![]() The chemical symbols identify the types of atoms. For example, the chemical formula for isopropyl alcohol is C 3H 8O. A chemical formula identifies the type and number of atoms that make up a molecule. Two of the most common are chemical and structural formulas. There are many ways to represent molecules. Some people define the smallest unit of sodium chloride as the sodium chloride compound, but a compound is a substance, not a cluster of atoms. Maybe that is okay since those smallest units are not permanent and constantly in flux, but it is inconvenient. However, sodium chloride, and any other pure substance formed by ionic or metallic bonds, does not have a name for its smallest unit. For water, and any other pure substance formed by covalent bonds, the smallest unit of the substance is a molecule. Sodium chloride (NaCl) is also a compound because it is a pure substance that consists of sodium and chlorine atoms. Water (H 2O) is a compound because it is a pure substance that consists of hydrogen and oxygen atoms. However, the oxygen atom is not the smallest unit of diatomic oxygen gas or ozone… the O 2 and O 3 molecules are, and all three of those pure substances have very different characteristic properties.Ī pure substance is classified as a compound when it consists of two or more different types of atoms (or elements). Oxygen atoms (O), diatomic oxygen gas (O 2), and ozone (O 3) are all elements since they consist only of oxygen atoms. Some textbooks define an atom as the smallest unit of an element, but that is not accurate. A pure substance is classified as an element when it consists of a single type of atom. Pure substances can typically be classified as either elements or compounds. Ionic bonds are chemical bonds because they form new substances intermolecular bonds do not. Sodium chloride is a completely different substance. Sodium is a soft, silvery metal that reacts violently in water (think explosions and poisonous gases) and chlorine is a yellow-green gas that acts as a strong bleach. And sodium chloride does not behave anything like sodium or chlorine. However, sodium chloride has a specific chemical composition (one sodium ion for each chlorine ion) and its own distinct characteristic properties, such as density, boiling and melting points, and heats of vaporization and fusion, something that mixtures do not have. We could say that ionic bonds are not chemical bonds at all, but intermolecular bonds… and that sodium chloride is a mixture and not a pure substance. ![]() That is why a water molecule is a molecule and a sodium chloride ion pair is not. The oxygen atom in a water molecule is paired with two specific hydrogen atoms, and those pairs do not change until a chemical reaction breaks the molecule apart. Unlike ionic bonds, covalent bonds cannot be broken without creating new molecules and substances. ![]() Two atoms that form a covalent bond are physically sharing electrons. While the hydrogen and oxygen atoms in a water molecule are also held together by electrostatic attraction, they also have overlapping orbitals. ![]()
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