Chapter 4: Chemical Bonds The Art of Deduction: Stable Electron Configurations

Chapter 4: Chemical Bonds

The Art of Deduction: Stable Electron Configurations

 Valence electrons – electrons in the outermost shell  Core electrons – electrons in all other shells

 Stable configuration example: Na+ and Cl-

• Na = 11 electrons, 1 in 3

rd shell

• Na gets rid of valence electron, giving Na the same electron configuration as neon

 Na now positively charged – Na ion: Na+

o Cl gains the electron, becoming negatively charged – Cl ion: Cl-

• Positive charge indicates loss of an electron and negative charge indicates gain of electron
• Isoelectronic – atoms that have the same electron configuration

Lewis (Electron-Dot) Symbols  Valence electrons are represented by dots in ions; element symbol represents the core of the atom (nucleus plus inner electrons)

o Equations from above example:

 Na Na +

• 1e

-

  Lewis Symbols and the Periodic Table

• Only the number of dots is important; do not need to be drawn in specific positions, except there should be no

more than 2 dots on any side of the chemical symbol

o Practice Example:

2  Write Lewis symbols for magnesium, oxygen, and phosphorus. You may use the periodic table. Magnesium is in group 2A, oxygen is in group 6A, and phosphorus is in group 5A. The Lewis symbols,

therefore, have two, six, and five dots, respectively. They are:

The Reaction of Sodium and Chlorine  Na – highly reactive metal and soft enough to be cut with a knife; reacts violently with water, becoming so hot that it melts  Chlorine (Cl2) – gas; used as disinfectant  If Na is dropped into flask containing Cl gas, violent reaction producing sodium chloride (salt)  Ionic bonds – the chemical bond that results when electrons are transferred from a metal to a nonmetal; the electrostatic attraction between ions of opposite charge.

• i.e. Na + Cl  NaCl

Using Lewis Symbols for Ionic Compounds

 Examples of Ionic Compound Lewis Symbols:

o Potassium (K) – metal in same family as Na, reacts with Cl:

o K + Br (same family as Cl)  Potassium bromide (KBr):

• Mg (2A metal) reacts with O to produce magnesium oxide (MgO)

 When an Mg atom becomes an Mg 2+ ion, its second shell becomes its outermost shell. The Mg 2+ ion is isoelectronic with the Ne atom and has the same stable

main-shell electron configuration: 2 8.

o Practice Example:

 Use Lewis symbols to show the transfer of electrons from magnesium atoms to nitrogen atoms to form ions with noble gas configurations.

3

• Usually, metallic elements in 1A and 2A react with nonmetallic elements in 6A and 7A to form ionic compounds;

products are stable crystalline solids  The Octet Rule

• Octet – set of 8 valence electrons

 Characteristic of all noble gases except He

• When atoms react, they tend to attain this stable noble-gas electron configurationthey follow the octet rule, or

“rule of eight”

o Examples: determining formulas by electron transfer

 What is the formula of the compound formed by the reaction of sodium and sulfur  Sodium is in group 1A; the sodium atom has one valence electron. Sulfur is in group 6A; the sulfur atom has six valence electrons.Formulas and Names of Binary Ionic Compounds  Names of simple positive ions (cations) – derived from those of their parent elements by addition of the word ion

• Sodium atomloses electronSodium ion (Na+)
• Mg atomloses 2 electrons Magnesium ion (Mg

2+ )  Names of simple negative ions (anions) – derived from those of their parent elements by changing the usual ending to –ide

• Cl atomgains electronChloride ion (Cl

- )

• Sulfur atomgains 2 electronsSulfide ion (S

2- )  Binary ionic compounds - simple ions of opposite charge combining

• To get correct formula for binary compound – write each ion with its charge (positive ion to the left), then

transpose the charge numbers (but not the + and – signs) and write them as subscripts

o Practice Examples:

 Write the formulas for (a) calcium chloride and (b) aluminum oxide.

4  (a) First, we write the symbols for the ions. (We write the charge on chloride ion explicitly as “1–” to illustrate the method. You may omit the “1” when you are comfortable with the process.)  Ca 2+ Cl 1–

 We cross over the charge numbers (without the charges) as subscripts. Then we write the formula. The formula for calcium chloride is: Ca1Cl2 or (dropping the “1”) simply CaCl2  (b) We write the symbols for the ions. Al 3+ O 2–

 We cross over the charge numbers as subscripts.

 Write the formula for aluminum oxide: Al2O3

 This method is the crossover method, works because it is based on the transfer of electrons and the conservation of charge. Two Al atoms lose three electrons each (a total of six electrons lost), and three O atoms gain two electrons each (a total of six electrons gained). The electrons lost must equal the electrons gained. Similarly, two Al 3+ ions have six positive charges (three each), and three O 2– ions have six negative charges (two each). The net charge on Al2O3is zero, just as it should be.

Covalent Bonds: Shared Electron Pairs

 Covalent bond – bond formed when atoms share electrons

o Single covalent bond – 1 pair of electrons are shared i.e. H:H

• Bonding pair – a pair of electrons shared by 2 atoms, forming a chemical bond i.e.
• Lone pairs – the electrons that stay on 1 atoms and are not shared aka nonbonding pairs

 Multiple bonds – some molecules must share more than 1 pair of electrons to follow the octet rule i.e. CO2:

• Double bond – covalent linkage in which 2 atoms share 2 pairs of electron

 Indicated by a double dash: =