CHEMICAL BONDS IONIC BONDS 2.1 The Ions That Elements Form 2.3 The Energetics of Ionic Bond Formation 2.4 Interactions Between Ions COVALENT BONDS 2.6 Lewis Structures of Polyatomic Species 2.8 Formal Charge General Chemistry I 1 Key Ideas and Definitions Bond formation is accompanied by a lowering of energy: it is a favorable process. Formation of chemical bonds results in ionic lattices, molecules,and metallic lattices. It is fundamental to the production of compounds and so is central to the science of chemistry. Energy lowering is due to attractions between oppositely charged ions (ionic bonding), or between nuclei and shared electron pairs (covalent bonding).
➢ Chemical bond is the link between atoms. Na+, Cl- - covalent bond i. NH3 - metallic bond i. Cu General Chemistry I 2 IONIC BONDS (Sections 2.4) ➢ Ionic model: the description of bonding in terms of ions An ionic solid is an assembly of cations and anions stacked together in a regular pattern, called a crystal lattice.1 The Ions That Elements Form ➢ Cations are formed by removal of outermost electrons in the order np, ns, (n-1)d.
➢ Main-group metal atoms lose their valence s- and/or p- electrons and acquire the electron configuration of the preceding noble gas atom. General Chemistry I 3 ➢ Anions: Add electrons until the next noble-gas configuration is reached General Chemistry I 4 Self-Tests 2.1B Write the electron configurations of (a) the Manganese (II) ion and (b) the lead (IV) ion.2B Write the chemical formula and electron configuration of the iodide ion. Solution: I needs to gain only one electron to achieve Xe electron structure, hence I- and [Xe] or [Kr] 4d10 4s2 4p6 General Chemistry I 5 2.2 Lewis Symbols Atoms and ions are conveniently represented by Lewis dot symbols, showing the element symbol, the valence electrons and charge, if any. Valence electrons – depicted as dots; a pair of dots for paired electrons.
- Atoms - Cations and anions Here, we can use Lewis dot symbols to show electron transfer in the formation of cations and anions. General Chemistry I 6 Gilbert N Lewis General Chemistry I 7 2.3 The Energetics of Ionic Bond Formation The formation of a typical ionic compound, such as sodium chloride can be broken down into three simple steps: 1. Ionization of gaseous metallic atom to give the cation. Formation of gaseous non-metallic anion.
Condensation of the gaseous ions into a crystal lattice. The energies involved in these processes illustrate the favorability of ionic compound formation (see Fig.4, next slide): Na(g) → Na+(g) + e- (g) 494 kJ·mol-1 Cl(g) + e- (g) → Cl-(g) -349 kJ·mol-1 Na+(g) + Cl- (g) → NaCl(s) -787 kJ·mol-1 Na(g) + Cl(g) → NaCl(s) -642 kJ·mol-1 General Chemistry I 8 Energetics of ionic compound formation. The difference in energy between the ions in the lattice and separated gaseous ions is called the lattice energy.4 Interactions Between Ions - In an ionic solid, each cation is attracted to all the anions to a greater or lesser extent. This is a “global” characteristic of the entire crystal ➢ Lattice energy: the difference in energy between the ions packed together in a solid and the ions widely separated as a gas - Strong electrostatic interactions in ionic solids → high melting points and brittleness General Chemistry I 10 - Coulomb potential energy of the interactions of two individual ions: Here, e is the fundamental charge; z1 and z2 are the charge numbers of the two ions; r12 is the distance between the centers of the ions; e0 is the vacuum permittivity constant.
- Molar potential energy of a three-dimensional crystal: d = distance between centers of neighboring ions (rcation + ranion) NA is the Avogadro number ➢ The factor A is the Madelung constant, dependent on how the ions are arranged about one another in the 3-dimensional lattice. General Chemistry I 11 One-Dimensional Crystal Model - In a one-dimensional crystal in which cations and anions alternate along a line: General Chemistry I 12 This implies Ep is most favorable for small ions with large charges. Also for a one dimensional model crystal, the Madelung constant A is 2 ln2 = 1. This compares well with values of A for real ionic crystals (Table 2.
For multi-charged ions (z1 and z2), z2 is replaced by the absolute value of z1z2 (its value without the negative sign). General Chemistry I 13 Self-Test 2.3A The ionic solids KCl and CaO crystallize to form structures of the same type. In which compound are the interactions between the ions stronger? Solution: The ions in CaO are both smaller and more highly charged, hence CaO has the stronger interactions. General Chemistry I 14 Attraction, Repulsion and the Born-Mayer Equation The previous discussion does not take into account cation-anion repulsion – the real potential energy of an ionic solid is a balance between attractive and repulsive interionic interactions.
If a cation and anion are brought together (see Fig.7, opposite), potential energy decreases to a minimum value. Further decrease in d, leads to serious unfavorable repulsive interaction. General Chemistry I 15 ➢ Born-Mayer equation with d* = 34.5 pm repulsive effect This equation allows for repulsive interactions at small values of d: EP,min increases (becomes less favorable) when d approaches d* and is actually positive when d* > d. General Chemistry I 16 COVALENT BONDS (Sections 2.5 Lewis Structures Some Definitions According to Lewis Theory ➢ Covalent bond - a pair of electrons shared between two atoms - Octet rule: in covalent bond formation, atoms go as far as possible toward completing their octets by sharing electron pairs.
- Valence of an atom is the number of bonds it can form. - A line (-) represents a shared pair of electrons. - Lone pairs of electrons – electron pairs not involved in bonding. - Lewis structure – atoms are indicated by chemical symbols, covalent bonds by lines, and lone pairs by pairs of dots.
