8 July 2020- What is Chemistry?- Part 2

MOLECULES AND CHEMICAL FORMULAS

Elements vary from those that are highly active to those that are almost completely inert. The tendency of an atom to combine is a chemical property of the atom, and it is controlled by electrical forces at the atomic level. Those forces forge the links, called chemical bonds, that hold two or more atoms together. The groups of atoms formed by chemical bonding are known as molecules. Molecules are the smallest units of a compound that can exist while still keeping the substance’s composition and chemical properties.

To make elements and compounds easier to describe in written form, chemists commonly indicate each element with an abbreviation called a chemical symbol. This is usually the initial or the first two letters of the common name or the Latin name of the element. Chemists use such shorthand to represent not only the name of an element but also one atom of that element. The symbols for single atoms of hydrogen, carbon, and iron, for example, are H, C, and Fe.

Atoms of one element can combine with each other to form molecules. For example, atoms of oxygen (O), hydrogen (H), nitrogen (N), and chlorine (Cl) tend to react with their own kind to form two-atom molecules. These molecules are represented by the abbreviations O₂, H₂, N₂, and Cl₂, respectively. The subscript 2 shows that two atoms of the element make up one molecule of the element. Oxygen also can form a three-atom molecule called ozone that has the abbreviation O₃. Abbreviations of molecules written in this way are known as chemical formulas. They show both the kinds of atoms and the number of each kind in the molecule.

Six gaseous elements—helium, neon, argon, krypton, xenon, and radon—were thought to be completely inactive chemically when they were first discovered and so were named inert gases. All are present in the air as single, uncombined atoms; thus in their gaseous forms their symbols (for example, He for helium) are written without subscripts. Since the early 1960s chemists have found that under proper conditions at least some of them can be made to form compounds. Today the preferred name of this family of elements is noble gases.

Chemical formulas are also used to represent compounds and their molecular compositions. One atom each of hydrogen and chlorine react to form the compound hydrogen chloride. The chemical formula for hydrogen chloride is HCl. Hydrogen atoms combined with carbon atoms in a proportion of four to one form the flammable gas methane, which is represented as CH₄. One atom of chlorine combines with three atoms of aluminum (Al) to form aluminum chloride (AlCl₃), which in a hydrated form (combined with molecules of water) is used in antiperspirants.

Even in the form of molecules, atoms of elements may still react with other elements. Oxygen molecules in the air (O₂) undergo a complex reaction with atoms of iron (Fe) when the iron is exposed to moist air. They combine with the iron to form rust, which is a chemical compound called ferric oxide (Fe₂O₃). In the same way, molecules of compounds can react with elements and with other compounds. A water molecule and an ammonia molecule (NH₃), for example, can combine into a new molecule called ammonium hydroxide (NH₄OH).

In reading chemical formulas, it is important to keep in mind what they do not show. Formulas usually say little about the chemical bonding of the atoms. In addition, they are necessarily written “flat” on paper or a chalkboard, although molecules actually exist in three dimensions in space. For example, the formula for methane, CH₄, does not show that all four hydrogen atoms are attached only to the carbon atom, or that the hydrogen atoms are arrayed in space around the carbon atom at equal distances and angles. Thus the molecule resembles a tetrahedron, or four-sided pyramid (including the base), with the carbon atom at the center and the hydrogen atoms at the corners. In solving chemical problems—such as designing a new plastic, for example—chemists often must consider the geometric structures of the molecules that will react and the molecules that will be produced.

CHEMICAL EQUATIONS

To express the reactions that form and break down molecules, chemists write chemical equations. These are shorthand versions of ordinary word description, and they make use of symbols and formulas for elements and compounds. For example:

Word description: Two molecules of hydrogen react with one molecule of oxygen to form two molecules of water.

Chemical equation: 2H₂ + O₂ → 2H₂O

Word description: Two molecules of ammonia react with each other to form one molecule of nitrogen and three molecules of hydrogen.

Chemical equation: 2NH₃ → N₂  + 3H₂

In chemical equations, the substances on the left side of the arrow—those undergoing the chemical change—are called reactants. The substances on the right side—the result of the reaction—are called products. The arrow can be read as “give,” “form,” or “yield.”

That's it for today. Tomorrow we will look at the history of chemistry. As always, until next time, stay curious, and stay sciencey!