Introductory Chemistry: A Foundation - Zumdahl S.S., DeCoste D.J. 2019
Chemical Foundations: Elements, Atoms, and Ions
Formulas of Compounds
· To learn how a formula describes a compound’s composition.
A compound is a distinct substance that is composed of the atoms of two or more elements and always contains exactly the same relative masses of those elements. In light of Dalton’s atomic theory, this simply means that a compound always contains the same relative numbers of atoms of each element. For example, water always contains two hydrogen atoms for each oxygen atom. In this context, the term relative refers to ratios.
The types of atoms and the number of each type in each unit (molecule) of a given compound are conveniently expressed by a chemical formula . In a chemical formula the atoms are indicated by the element symbols, and the number of each type of atom is indicated by a subscript, a number that appears to the right of and below the symbol for the element. The formula for water is written , indicating that each molecule of water contains two atoms of hydrogen and one atom of oxygen (the subscript is always understood and not written). Following are some general rules for writing formulas:
Rules for Writing Formulas
1. Each atom present is represented by its element symbol.
2. The number of each type of atom is indicated by a subscript written to the right of the element symbol.
3. When only one atom of a given type is present, the subscript is not written.
Interactive Example 4.1. Writing Formulas of Compounds
Write the formula for each of the following compounds, listing the elements in the order given.
a. Each molecule of a compound that has been implicated in the formation of acid rain contains one atom of sulfur and three atoms of oxygen.
b. Each molecule of a certain compound contains two atoms of nitrogen and five atoms of oxygen.
c. Each molecule of glucose, a type of sugar, contains atoms of carbon, atoms of hydrogen, and atoms of oxygen.
Self-Check: Exercise 4.1
· Write the formula for each of the following compounds, listing the elements in the order given.
a. A molecule contains phosphorus atoms and oxygen atoms.
b. A molecule contains one uranium atom and six fluorine atoms.
c. A molecule contains one aluminum atom and three chlorine atoms.
See Problems 4.19 and 4.20.
Chemistry in Focus A Four-Wheel-Drive Nanocar
A special kind of “microscope” called a scanning tunneling microscope (STM) has been developed that allows scientists to “see” individual atoms and to manipulate individual atoms and molecules on various surfaces. One very interesting application of this technique is the construction of tiny “machines” made of atoms. An example of this activity was performed by a group of scientists from the University of Groningen in the Netherlands. They used carbon atoms to construct the tiny machine illustrated in the accompanying photo. Notice that the “wheels” are paddle-like structures. When electrons are fired at the tiny car, the bonding between the carbon atoms changes in such a way that the paddles twist and propel the car forward. The scientists have been able to move the car forward as much as car lengths on the copper surface.
Although at this point it is not quite clear how the little car could perform useful tasks, the research is yielding useful results on how the carbon-based structure interacts with the copper atoms comprising the surface.
See Problem 4.110