Ionic Bonding and Ionic Compounds Or How I Learned to Love Electrostatic Forces
Ionic Bonds The periodic table is based upon the properties of the elements. After the discovery of the electron, scientists found that all elements in the same group have the same number of valence electrons. Valence electrons are the electrons in the highest occupied energy level of an element’s atoms. The number of valence electrons determines the chemical properties of an element . The number of valence electrons is related to the group numbers in the periodic table.
The elements in Group 1A (hydrogen, lithium, etc.) all have one valence electron. H 1 s 1 Na 1 s 2 2 s 2 2 p 6 3 s 1 Li 1 s 2 2 s 1 K 1 s 2 2 s 2 2 p 6 3 s 2 3 p 6 4 s 1 Carbon and silicon, in Group 4A, have four valence electrons. C 1 s 2 2 s 2 2 p 2 Si 1 s 2 2 s 2 2 p 6 3 s 2 3 p 2 Groups 5A has five (5) valence electrons and Group 6A has six (6) valence electrons, etc.
Valence electrons are usually the only electrons used in the formation of chemical bonds. A shorthand way of showing valence electrons is through electron dot structures. Electron dot structures show valence electrons as dots. The inner electrons are represented by the symbol for the element being considered. P.188
Formation of Cations & Anions As we have talked about before, noble gases are quite unreactive. This is because they have filled outer energy levels. Atoms form ions & molecules for this reason, they want filled outer energy levels. As such, they follow a rule called the octet rule: The octet rule states that atoms in compounds tend to have the stable electron configuration of a noble gas. Na -- 1 s 2 2 s 2 2 p 6 3 s 1 Na· Na + -- 1 s 2 2 s 2 2 p 6 Na + octet
Metals lose electrons to obey this rule, and nonmetals share or gain electrons to obey this rule. Sodium is a metal and, as such, loses electrons to form a cation & have eight valence electrons. Nonmetals like chlorine gain electrons & form anions . Cl -- 1 s 2 2 s 2 2 p 6 3 s 2 3 p 5 This is the same electron configuration as argon. It is easier for nonmetals to gain electrons to become an anion to have a filled outer energy level than to lose electrons. Oxygen atoms have six valence electrons. By gaining two more they have the same electron configuration as neon. Cl - -- 1 s 2 2 s 2 2 p 6 3 s 2 3 p 6 Cl - octet
Ionic Bond Formation Anions and cations have opposite charges. They attract one another by electrostatic forces. The forces of attraction that bind oppositely charged ions together are called ionic bonds . Ionic compounds are also called salts. In any sample of an ionic compound, the positive charges of the cations must equal the negative charges of the anions. Sodium chloride is a good example of how ionic bonds are formed.
The chemical formula, NaCl, is a formula unit. It shows that one sodium ion combines with one chlorine ion. 1 s 2 2 s 2 2 p 6 3 s 1 1 s 2 2 s 2 2 p 6 3 s 2 3 p 5 Na + Cl - or NaCl Chemical formula Na·
Predict the formula when aluminum combines with bromine to make aluminum bromide. + Three bromine react with one aluminum ion. The formula is, therefore, AlBr 3 , and the name always ends in “-ide”: aluminum bromide (metal first, then nonmetal with -ide ending). Example: Predict the formula, and name, using electron dot structures for (a) potassium and oxygen (b) magnesium and nitrogen Al · · · Br : .. .. .
K + K + O 2- + + N 3- N 3- Website o ’understanding K· K· or K 2 O potassium oxide Mg • • Mg • • Mg • • Mg 2+ Mg 2+ Mg 2+ Mg 3 N 2 magnesium nitride
Polyatomic ions Now that we have studied some of the more simple ionic bonds let us review and study some of the more complex ionic elements. We have learned that the group “A” elements’ ionic charge is determined by its group # (i.e., Group 1A = +1; Group 2A = +2, etc.) On your paper, make a chart of all of the Group A elements and their ionic charges 1A 2A 3A (ignore 4A) 5A 6A 7A Li + Be 2+ Al 3+ etc. p.190
Unlike the cations of Group 1A, 2A, and 3A metals, many of the transition metals have more than one common ionic charge. P.R52
Notice that lead, Pb; Iron, Fe; Tin, Sn; Copper, Cu; and, Mercury, Hg, have more than one charge. Pb = +2 and +4 Tin = +2 and +4 etc. These are known as the “CMILT” ions. (C=copper, M=mercury, etc.) When you say the name of these ions, you say the element plus whatever is in parentheses (i.e., Pb 4+ = “lead four” ion) = lead (IV). Example: lead(II) oxide = PbO = Pb 2+ + O 2- All of the ions mentioned so far have been single atoms. Such ions are called monoatomic ions.
Ions made of two or more atoms are called polyatomic ions . Polyatomic ions are a tightly bound group of atoms that behave as a unit and carry a charge. Example: Most polyatomic ions end in “-ite” or “-ate”. “-ite” indicates one less oxygen atom than “-ate”. Charge is still the same for both, however. Some common polyatomic ions NO 3 - 1 = nitrate ion NO 2 - 1 = nitrite ion
Polyatomic Ion Formation In writing the formula of a compound that contains a polyatomic ion, if more than one of that ion is needed parentheses are written around it . Example: Determine the formula for calcium nitrate The () means 2 NO 3 -1 ’s are needed. Without (), it would look like “CaNO 32 ” or that there are 32 O’s. Calcium ion = Ca +2 Nitrate ion = NO 3 -1 Calcium nitrate = Ca(NO 3 ) 2
copper(I) nitrite tin(II) perchlorate Cu +1 NO 2 -1 Sn 2+ ClO 4 -1 CuNO 2 Sn(ClO 4 ) 2 Write the name for the following compounds: PbCl 2 Sn 3 N 4 Lead(II) chloride Tin(IV) Nitride
20. When an ionic bond is formed, the atom that transfers its valence electron becomes an ion with 1) positive charge and more protons. 2) positive charge and no change in the number of protons. 3) negative charge and more protons. 4) negative charge and no change in the number of protons. 22. What monoatomic ion has 26 protons, 30 neutrons and 24 electrons? 1) chromium (II) 2) copper (II) 3) iron (II) 4) zinc (II) 19. Which part of the atom is involved in chemical bonding? 1) only the nucleus. 2) only the protons. 3) only the electrons. 4) only the valence electrons.
Properties of Ionic Compounds Most ionic compounds are crystalline solids at room temperature. Other properties of ionic compounds are: 1. High melting point 2. Soluble (dissolves) in water 3. Well-defined crystals 4. Molten form conducts electricity
Each ion is strongly attracted to its neighbor and repulsions are minimized.
Metallic Bond Another bond that deals with cations but is no where similar to ionic bonds is the metallic bond. Metallic bonds consist of the attraction of the free-floating valence electrons for the positively charged metal ions. The electrons are often referred to as forming an electron sea.
Metals are thought to have electrons that are delocalized - they don’t belong to any one atom, but are free to move about. Another view
This model of metals explains several physical properties. Metals are good conductors of electrical current (flow of electrons) because as electrons enter one end of a bar of metal, an equal number leave the other end. Another physical property that has revolutionized the industrial world is the fact that metals are ductile. Being ductile means that it can be drawn into thin wires.