the elements can form different compounds.comment

Any substance that contains only one kind of an atom is known as an element. They cannot be broken down into simpler substances.

The elements can be divided into three categories that have characteristic properties: metals, nonmetals, and semimetals.

Two or more elements combine together to form compounds. The atoms of different elements are held together in compounds by chemical bonds (mostly ionic and covalent).

Metals often react with nonmetals to form ionic compounds (with ionic bonds). These compounds are composed of positive and negative ions formed by adding or subtracting electrons from neutral atoms and molecules.

2Na + Cl2 -----------> 2NaCl

Nonmetals combine with each other to form covalent compounds (covalent bonds), which exist as neutral molecules.

2H2 + O2 -----------> 2H2O

The properties of compounds are often very different from the properties of the elements that make them. For example, water is made from two atoms of hydrogen bonded to one atom of oxygen. At room temperature, hydrogen and oxygen are both colorless, odorless gases, and they remain gases down to extremely low temperatures. Water, however, is a liquid at temperatures up to 100°C (212°F) and a solid

below 0°C (32°F).