2. KEY VOCABULARY Atom The basic structural unit of matter; the smallest particle of an element that can enter into a reaction. Bond The electrical attraction between two different atoms (elements) that forms a molecular or ionic compound. Chemical bond The electrical attraction between two different atoms (elements) that forms a molecule (compound) Chemical formula A shorthand notation to describe elements and compounds and their reactions. Each element is identified by one or two letters. For example, H represents hydrogen, Cl represents chlorine, and O represents oxygen. Compound A homogeneous, pure substance composed of two or more essentially different elements that are chemically combined and that are thus present in definite proportions. For example, H2O contains two atoms of hydrogen and one atom of oxygen chemically combined to produce a new substance with properties quite different from either hydrogen or oxygen.

3. Covalent bonds Chemical bonds in which atoms share electrons. Element A collection of atoms of one type that cannot be decomposed into any simpler units except by spontaneously changing into other units by radioactive processes. Oxygen, hydrogen, iron, and carbon are examples of elements. Ion An atom or group of atoms that has gained or lost one or more electrons and is now either positively or negatively charged. ionic bond A bond that forms when electrons are transferred from one atom to another, resulting in two oppositely charged ions that now attract each other. Molecule The smallest particle of an element or compound that displays the properties of the substance in quantity.

4. Atoms and Molecules An atom is defined as the smallest particle of an element that retains the properties of the element. Elements are the simplest chemical forms of matter: they cannot be broken down into simpler substances by chemical processes. The 90 naturally occurring elements are listed in the Periodic Table of the Elements. A molecule consists of two or more atoms that are held together through covalent bonds. Some elements and all covalent compounds are made of molecules. Most of the elements that are commonly found in molecular form are diatomic—their molecules contain two identical atoms. This activity involves three elements—hydrogen, oxygen, and nitrogen. Types of Substances A compound is a substance that consists of two or more elements chemically combined through bonding. There are two major classes of compounds: molecular and non-molecular. In covalent bonds, atoms share electrons. The bond between two identical atoms, such as the bonds in H2, O2, and N2, are pure, non-polar covalent bonds. Other bonds between non-metal elements, such as the bond between oxygen and hydrogen in water, involve a more or less unequal sharing of electrons. These bonds are called polar covalent bonds.

5. In contrast to a covalent bond is an ionic bond. In an ionic bond, electrons are transferred Or donated from one atom to another. The atoms that have gained or lost electrons are no longer neutral and are now called ions. (An ion is an atom or group of atoms that has a positive or negative charge.) Ionic bonds form from an electrostatic attraction between the positive and negative ions.

10. In your science notebook, write what you think the formula for this molecule would be. In your science notebook, draw and label a diagram of your model of an oxygen molecule.

11. In your science notebook, write what you think the formula for this molecule would be. In your science notebook, draw and label a diagram of your model of an nitrogen molecule.

13. In your science notebook, write the name and chemical formula each molecule in the table above. Use your materials to create a model of each molecule, then draw and a sketch of each.