined with the explosive metal sodium to form the white salt crystals we use in our food.

Chemical reactions concern outer orbits

The essence of Chemistry relates primarily to the outer orbits of the elements involved. In other words, chemical activity is determined by the number of electrons in the outer orbits of the atoms. Those electrons are often called valence electrons.

Electrons of atom

Each atom has electrons arranged in orbits, shells or levels around its nucleus. The Bohr or solar system model of the atom is no longer used, but it is convenient for visualizing the electrons in orbit, similar to the planets in our Solar System. One difference is that there are usually more than one electron in an orbit, as opposed to our Solar System with one planet per orbit.

Rules on orbits

There are rules for the maximum number of electrons in each orbit or shell. There is also a rule on filling an orbit.

Maximum number

A maximum of 2 electrons are allowed in the first orbit. For example, Hydrogen has 1 electron and Helium has 2 electrons in the first orbit or shell.

The maximum number of electrons in the second orbit is 8. Element number 3, Lithium has 2 electrons in the first orbit and 1 in the second orbit. Neon, number 10, has 2 electrons in the first orbit and 8 in the second.

The third orbit is considered "full" with 8 electrons, but it allows a maximum of 18 electrons

The fourth orbit is also considered "full" with 8 electrons, but can have no more than 32.

The electrons must usually fill up the lower orbits before starting on a higher orbit. With larger atoms, more complex rules on how orbits are filled take hold.

(See the Periodic Table for more about orbits or shells.)

Filling orbit

This rule states that elements "like" to have their outer orbit either filled or empty of electrons. For example, Chlorine has three orbits or shells, with 7 electrons in the outer orbit. It would like to have 8. Likewise, Sodium has three orbits, with only 1 electron in its outer orbit. Sodium would like to get rid of that electron, so it would have 8 in its outer (second) orbit.

Outer shell determines reactions

The chemical combination of atoms to form a molecule is primarily based on the number of electrons in the outer orbit or shell of the each atom.

Chemical compounds are formed when elements can trade or share electrons in their outer orbits, such that the shells or each element are either filled to the maximum or completely empty. In the case of Sodium Chloride (NaCl) or common table salt, Sodium gives up its one outer electron to fill the Chlorine's outer shell.


Chemistry is a science of substances, their properties, and how and why materials combine or separate to form different substances. Atoms, molecules and compounds are involved in the study of Chemistry. The outer electron orbits or shells primarily determine the chemical characteristics of a material and whether materials will chemically combine.

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