Chapter 11 Chemical Bonds: The Formation of Compounds from Atoms Advanced Chemistry Periodic Trends in Atomic Properties Learning Objective

Transcription

1 Chapter 11 Chemical Bonds: The Formation of Compounds from Atoms Advanced Chemistry 11.1 Periodic Trends in Atomic Properties Discuss the atomic trends Metals are located on the left side of the periodic table, nonmetals for metals and nonmetals, on the right. atomic radius, and ionization energy. Atomic radius increases down a group and decreases across a period (from left to right).

2 Ionization energy decreases down a group and increases across a period (from left to right). Key Term Ionization energy The energy required to remove an electron from an atom, an ion, or a molecule. A + ionization energy à A + + e Lewis Structures of Atoms Draw the Lewis structure To determine a Lewis structure for representative elements, use for a given atom the group number as the number of electrons to place around the symbol for the element. Key Term Lewis structure A method of indicating the covalent bonds between atoms in a molecule or an ion such that a pair of electrons (:) represents the valence electrons forming the covalent bond.

3 11.3 The Ionic Bond: Transfer of Electrons From One Atom to Another Discuss the formation of The goal of bonding is to achieve stability. For representative an ionic bond and the elements, this stability can be achieved by attaining a valence chemical change that electron structure of a noble gas. results from the bond. In an ionic bond stability is attained by transferring an electron from one atom to another: o The atom that loses an electron becomes a cation. Positive ions are smaller than their parent atoms. Metals tend to form cations. o The atom gaining an electron becomes an anion. Negative ions are larger than their parent atoms. Nonmetals tend to form anions. o Ionic compounds do not exist as molecules. Ions are attracted by multiple ions of the opposite charge to form a crystalline structure. Key Term Ionic bond The chemical bond between a positively charged ion and a negatively charged ion.

4 11.4 Predicting Formulas of Ionic Compounds Predict the formulas of Chemical compounds are always electrically neutral. ionic compounds from Metals lose electrons and nonmetals gain electrons to form their position on the compounds. periodic table. Stability is achieved (for representative elements) by attaining a noble gas electron configuration The Covalent Bond: Sharing Electrons Draw the electron structure of a covalent bond. Key Terms Covalent bond Polar covalent bond Covalent bonds are formed when two atoms share a pair of electrons between them. This is the predominate type of bonding in compounds. True molecules exist in covalent compounds. Overlap of orbitals forms a covalent bond. Unequal sharing of electrons results in a polar covalent bond. A chemical bond formed between two atoms by sharing a pair of electrons. A covalent bond between two atoms with differing electronegativity values, resulting in unequal sharing of bonding electrons.

5 11.6 Electronegativity Explain how electronegativities of component atoms in a molecule determine the polarity of the molecule. Electrons spend more time closer to the more electronegative atom in a bond forming a polar bond. The polarity of a bond is determined by the electronegativity difference between the atoms involved in the bond. o The greater the difference, the more polar the bond is. o At the extremes, large differences result in ionic bonds and tiny differences (or no difference) result(s) in a nonpolar covalent bond. o A molecule that is electrically asymmetrical has a dipole, resulting in charged areas within the molecules. o If the electronegativity difference between two bonded atoms is greater than , then the bond will be more ionic than covalent. o Polar bonds do not always result in polar molecules.

6 Key Terms Electronegativity The relative attraction that an atom has for a pair of shared electrons in a covalent bond. Nonpolar covalent bond Dipole A covalent bond between two atoms with the same electronegativity value; thus, the electrons are shared equally between the two atoms. A molecule that is electrically asymmetrical, causing it to be oppositely charged at two points. For example:

7 11.7 Lewis Structures of Compounds Draw the Lewis structure PROBLEM-SOLVING STRATEGY: Writing a Lewis Structure of a covalent compound. 1. Obtain the total number of valence electrons to be used in the structure by adding the number of valence electrons in all the atoms in the molecule or ion. If you are writing the structure of an ion, add one electron for each negative charge or subtract one electron for each positive charge on the ion. 2. Write the skeletal arrangement of the atoms and connect hem with a single covalent bond (two dots or one dash). Hydrogen, which contains only one bonding electron, can form only one covalent bond. Oxygen atoms are not normally bonded to each other, except in compounds know to be peroxides. Oxygen atoms normally have a maximum of two covalent bonds (two single bonds or one double bond). 3. Satisfy the octet rule. Distribute pairs of electrons (pairs of dots) around each atom (except hydrogen) to give each atom a noble gas structure. 4. Count the total number of electrons used and compare it to the total number of valence electrons available as calculated in Step 1. If there are too many used, change single bonds between atoms to double or triple bonds by shifting the unbounded pairs of electrons as needed. Check to see that each atom has a noble gas electrons structure (two electrons for hydrogen and eight for the others). A double bond counts as four electrons for each atom to which it is bonded, and triple as six Complex Lewis Structures Draw the resonance When a single Lewis structure cannot be drawn for a molecule, structures for a resonance structures (multiple Lewis structures) are used to polyatomic ion. represent the molecule. For example, the nitrate ion NO3 - Key Term Resonance structure A molecule or ion that has multiple Lewis structures.

8 11.9 Compounds Containing Polyatomic Ions Describe a compound that Polyatomic ions behave like a single unit in many chemical contains both ionic and reactions. covalent bonds. The bonds within a polyatomic ion are covalent. For example, Na2CO3(s)!"#$% 2 Na + (aq) + CO3 2- (aq) Molecular Shape Determine the shape of a compound by using VSEPER method. Key Terms Linear structure Trigonal planar structure Tetrahedral structure Bent structure Lewis structures do not indicate the shape of a molecule. PROBLEM-SOLVING STRATEGY: Determining Molecular Shape Using VSEPR Draw the Lewis structure for the molecule. Count the electron pairs around the central atom and arrange them to minimize repulsions (as far apart as possible). This determines the electron pair arrangement. Determine the positions of the atoms. Name the molecular structure from the position of the atoms. In the VSEPR model, an arrangement where the pairs of electrons are arranged 180 apart for maximum separation. An arrangement of atoms in the VSEPR model where the three pairs of electrons are placed 120 apart on a flat plane. An arrangement of the VSEPR model where four pairs of electrons are placed degrees apart to form a tetrahedron. Bent structure in which the molecule looks like a V. The electron structure is tetrahedral but there are 2 pairs of nonbonding electrons.

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