CH-121 Chapter 4
Steps for Making a Lewis Structure
1) Calculate total valence electrons (if it's a cation, subtract electrons, if it's an anion, add electrons). 2) Identify the least electronegative atom and put that in the middle and others on the outside (H should never be in the middle) 3) Put one pair (single bond) from all peripheral atoms to the central atom. 4) Put all remaining valence electrons on the peripheral atoms as lone pairs (except H atoms - they should never have lone pairs or more than one bond). If there are leftover lone pairs, place them in the central atom. 5) Turn lone pairs into double or triple bonds to give octets to as many as possible to make sure all atoms that should have 8 electrons have 8 electrons. 6) If the structure is for an ion, enclose the structure in brackets with the charge of the ion in the upper right corner.
Octet Rule Exceptions
1) Odd-Electron Molecules The odd number immediately tells us that we have a free radical, so we know that not every atom can have eight electrons in its valence shell. Rearrange the electrons to make multiple bonds with the central atom in order to obtain octets wherever possible. 2) Electron-Deficient Molecules We will also encounter a few molecules that contain central atoms that do not have a filled valence shell. Generally, these are molecules with central atoms from groups 2 and 13, outer atoms that are hydrogen, or other atoms that do not form multiple bonds. 3) Hypervalent Molecules Molecules containing at least one main group element that has more than eight electrons in its valence shell. -Hydrogen wants 2 electrons. -Helium wants 2 electrons. -Lithium wants 2 electrons. -Beryllium wants 4 electrons. -Boron wants 6 electrons.
Bond Polarity
BOND TYPE / ELECTRONEGATIVITY DIFFERENCE Pure Covalent (Nonpolar) - less than 0.4 Polar Covalent - 0.4-1.8 Ionic - greater than 1.8
Binary Compound
Compound containing two different elements.
Binary Acid
Compound that contains hydrogen and one other element, bonded in a way that imparts acidic properties to the compound (ability to release H+ ions when dissolved in water).
Oxyacid
Compound that contains hydrogen, oxygen, and one other element, bonded in a way that imparts acidic properties to the compound (ability to release H+ ions when dissolved in water).
Covalent Bond vs. Ionic Bond
Covalent Bond - Attractive force between the nuclei of a molecule's atoms and pairs of electrons between the atoms. Ionic Bond - Electrostatic forces of attraction between the oppositely charged ions (cations and anions) of an ionic compound. There is a transfer of electrons. Usually happen between a metal and a nonmetal.
Covalent Compound vs Ionic Compound
Covalent Compound - (also, molecular compound) Composed of molecules formed by atoms of two or more different elements. Bond formed when electrons are shared between atoms. Usually happen between a nonmetal and a nonmetal. Covalent bonding can occur between two neutral atoms. Molecular compounds are composed of discrete molecules. Molecular compounds typically have low melting and boiling points because the attraction between molecules (which are electrically neutral) is weaker than that between electrically charged ions. Under normal conditions molecular compounds often exist as gases, liquids, or low-boiling solids. They do not conduct electricity or heat well. They are held together by covalent bonds. Ionic Compound - Compound composed of cations and anions combine in ratios, yielding an electrically neutral substance. A metal and a nonmetal usually form ionic compounds. Ionic compounds are electrically neutral because they consist of ions with opposite charges and equal magnitude. Regarding ionic compounds, the total positive charge equals the total negative charge. They are held together by ionic bonds. Molecular and ionic compounds are both neutral substances. The ions in an ionic compound balance the charge, whereas molecular compounds are simply uncharged. Neither type of compound conducts electricity in the solid phase. Ions typically exhibit dramatically different properties from those of the corresponding neutral compound. When naming binary covalent compounds, the element listed first will be the one that is more metallic.
Bond Length
Distance between the nuclei of two bonded atoms at which the lowest potential energy is achieved. When a covalent bond forms, each electron interacts with each nucleus, which brings the system to a lower energy. The ideal bond length is determined by the lowest energy that can be attained as the atoms approach each other. In a polar covalent bond, the atom that attracts the electrons less strongly has the partial positive charge. A polar covalent bond generates partial charges, and chlorine will have the electron excess in a molecule of HCl, thus it will be partially negative.
