Unit 1: Back to the Basics

Section 3: Molecules, Ions, and the Periodic Table

Introduction

Molecules

Ways of Showing the Composition of Compounds

Ions

The Periodic Table

Introduction

In the last section, Section 2, atoms were defined as the smallest division of all substances. For example, water can be broken down into hydrogen atoms and oxygen atoms. This unit deals with the questions, What holds atoms together? How are substances created using atoms? How do people know what atoms and how many are in certain compounds? And how are atoms related to each other?

Molecules

Molecules are compounds that contain more than one atom of the same or different type of atom. Examples of molecules are CO, CO₂, SiO₂, N₂, O₂, and F₂. (Notice that the main criteria is that there are more that one atom present, but it doesn't matter if the atoms are the same or different.) The atoms in molecules are held together by chemical bonds, or more specifically, covalent bonds. Chemical bonds are the forces that hold atoms together. Covalent bonds are a specific type of chemical bond and this type of chemical bond occurs when elements "share" electrons. Sharing electrons means that one atom uses the electron for a while and then the other atom uses the electron for a while and it keeps switching back and forth.

Ways of Showing the Composition of Compounds

All compounds are composed of different elements and different numbers of them. To identify which elements and how many of them are present in different compounds, a set of rules were set up to identify compounds. The rules are very simple. The first rule is that chemical symbol is written to tell what element is used in a compound and the second rule is a subscript that follows the symbol to tell how many of these elements are present. The name of the substance using this method of naming is called the molecular formula. An example of the molecular formula are H₂O (2 atoms of hydrogen and 1 atom of oxygen), CO₂ (1 atom of carbon and 2 atoms of oxygen), and Br₂ (2 atoms of bromine).

TRY IT FOR YOURSELF: Practice Problems

There are still other ways of showing what atoms and how many are in compounds. Another way might be to draw the structural formula. The structural formula is a picture that shows what atoms are bonded to what atoms and what the relative positions of the atoms are in the compound. In this type of formula lines are drawn between the atoms to show how atoms are bonded together.

Ions

Ions are atoms or groups of atoms with a net positive or negative charge. For example, H⁺, OH^-, H₃O⁺, Na⁺, Mg⁺², B+³, F^- are all examples of ions. Ions are created by elements giving away or gaining an electron or two. An example might be that Na (sodium) might give away and electron which would make it Na⁺ [Remember that Na starts out neutral and then gives away a negatively charged electron so it becomes positive.]. Then Cl (Chlorine) might come along and pick up that extra electron and then it would become Cl^- [Remember that Cl starts out neutral and then gains a negatively charged electron, so it becomes negative.] An ion with a positive charge on it is called a cation. An ion with a negative charge on it is called an anion. (People remember that the cation is positive because the "t" looks like a + sign, and the anion is negative because there is a "n" in it.

Above, in the molecules section, chemical bonds were described as the forces that hold atoms together and covalent bonds were described as a specific type of chemical bond in which electrons are being "shared." There is another type of bonding and that is called ionic bonds. Ionic bonds involve ions of different charges, and these ions would pull each other together because of the opposite charges. They interact with each other much like the positive side of a magnet would react with the negative side of a magnet. An example of ionic bonding would be when Na⁺ and Cl^- interact with each other to produce NaCl. (Notice that there is not charge on the final product. This is because the negative charge balances out the positive charge to leave a charge of zero.)

The Periodic Table

The periodic table is a table that shows a list of all the elements, their weights, and the number of protons of each element in an organized manner. The periodic table is used to show trends, similarities, and differences of elements. The periodic table is exactly what its name is, a table, so it has columns and rows. The rows are called periods. The columns are called families or groups. Each group (column) of elements has similar chemical properties. Later in this On-Line Text Book, more in depth information will be presented regarding the periodic table, but for now as you learn about different elements through the examples, make a mental note of what position that element is in on the periodic table.