Chemistry March 12, 2012 -Polyatomic ions are groups of atoms that have gained a charge. Typically include a nonmetal. Naming Polyatomic Ions .The base name for the polyatomic ions is based on the element other than oxygen .The names of the most common polyatomic ions end in â€“ate .The name of a related ion with one less oxygen uses the â€“ite ending instead of â€“ate .Recognizing these ending will help you identify polyatomic ions in the name of compound Polyatomic ions .There are exceptions to this naming pattern - OH- hydroxide ion - CN- cyanide ion .There is no easy way to learn these .You will have to just memorize them Writing formula for polyatomic ions .These polyatomic ions do not exist by itself .Must be associated with ions of opposite charge .To write the correct formula follow the same charge balancing methods used for single atom ions Writing formula .Parentheses are used to indicate that more than one polyatomic ion is needed to balance the charge .The number of polyatomic ions is written in the subscript outside the parentheses .Ex: magnesium nitrate -Mg2+ and NO3-Two nitrates are needed to balance the charges -Mg(NO3)2 Practice .Aluminum carbonate .Ammonium sulfate
The formula for calcium phosphate is: Ca3(PO4)2 The chemical formula for iron (II) nitrate is: Fe(NO3)2 Naming compounds .Write the positive ion first .Then write the name of the polyatomic ion .It is important that the polyatomic ion is recognized when writing the formula Practice .Cu(NO2)2 â€“ copper (II) nitrite .AlPO4- aluminum phosphate The name of CaSO4 is: A. Calcium sulfate The name of NaHCO3 is: C. Sodium bicarbonate Covalent Bonds .Atoms of two nonmetal elements are joined together in covalent compounds .Held together by sharing electrons .In most covalent compounds each of the atoms fills its octet of 8 electrons -Ex. F2 Diatomic Molecules .There are seven atoms that form diatomic molecules -H, N, O, F, Cl, Br, I Molecule with more than one element .The number of electrons that an element shares = the number of covalent bonds an atom = the number of electrons needed to acquire noble gas arrangement
Multiple covalent bonds .Covalent compounds are capable of sharing 2 or 3 pairs of electrons -C, O, N and S are the most likely to form double or triple bonds -Ex: CO2 -Ex: N2 Naming covalent compounds .The first nonmetal is named by its elemental name .The second nonmetal is named by its elemental name with the ending changed to – ide .Prefixes are used to designate the number of atoms of each element in a compound .The prefix mono- is omitted from the first element .If two vowels (a & o) are back to back the first vowel is omitted Table 3.4 page 91 Prefixes Used to Denote Numbers of Atoms in a Compound Practice .SF6 – sulfur hexafluoride .N2O4 – dinitrogen tetroxide What is the correct name for P4O10? D. Tetraphosphorus decoxide What is the correct name for IF5? B. Iodine Pentafluoride Covalent compounds .Writing the chemical formulas -Write the symbol for each element as listed in the name -Use subscripts to show how many of each are present Bond Polarity .Bonds do not always involve equal sharing of the electrons .The electron will be pulled more by one of the elements than the other .Electronegativity – ability of an atom to attract electrons in a bond Page 86 Figure 3.5 Electronegativities of the elements
Decreases in electronegativity as we go down a group. Increases across a period. Bond Polarity .In covalent bonds the electrons can be shared either equally or unequally -Depends on the electronegativity of the atoms in the bonds Bond where unequal sharing of electrons is a polar covalent bond. Equal sharing of electrons in a nonpolar covalent bond. Properties of compounds .Physical state of compounds .Melting point -Temperature where solid liquid (always higher in ionic compounds) .Boiling point -Temperature where liquid gas .Structure of solids .Solutions Ionic compounds are always going to be solids, covalent compounds can be a solid, liquid, or gas. Lewis Structures of Molecules .Write skeletal structure with chemical symbols -Least EN element at center .Determine number of valence electrons in each atom .Connect central atom to surrounding atoms .Ensure octet rule is satisfied for each atom .Perform a final electron count -Same rules for polyatomic ions -Hydrogen is always on the outside. Practice .Draw a Lewis structure for each of the covalent compounds -N2 -C2H6 -NH4+ -CH3COOLewis structures and stability .H2, O2, and N2 – All covalent molecules
-Stability and reactivity quite different -Bonding of the molecules .Bond energy â€“ amount of energy required to break a bond .Bond length â€“ distance between two nuclei The shorter the bond length, the stronger the bond/more energy required to break the bond. Exceptions to octet rule .Only with nonmetal atoms .Helium like elements .Expanded Octets (happens with elements in period 3 and below) .Molecules with odd number of valence electrons Shapes of molecules .Valence-shell electron-pair repulsion (VSEPR) theory -Uses the geometry of electron groups around a central atom to determine the shape of the molecule -According to VSEPR the electron pairs stay as far apart as possible Determining shape of molecule .First write the electron dot structure .Second -Determine the number of electron pairs in the molecule bonded to atoms -Determine the number of nonbonded electron pairs around the central atom -An electron group is a pair of electrons where they are in a bonding pair or a nonbonded pair *Page 109 Table 3.5 Molecular Structure: The Geometry of a Molecule Is Affected by the Number of Nonbonded Electron Pairs Around the Central Atom and the Number of Bonded Atoms *Should know for the test. Do not need to know the bond angles. Know bonded and nonbonded pairs. Rules for Using VSEPR 1. 2. 3. 4. 5.
Write the electron dot structure Count the electron groups around the central atom Use VSEPR to determine the arrangement of the electron groups Count the number of atoms to bonded to the central atom Use the bonded atoms to identify the shape of the molecule
An electron pair can be an electron group, meaning you can treat double and triple bonds the same as an electron group. Polarity of Molecules .Covalent bond can be polar or nonpolar .Molecules can be polar or nonpolar depending on the bonding structure .Molecules can be nonpolar even if they do contain more than one polar bond -If the bonds are symmetrical they are nonpolar Nonpolar bonds always produce nonpolar molecules. .The atom with the greater electronegativity is more negative and the other atom is more positive .The separation of charge is called the dipole -Represented by the symbols (Î´+ & Î´-) or by the symbol ďƒ Polar Molecules .In these molecules one end is more negatively charged than the other end .Polarity occurs when the polar bonds do not cancel out .The cancellation process depends on the type of atoms, the electron pairs around the central atom and the shape of the molecule .Ex. HCl is polar because the electrons are shared unequally in a polar covalent bond A nonpolar molecule always has a central atom bonded to the same element bonded at every position and no nonbonded pairs of electrons at the central atom. .In polar molecules with three or more atoms the shape of the molecule determines whether the dipoles cancel or not -Often there are lone electron pairs around the central atom -If the dipoles do not cancel the molecule becomes more positive at one end and more negative at the other end -This gives the molecule a dipole Polar molecules have a central atom that have different atoms bonded to it. Nonpolar molecules have a central atom where dipoles cancel each other out. Nonpolar molecules .When the polar bonds or dipoles cancel each other the molecule is nonpolar .CO2 and CCl4 contain polar bonds but the symmetry of the bonds cancels the dipole Properties of molecules
.Intramolecular forces â€“ force within the molecules .Intermolecular forces â€“ forces between the molecules -These determine the macroscopic properties Solubility .Like dissolves like -Polar solutes dissolve in polar solvents -Nonpolar solutes dissolve in nonpolar solvents Boiling and melting points .Energy is required for both of these processes -Amount of energy directly related to intermolecular forces and mass Page 115. Table 3.6 Melting and Boiling Points of Selected Compounds in Relation to Their Bonding Type