General Chemistry I 17 Self-Test 2.4A Write the Lewis structure for the “interhalogen” compound chlorine monofluoride, ClF, and state how many lone pairs each atom possesses in the compound. Cl: or : F Cl:. Three lone pairs on both atoms General Chemistry I 18 2.6 Lewis Structures of Polyatomic Species – A Lewis structure does not portray the 3D shape of a molecule or ion, but simply displays which atoms are bonded together. - - bond order: the number of bonds that link a specific pair of atoms.
General Chemistry I 19 ➢ Writing a Lewis Structure 1. Count the total number of valence electrons, from the group numbers E. CO2 4 + 2 x 6 = 16 (C in group IVA; O in group VIA) NOTE: if an anion (-), add the value of the charge; if a cation (+), subtract that value. Calculate the total number of electrons that are needed if each atom had its own noble gas shell of electrons (2 for H and 8 for all others).
for CO2 there are 3 atoms (no H) and hence 24 noble gas shell electrons. Subtract the number in 1 from the number in 2: this gives the number of shared (bonding) electrons present (and number of bonds = 1/2 that number). Add electron pairs to “complete the octets”, as necessary. Represent each bond by a line.
for CO2, the figure is 24 – 16 = 8 (4 : O : C : O: bonds: two “double bonds”) Hence the Lewis diagram is or. :O C O: The above works well only for molecules that obey the octet rule. For certain molecules (like BF3, radicals, and high valence compounds like SF6), this rule is ignored. General Chemistry I 21 ➢ Writing a Lewis Structure: Rules of Thumb - Terminal atom: bonded to only one other atom - Central atom: bonded to at least two other atoms – Usually, element with lowest I1 or lowest electronegativity is central atom., in HCN, carbon has lowest I1 and is least electronegative: hence it is the central atom.
– Usually, there is symmetrical arrangement around central atom., in SO2, OSO is symmetrical, with S central and O terminal. – Oxoacids have H atoms mostly bonded to O atoms., H2SO4 is actually (HO)2SO2, with two O atoms and two OH groups bonded to S. – For organic compounds, the atoms are arranged into groups, as suggested by the standard molecular formula., CH3COOH = one CH3 group and one COOH group. General Chemistry I 22 Examples.
Phosphorous acid, Acetone, (CH3)2CO P(OH) 3. : H N H H H Ammonium carbonate, (NH4)2CO 3 General Chemistry I 23 Self-Test 2.5A Write a Lewis structure for the cyanate ion, NCO- (sometime written CNO-). General Chemistry I Self-Test 2.6A Write a Lewis structure for the urea molecule, (NH2)2CO Solution H .7 Resonance – Multiple Lewis structures: many compounds can be represented by different Lewis structures in which the location of electrons (but not nuclei) differ. They are known as resonance structures, each making a contribution to the real structure of the molecule (called a “resonance hybrid”).
– The resonance symbol is a double-headed arrow (↔), indicating a blend of the contributing structures: – Resonance implies delocalization: in which a shared electron pair is distributed over several pairs of atoms and cannot be identified with just one pair of atoms. General Chemistry I 26 ➢ Benzene, C6H6 - No reactions typical of compounds with double bonds - All the carbon-carbon bonds with the same length - Only one 1,2-dichlorobenzene exists. General Chemistry I 27 Self-Test 2.7B Write Lewis structures contributing to the resonance _ hybrid for the nitrite ion, NO2 Solution _. The resonance hybrid is O N O _ General Chemistry I 28 Additional Self-Test Write Lewis (resonance structures) for the ozone molecule (O3).
Comment on the predicted bond lengths. :O O O : :O O O : (Without formal charges: see 2. Suggests bonds are equal in length: O O O r(O=O) < r(O O) < r(O O) Resonance hybrid General Chemistry I 29 2.8 Formal Charge ➢ Formal charge – the charge an atom would have if the bonding were perfectly covalent in the sense that the atom had exactly a half- share of the bonding electrons. V = the number of valence electrons in the free atom L = the number of electrons present on the bonded atom as lone pairs B = the number of bonding electrons on the atom - A Lewis structure in which the formal charges of the individual atoms are closest to zero typically represents the lowest energy arrangement of the atoms and electrons.
General Chemistry I 30 – Formal charge exaggerates the covalent character of bonds by assuming that the electrons are shared equally. – Oxidation number (state) exaggerates the ionic character of bonds. It represents the atoms as ions, and all the electrons in a bond are assigned to the atom with the lower ionization energy. formal charge oxidation state General Chemistry I 31 Self-Test 2.8B Suggest a likely structure for the oxygen difluoride molecule.
Write its Lewis structure and formal charges. : General Chemistry I 32 Chapter 2. CHEMICAL BONDS EXCEPTIONS TO THE OCTET RULE 2.9 Radicals and Biradicals 2.10 Expanded Valence Shells 2.11 The Unusual Structures of Some Group 13/III Compounds IONIC VERSUS COVALENT BONDS 2.12 Correcting the Covalent Model: Electronegativity 2.13 Correcting the Ionic Model: Polarizability THE STRENGTH AND LENGTHS OF COVALENT BONDS 2.15 Variation in Bond Strength 2.16 Bond Lengths General 2012 General Chemistry Chemistry I I 33 2.9 Radicals and Biradicals ➢ There are three types of molecules for which the octet rule of Lewis has to be dropped: 1. Odd-electron molecules (radicals) 2.
High valence molecules (hypervalent compounds) (See 2. Low valence molecules (especially of group IIA and IIIA elements) (See 2.11) General Chemistry I 34 ➢ Radicals are species with at least one unpaired electron. They are often highly reactive. :N O: Biradicals: molecules with two unpaired electrons General Chemistry I Self-Test 2.