Ionic Compounds Atoms are rarely found in nature in their pure state. Most often they are combined with other elements in compounds. Two major types of compounds we will study are molecular and ionic compounds. These two major groups of compounds are different by the way they combine. Metals and nonmetals combine to form binary ionic compounds. Nonmetals combine with other nonmetals to form compounds and polyatomic ions.

Forming Ions Ions are elements which have gained or lost one or more electrons. Elements gain or lose electrons to achieve greater stability. Many atoms gain or lose electrons to become isoelectronic with the noble gases. Electrons are gained or lost from the outermost electron shell. A general rule is “Last one in is the first one out”.

Forming Ions If atoms lose electrons they become positively charged because they have more protons than electrons. These ions are called Cations. The metals of the periodic table form cations. Lithium loses its outer electron and becomes isoelectronic with the noble gas helium. The lithium ion is then written as Li + An ion and its element have different physical and chemical properties and is named different.

Forming Ions If atoms gains electrons they become negatively charged because they have more electrons than protons. These ions are called Anions. The nonmetals of the periodic table form anions. Fluorine gains an outer electron and becomes isoelectronic with the noble gas Neon. The fluorine ion is then written as F - An ion and its element have different physical and chemical properties and is named different.

Forming Ions Many metals and nonmetals form predictable ions. All alkali metals, with hydrogen, form +1 cations. All alkaline earth metals form +2 cations. Aluminum forms a +3 cation, Al 3+. All halogens form -1 anions. Column 16 nonmetals form -2 anions. Column 15 nonmetals form -3 anions. Carbon does not form an ion, nor do the metalloids with the exception of arsenic, As 3-.

5.1 Ions Transition and post transition metals form cations that are not as predictable as column I & II elements. Transition metals may form more than one type of cation. Most transition metals form ions to achieve stability in some other way than being isoelectronic with the noble gases.

Losing electrons is also called oxidation. The chart below shows the possible oxidation states of the transition metals. Notice that most of them form multiple ions.

Forming Ionic Compounds Oppositely charged objects attract. Cations (+) and anions (-) attract to each other to form ionic compounds. Compounds form in a ratio to charge balance –The sum of the positive and negative charges must equal zero.

Ionic Compounds Na forms cation Na+ Cl forms anion Cl- They form a compound in a 1:1 ratio NaCl – sodium chloride –Named by adding the suffix –ide to the end of the anion.

Ionic Compounds Calcium forms cation Ca 2+ Bromine forms anion Br - In order to charge balance, 2 bromide ions are needed for every calcium ion. CaBr 2 Calcium Bromide –In ionic formulas, Cations are always listed first.

Ionic Compounds Aluminum forms cation Al 3+ Oxygen forms an anion O 2- In order to charge balance, you must find a common multiple of ions. 2 Al 3+ is a charge of 6+ and 3 O 2- is a charge of 6-. Al 2 O 3 is Aluminum Oxide –In ionic formulas, the lowest ration of ions is always listed.

Ions – Polyatomic ions Molecular groups can also be charged upon forming. These groups of many atoms (poly) are called polyatomic ions Polyatomic cations are positively charged NH 4 + Polyatomic anions are negatively charged NO 3 - Polyatomic ions combine with monatomic ions or other polyatomic ions to form ionic compounds.

Table 4.4 in your book lists more common polyatomic ions