3.05 Ionic Bonding and Writing Formulas

Formula Unit

The formula an an ionic compound that represents the simplest whole number ratio of ions in the compound, not the actual number of each ion present in the crystal.

gaining electrons

The positive sodium ion and the negative chloride ion attract each other. This attraction is a type of chemical bond called an ionic bond. An ionic bond forms between ions with opposite charges. A compound that contains ionic bonds is called an ionic compound. In this example, the ionic compound sodium chloride (table salt) is formed.

losing electrons

When a sodium atom loses its outer electron, it becomes a sodium ion. The symbol for the sodium ion is Na+. Recall that an ion is a positively or negatively charged particle. Because it has lost an electron, the sodium ion has more protons than electrons. Therefore, it has a positive charge. It has lost only one electron, so it gas a charge of +1. The symbol Na+ shows this charge.

Examples

- what is the charge for K when it becomes an ion? A= + - what is the charge for Mg when it becomes an ion? A= 2+ - what is the charge for N when it becomes an ion? A= 3- - what is the charge for O when it becomes an ion? A= 2- - what is the charge for F when it becomes an ion? A= - - what is the formula unit for a compound made from Cl and Al? A= AlCl 3 - what is the formula unit for a compound made from Pb4+ and O? A= PbO2 - what is the formula unit when calcium bonds with nitrogen? A= Ca 3 N 2

ionic compounds

-As you know, opposite charges attract. The attraction between positive and negative ions is called an ionic bond. Even though ions are charged, they combine in ratios that cancel those changes and make the ionic compound neutral. When you write the formula of an ionic compound, subscripts are used to represent the ratio of positive and negative ions in the ionic compound. -Ionic Compound: A compound composed of positive and negative ions that are combined in a ratio where the positive and negative charges cancel out.

Using the periodic table

-Elements in the same group of the periodic table have the same number of electrons in their valence energy levels, so they tend to have the same charge when they form naturally occurring ions. Just as the periodic table was your guide for electron configurations and valence electrons, it also helps you determine the ionic charges of the main group elements. -You may have noticed that the elements in groups 3 through 12 do not have common charges listed on the periodic table. These transition metals tend to form positive ions, but the number of electrons lost varies. For example, iron can form an ion with either a +2 or +3 charge, depending on the conditions.

naturally occurring ions

-atoms are neutral when the number of protons and electrons are equal and their charges cancel. -In an atom that loses electrons, protons outnumber the electrons and the atom becomes a positively charged ion. -If the atom gains electrons, the electons outnumber the protons and form a negatively charged ion. -We write the charge as a superscript to the right of the atomic symbol, showing the magnitiude and sign of the charge. -notice that the number is written before the sign of the charge, and if the number is a one (1), it is not written. -atoms gain or lose electrons unitl they have a full valence energy level. If an atom only has a few electrons in its outermost energy level, it tends to lose those electrons so that the energy level underneath, which is already full, becomes the outermost valence. -if the outermost energy level of an atom is almost full, that atom can attract electrons from other atoms until the outer valence is full.

Steps 1-5 ^

1) Identify the charge of each of the ions in the compound. Remember that metals tend to lose all of the electrons in their outermost energy level and nonmetals gain electrons to fill their outermost energy level. 2) The positive ion is always written first, followed by the negative ion. This means that in most ionic compounds the metal ion is written on the left and the nonmetal ion on the right (corresponding to their locations of the periodic table). 3) Determine the ratio of positive to negative ions needed to make the compound neutral. Use the correct combination of ions to produce a neutral compound with a net charge of zero. One way to do this is to use a trick called "crossing the charges." This means that whatever the charge of the cation, that number (not the sign) becomes the subscript of the anion and vice versa. 4) Make sure that the subscripts represent the lowest whole number ratio for the ionic compound. The formula of an ionic compound does not represent an individual unit that can be isolated or exist individually. Instead, it represents a ratio of how the positive ions and negative ions combine in an ionic crystal. The formula of an ionic compound is called a formula unit, which represents that simplest whole number ratio of positive to negative ions in the ionic compound. 5) Check your work: make sure that the net charge of the formula is zero. The charge of each ion is multiplied by its subscript to give you the total charge from that type of ion. Adding the overall charge from both types of ions should add up to a total of zero.