Ionic & Metallic Bonding

Ionic & MetallicBonding Chemistry Chapter 7

Valence Electrons • Valence electrons - electrons in the highest occupied energy level of an element’s atom. • # of valence electrons determines the chemical properties of an element. • Group # of representative elements = # of valence electrons . • Valence electrons are usually the only electrons used in chemical bonds.

Electron (Lewis) Dot Structures • Electron dot structures are diagrams that show valance electrons as dots around the chemical symbol.

Draw the dot structure Magnesium Carbon Mg C Aluminum Chlorine Al Cl

The Octet Rule • Octet comes from the Greek work okto meaning 8. How does it relate to chemistry? • Nobel gases have 8 electrons in their outermost energy level (except He). • In forming compounds atoms tend to achievethe electron configuration of a noble gas; that is an octet of electrons in the highest energy level, hence the Octet Rule (Proposed by chemist Gilbert Lewis).

How Atoms Achieve an Octet • Atoms of themetallic elements tend to lose their valence electrons, leaving a complete octet in the next-lowest energy level. • Atoms ofnonmetallic elements tend to gain electronsor(to share electrons with another nonmetallic element) to achieve a complete octet.

Formation of Cations • An atom’s loss of valence electrons produces a cation, or positively charged ion. • For metallic elements the name of the cation is the same as the element. • For transition metals, the charge of the cation may vary, that is it does not always lose enough electrons to have a electron config of a noble gas. • A Roman numeral is used after the name to indicate the value of the charge(# of electrons lost) • Copper(I) or Copper(II)

More on Cations • Pseudo noble-gas electron configuration • Cadmium - how many valence electrons? • 1s22s22p63s23p63d104s24p64d105s2 • How many electrons would Cd have to lose to to have a noble gas config? How many would it have to gain? • Ions with a charge of greater than 3 are extremely unlikely; thus Cd can’t attain noble gas config., but if it loses two it has 18 valence electrons and is relatively favorable in compounds. • 18 valence electrons is know as a pseudo noble gas electron config. • Generally occurs with elements on the right side of the d block.

Formation of Anions • The gain of negatively charged electrons by a neutral atom produces an anion. • The name of an anion of a nonmetallic element is not the same as the element name; anion names typically ends with “ide”. • Ions produced when halogens (group 7A) gain electrons are called halide ions. How many electrons do the halogens have to gain to attain a noble gas electron config?

Ionic Bonds • Anions & cations have opposite charges and are attracted to each other by electrostatic forces. This electrostatic force is calledionic bondsand it hold ions together to create compounds.

Chemical Formula & Formula Units • Chemists represent the composition of substances by writing chemical formulas. • A chemical formulashows the kind and # of atoms in the smallest representative unit of a substance. I.e. NaCl. • Chemical formulas refer to a ratio know as a formula unit. A formula unitis the lowest whole-number ratio of ions in an ionic compound. • One Na+ to each Cl- or 1 to 1 ratio or NaCl. • One Mg2+ to two Cl-, what would the ratio be? • 1:2 or MgCl2

Formation of Ionic Compounds • Compounds composed of cations & anions are called ionic compounds. Usuallycomposed of metal cations and nonmetal anions. • Although they are composed of ions, ionic compounds areelectrically neutral. • Formula must be balanced to show it is electrical neutral. • Ionic bonding is responsible for the formation of all types of salts; ionic compounds are often referred to as salts.

Balancing (Ionic) formulas • Crisscross method • The numerical value of the charge of each ion is crossed over and becomes the subscript for the other ion.

Polyatomic Ions (see handout) • A polyatomic ionis a tightly bound (bonded) group of atoms that has a positive or negative charge and behaves as a unit, i.e. ammonium ion (NH4+).

Naming Ionic Compounds • Binary Ionic Compounds are compounds composed of two elements. • To name any binary ionic compound, state the cation name first, followed by the anion name. • Terniary Ionic Compounds are composed of more than two elements (contain a polyatomic ion) • To name any ternary ionic compound state the cation name first, followed by the anion name.

Practice Naming ofBinary Ionic Compounds • Easy one first! • NaCl • Sodium Chloride • K2S • Potassium Sulfide • LiBr • Lithium Bromide • Cs2O • Cesium oxide • A little harder! • CuO • Copper(II) oxide • FeO • Iron(II) oxide • SnS2 • Tin(IV) sulfide

Writing Formulas forBinary Ionic Compounds • If you know the name of a binary ionic compound, you can write the formula. • Write the symbol of the cation and then the anion. Add whatever subscripts are needed to balance the charges. • Remember the Crisscross method

Practice Writing Formulas ofBinary Ionic Compounds • Easy ones first • Sodium Chloride • NaCl • Potassium Chloride • KCl • Lithium Fluoride • LiF • A little Harder • Calcium bromide • CaBr2 • Potassium Nitride • K3N • Tin(IV) sulfide • SnS2 • Iron(III) oxide • Fe2O3

Practice Writing Formulas for Polyatomic (Ternary) Ionic Compounds • A few easy ones • Sodium nitrate • NaNO3 • Calcium carbonate • CaCO3 • Lead(II) sulfate • PbSO4 • Lets try a couple of harder ones • Calcium Nitrate • Ca(NO3)2 • Lithium Carbonate • Li2(CO3)

Properties of Ionic Compounds • Most ionic compounds are crystalline solids at room temperature. • Ionic compounds generally have high melting points. • Results from a repeating 3-dimensional arrangement of ions that maximizes attractions and minimizes repulsions. Large attractive forces make ionic compounds very stable. • I.e. melting point of NaCl is 800oC. • The coordination # of an ion is the number of ions of the opposite charge that surround it in a crystal solid

Coordination Number

Ionic Compound NaCl

Properties of Ionic Compounds (cont) • Another property of ionic compounds has to do with conductivity. Ionic compounds can conduct an electric current when melted or dissolved in water.

Metallic Bonds • Metals are made up of closely packed cations rather than neutral atoms. • The valence electrons of metal atoms can be modeled as a “sea of electrons”. • valence electrons are mobile and can drift freely from one part of the metal to another. • Metallic bonds - consist of the attraction of the free floating valence electrons for the positively charged metal ions.

The metallic bond consists of a series of metals atoms that have all donated their valence electrons to an electron cloud that permeates the structure. This electron cloud is frequently referred to as an electron sea. It might help to visualize the electron sea model as if it were a box of marbles that are surrounded by water. The marbles represent the metal atoms and the water represents the electron sea. Metallic Bond

Metallic Properties • The free flowing valance electrons allow metals to have certain properties not found in non-metals. • Conductive – conduct energy • Ductile – can be drawn into wire • Malleable – can be shaped (hammered)

Electrical Conductivity in Metals

Crystalline Structure of Metals • Like ionic compounds metals also are crystalline. • Metals that contain just one kind of atoms are among the simplest forms of all crystalline solids. • Metals atoms are arranged in very compact and orderly patterns. • Because metal atoms are spherical there are several types of crystalline arrangements possible: • Body-centered cubic • Face-centered cubic • Hexagonal close packed

Alloys • Alloys are mixtures composed of two or more elements, at least one of which is a metal. • Alloys are important because their properties are often superior to those of their component elements. • Sterling sliver – Ag and Cu • Bronze – Cu and Sn • Cast iron – Fe and C • Stainless steel – Fe, Cr, C, and Ni • Surgical steel – Fe, Cr, Ni, an Mo • Nonferrous (non-iron) alloys used in coins

Ionic & Metallic Bonding