Polyatomic Ions to Memorize
ELEMENT // FORMULA // CHARGE Ammonium .... NH4 ............ +1 Acetate ............ C2H3O2 ... -1 Bicarbonate ... HCO3 ........ -1 Carbonate ...... CO3 ........... -2 Cyanide ............ CH .............. -1 Hydroxide ....... OH .............. -1 Nitrate ............... NO3 ........... -1 Phosphate ....... PO4 ............ -3 Sulfate ............... SO4 ............ -2
Metallic Bond
Electrons are no longer localized on a single atom. They are pooled together.
Octet Rule
Guideline that states main group atoms will form structures in which eight valence electrons interact with each nucleus, counting bonding electrons as interacting with both atoms connected by the bond. Because hydrogen only needs two electrons to fill its valence shell, it is an exception to the octet rule. The transition elements and inner transition elements also do not follow the octet rule.
Chemical Nomenclature
Ionic (forms one type of ion) - (name of cation/metal)(base name of anion/nonmetal + ide) Ionic (forms multiple ions - d block) - (name of cation/metal)(charge of cation in roman numerals)(base name of anion/nonmetal + ide) Molecular - (prefix)(name of first element)(prefix)(base name of second element + ide) An ionic bond occurs between a metal and a nonmetal. A molecular bond occurs between nonmetal and nonmetal.
How do the properties of ionic compounds shed some light on the nature of ionic bonds?
Ionic solids exhibit a crystalline structure and tend to be rigid and brittle; they also tend to have high melting and boiling points, which suggests that ionic bonds are very strong. Ionic solids are also poor conductors of electricity for the same reason. Most ionic solids, however, dissolve readily in water.
Lewis Structure and Lewis Symbol
Lewis Structure - Diagram showing lone pairs and bonding pairs of electrons in a molecule or an ion. Lewis Symbol - Symbol for an element or monatomic ion that uses a dot to represent each valence electron in the element or ion. Valence electronic structures can be visualized by drawing Lewis symbols (for atoms and monatomic ions) and Lewis structures (for molecules and polyatomic ions). Lone pairs, unpaired electrons, and single, double, or triple bonds are used to indicate where the valence electrons are located around each atom in a Lewis structure. Most structures - especially those containing second row elements - obey the octet rule, in which every atom (except H) is surrounded by eight electrons.
Free Radical
Molecule that contains an odd number of electrons.
Monatomic Ion, Polyatomic Ion, and Oxyanion
Monatomic Ion - Ion composed of a single atom. Polyatomic Ion - Ion composed of more than one atom. Oxyanion - Polyatomic anion composed of a central atom bonded to oxygen atoms.
Pure Covalent Bond vs. Polar Covalent Bond
Pure Covalent Bond - (also, nonpolar covalent bond) Covalent bond between atoms of identical electronegativities. Polar Covalent Bond - Covalent bond between atoms of different electronegativities; a covalent bond with a positive end and a negative end. In a polar covalent bond, the more electronegative atom will be partially negative.
How are simple cations named?
Simple cations (monatomic cations of elements of only one possible charge) are named with their elemental name followed by the word "ion." For instance, the Na+ is named the sodium ion. A Lewis symbol with no dots must be a cation.
Single Bond, Double Bond, and Triple Bond
Single Bond - Bond in which a single pair of electrons is shared between two atoms. Double Bond - Covalent bond in which two pairs of electrons are shared between two atoms. Triple Bond - Bond in which three pairs of electrons are shared between two atoms.
Electronegativity
Tendency of an atom to attract electrons in a chemical bond to itself. Electronegativity trend on the periodic table is up and right. What use electronegativity to know for certain what bond is formed, because sometimes it is not what you would expect.
Inert Pair Effect
Tendency of heavy atoms to form ions in which their valence s electrons are not lost. The noble gases are also called the inert gases because they are very unlikely to react and least likely to ionize. This is because they have a full valence shell of electrons, which means they do not want to gain or lose any electrons. The other groups listed all readily ionize.
Lone Pair
Two (a pair of) valence electrons that are not used to form a covalent bond.
When an element loses its valence electrons to form a cation, it assumes the electronic structure of what?
When an element loses its valence electrons to form a cation, it assumes the electronic structure of the preceding noble gas. To determine what charge an element will have, go back or forward to the closest noble gas. The electrons involved in chemical bonds are valence electrons. Atoms gain or lose electrons to form ions with particularly stable electron configurations. The charges of cations formed by the representative metals may be determined readily because, with few exceptions, the electronic structures of these ions have either a noble gas configuration or a completely filled electron shell. The charges of anions formed by the nonmetals may also be readily determined because these ions form when nonmetal atoms gain enough electrons to fill their valence shells.
