Chapter 5 Ionic and Covalent Compounds I II A 1 2 3 4 Compounds are substances composed of two or more elements combined in a specific ratio and held together by chemical bonds For example water is always two atoms of hydrogen combined with one atom of oxygen If this ratio is changed it is no longer water Lewis Dot Symbols About Lewis Dot Symbols As was discussed in the previous chapter atoms gain or lose electrons to acquire the configuration of a noble gas because that renders the ion stable Na Ne 3s1 Na Ne Cl Ne 3s23p5 Cl Ar Mg Ne 3s2 Mg2 Ne S Ne 3s23p4 S2 Ar Al Ne 3s23p1 Al3 Ne P Ne 3s23p3 P3 Ar When atoms interact to form compounds only their valence electrons are involved in bond formation Lewis dot symbols consist of the element s symbol surrounded by as many dots as there are valence electrons For the main group elements the number of valence electrons is the same as the group number Na Mg Al C Si P S Cl Ar Refer to the Figure below to determine how to distribute dots around a symbol 1 5 6 7 Monotomic ions can be represented by Lewis dot symbols by adding electrons to form anions or taking away electrons to form cations The number of bonds nonmetals of the second period can form is equal to the number of unpaired electrons in their Lewis symbol B 3 C 4 N 3 O 2 F 1 The transition metals the actinides and lanthanides have incompletely filled inner shells and simple Lewis dot symbols cannot be written for these elements B Practice Exercises 1 Write the Lewis dot symbol for K Ba In Sn Sb Se Br Xe 2 Write the Lewis dot symbol for Na Mg2 Al3 N3 S2 I 3 If the Lewis dot symbol of an element has five dots which of the following element is it C Ga Ca P Cl Se 4 How many bonds can each of the following atoms form S P Cl N 2 III Ionic Compounds and Bonding General Information A 1 2 3 B 1 Compounds can be divided into two broad categories ionic and covalent Elements with low ionization energies metals tend to form cations and those with high electron affinities nonmetals form anions When anions and cations combine by electrostatic attraction a neutral species an ionic compound forms Ionic Bonding Atoms having 1 2 or 3 valence electrons tend to lose these electrons to form cations with the electron configuration of the previous noble gas 2 Atoms having 5 6 or 7 valence electrons tend to acquire 1 2 or 3 electrons to form anions with the electron configuration of the following noble gas Na Ne 3s1 Mg Ne 3s2 Al Ne 3s23p1 Cl Ne 3s23p5 S Ne 3s23p4 P Ne 3s23p3 Na Ne Mg2 Ne Al3 Ne Cl Ar S2 Ar P3 Ar 3 In ionic bond formation the atom with few valence electrons the metal atom donates electrons to the atom with larger number of valence electrons the nonmetal atom forming a cation and an anion respectively The electrostatic attraction between these ions constitutes the ionic bond 3 C 1 2 3 4 a Lattice energy Elements that are most likely to form ionic compounds can be predicted from their position in the periodic table Elements from the two extremes of the periodic table form ionic compounds Ionic compounds are solids at room temperature and their stability depends not only on the environment of anions and cations but also on the three dimensional arrangement of these ions known as the crystal lattice Lattice energy is the amount of energy required to convert 1 mol of ionic solid into its constituent ions in the gas phase NaCl s Na g Cl g 788 kJ mol Lattice energy depends on the magnitude of the charges and on the distance between the ions Effect of distance between ions if charges are the same LiCl NaCl and KCl have the same anion Cl so the radius of the anion is the same but the radius of the cation increases from Li to Na to K therefore the lattice energy decreases as the distance between the nuclei of these ions increases LiCl NaCl KCl 834 kJ 787 kJ 701 kJ b Effect of charges if distances between ions are about the same The internuclear distance between LiF MgO and ScN are about the same however the charges of the ions increase from 1 for Li to 3 for Sc The higher charges result in stronger attraction between ions and the higher the charge the larger the lattice energy LiF 1017 kJ mol MgO 3890 kJ mol ScN 7547 kJ mol 5 Assuming similar distances between ions which compound has the larger lattice energy MgCl2 or ScCl3 4 IV Naming Ions and Ionic Compounds Naming Ions The name of a cation is the same as the name of the element Na sodium Al aluminum Na sodium ion Al3 aluminum ion Because transition metals can have more than one oxidation state the charge of the ion must be specified Fe2 Fe3 iron II ion iron III ion Cu Cu2 copper I ion copper II ion 3 To name anions take the stem of the name and add the ending ide The number of positive charges of the cation must equal the number of negative charges of the anion in order to form a neutral compound A 1 2 B 1 2 F S2 fluoride ion sulfide ion O2 N3 oxide ion nitride ion Formulas of Ionic Compounds Examples a b c d Na Na Na Mg Mg Mg Al Al Cl O N N O O Na Cl Na O Mg N Al O 5 C Naming Ionic Compounds O Name the cation first 1 2 3 4 5 6 Name the anion according to the rules discussed above MgCl2 magnesium chloride Al2O3 aluminum oxide Ca3N2 calcium nitride If the cation has variable oxidation states indicate the oxidation number with Roman numerals enclosed in parentheses FeCl2 iron II chloride FeCl3 iron III chloride MnO manganese II oxide MnO2 manganese IV oxide The oxidation states of most of the common monatomic ions can be determined by their position in the Periodic Table Table 5 2 page 147 lists some of the common monatomic ions Examples of ionic compounds PbBr2 CaCl2 lead II bromide Na2O sodium oxide calcium chloride Note that the numbers of atoms in ionic compounds are not indicated in the name These numbers are determined by the charges of the ions in order to form a neutral species 7 Name the following ionic compounds a b c d e FeO NaH Li3N MnO2 Na2S 6 V A 1 2 3 B 1 2 3 4 5 6 Covalent Bonding and Molecules General Information Metals and nonmetals form ionic compounds by the transfer of electrons Elements of similar properties form compounds by sharing electrons in order for each atom to have an octet of electrons The shared electron pair is known as a covalent bond and the resulting species is called a molecule Cl Cl Cl2 …