Chapter 6 Chemical Bonds

Why Bond? An atom’s goal is to be stable This means that the highest occupied energy level is filled with electrons For most elements the highest energy level is filled when it contains 8 electrons. (i.e. Noble Gases) 2

Examples of electron dot configurations 3

1)Ionic Bonds- chemical bond between a metal and a nonmetal.  Electrons are transferred from metal to nonmetal  Results in positively charged metal cation and negatively charged nonmetal anion 4

What does this look like? When sodium reacts with chlorine, an electron is transferred from a sodium atom to a chlorine atom. Each atom ends up with a more stable electron arrangement than it had before the transfer. 5

Formation of Ions When an atom gains or loses an electron, the number of protons is no longer equal to the number of electrons. The charge on the atom is not balanced, and the atom is not neutral. An atom that has a net positive or negative electric charge is called an ion. The charge on an ion is represented by a plus or a minus sign. If an atom gains an electron the charge is 1- If an atom loses an electron the charge is 1+ 6

Examples of common ions… Anions:O 2- F - S 2- I - N 3- P 3- Cations: Cr 2+ Mn 2+ Fe 3+ Co 3+ Cu + Cu 2+ 7

Ionization Energy The amount of energy used to remove an electron is called ionization energy It varies from element to element It increases from left to right in the periodic table. It takes more energy to remove an electron from a nonmetal than a metal It generally decreases from top of a group to bottom. 8

2) Covalent Bonds- chemical bond between two nonmetals.  Electrons are shared, but not always equally equally  Non-metals bond covalently to achieve stability (8 valence electrons) stability (8 valence electrons)  Not as strong as ionic bond

This illustration shows four ways to represent a covalent bond between two hydrogen atoms. This illustration shows four ways to represent a covalent bond between two hydrogen atoms. 10

Many nonmetal elements exist as diatomic molecules. Diatomic means “two atoms.” 11

Covalent Bonding: sharing electrons  When two atoms share one pair of electrons, it is called a single bond. Example Cl:Cl or Cl-Cl  Two atoms sharing two pairs of electrons = double bond.  Two atoms sharing three pairs of electrons = triple bond. 12

Polar Covalent Bonds A covalent bond in which electrons are not shared equally. When atoms form a polar covalent bond, the atom with the greater attraction for electrons has a partial negative charge. The other atom has a partial positive charge The type of atoms in a molecule and its shape are factors that determine whether a molecule is polar or nonpolar. 13

Attraction Between Molecules In a molecular compound there are forces of attraction between molecules. Attraction between polar molecules are stronger than between nonpolar molecules. When a molecule has only two atoms, it will be polar. When molecules have more than two atoms, the answer is not obvious. 14

Naming Compounds & Writing Formulas The name of an ionic compound must distinguish the compound from other ionic compounds containing the same elements. The formula of an ionic compound describes the ratio of the ions in the compound. 15

Binary Ionic Compounds A compound made from only two elements is a binary compound. The names have a predictable pattern: the name of the cation followed by the name of the anion. 16

This table lists eight common anions. The name of an anion is formed by adding the suffix –ide to the stem of the name of the nonmetal. 17

Metals With Multiple Ions Many transition metals form more than one type of ion. Many paint pigments contain compounds of transition metals. 18

Polyatomic Ions A covalently bonded group of atoms that has a positive or negative charge and acts as a unit is a polyatomic ion. Most simple polyatomic ions are anions. 19

Time to try one… What is the formula for the ionic compound calcium chloride? –Start off by listing the symbols and charges for the cation and anion. 20

List the symbols and charges for the cation and anion. Ca with a charge of 2+ and Cl with a charge of 1– 21

Determine the ratio of ions in the compound. It takes two 1– charges to balance the 2 + charge. There will be two chloride ions for each calcium ion. 22

Answer… CaCl 2 23

Naming Molecular Compounds The general rule is that the most metallic element appears first in the name. These elements are farther to the left in the periodic table. If both elements are in the same group, the more metallic element is closer to the bottom of the group. The name of the second element is changed to end in the suffix -ide, as in carbon dioxide. 24

There may be more than one molecular compound that can exist with the same two elements. The Greek prefixes in the table are used to name molecular compounds. The prefix octa-means “eight,” as in the eight tentacles of an octopus. 25

Two compounds that contain nitrogen and oxygen have the formulas N 2 O 4 and NO 2. The name of the compound with the formula N 2 O 4 is dinitrogen tetraoxide. The name for the compound with the formula NO 2 is mononitrogen dioxide. The prefix mono- often is not used for the first element in the name, so a more common name is nitrogen dioxide. 26

Let’s try one… What is the formula for diphosphorus tetrafluoride? 27

Because the compound is molecular, look for elements on the right side of the periodic table. Phosphorus has the symbol P. Fluorine has the symbol F. Di- indicates two phosphorus atoms, and tetra- indicates four fluorine atoms. The formula for the compound is P 2 F 4. 28

Last but not least… Metallic Bonds Last but not least… Metallic Bonds A metallic bond is the attraction between metals. Valence electrons move freely among atoms of metals in a sea of shared electrons. The mobility of electrons explains some of metals properties such malleability and being good conductors of heat and electricity Although the electrons are moving among the atoms, the total number of electrons does not change. So, overall, the metal is neutral. 29

Scientists can design alloys with specific properties by varying the types and amounts of elements in an alloy. An alloy is a mixture of two or more elements, at least one of which is a metal. Alloys have the characteristic properties of metals. Examples: Brass & bronze are alloys of Copper. Steel is an alloy of iron 30