IONIC AND METALLIC BONDING Chapter 7

Section Overview 7.1: Ions 7.2: Ionic Bonds and Ionic Compounds 7.3: Bonding in Metals

IONS Section 7.1

Valence Electrons Valence electrons are the electrons in the highest occupied energy level of an element’s atoms. To find the number of valence electrons in an atom of an element, simply look at its groups number. For example, all elements in Group 1A have one valence electron. The only exception is helium (Group 8A) which has two. Valence electrons are usually the only ones used in chemical bonds. Electron dot diagrams show valence electrons as dots.

Electron Dot Diagrams

The Octet Rule The octet rule states that in forming compounds, atoms tend to achieve the electron configuration of noble gases, which have eight valence electrons. Atoms of the metallic elements tend to lose their valence electrons, leaving a complete octet in the next-lowest energy level. Atoms of some nonmetallic elements tend to gain electrons or to share electrons with another nonmetallic element to achieve a complete octet. The octet rule applies to atoms in most compounds.

Formation of Cations An atom’s loss of valence electrons produces a cation, or a positively charged ion. There are usually many chemical differences between metals and their cations (ex. Sodium reacts explosively with water but sodium cation is unreactive). Most of the atoms involved in the formation of cations have one to three valence electrons which are easily removed. Cations formed by metals in 1A are 1+ and in 2A are 2+. Ex: Na 1s 2 2s 2 2p 6 3ds 1  Na 1s 2 2s 2 2p 6 The charges of cations for transition metals vary and some do not have noble-gas electron configurations. e-e-

Formation of Anions An atom’s gain of valence electrons produces a anion, or a negatively charged ion. The name of an anion typically ends in –ide. Ex: Chloride Cl 1s 2 2s 2 2p 6 3s 2 3p 5  Cl 1s 2 2s 2 2p 6 3s 2 3p 6 The ions that are produced when atoms of chlorine and other halogens gain electrons are called halide ions. All halogens have seven valence electrons and need to only gain one electron. +e -

IONIC BONDS AND IONIC COMPOUNDS Section 7.2

Formation of Ionic Compounds Compounds composed of cations and anions are called ionic compounds. Although they are composed of ions, ionic compounds are electrically neutral. The total positive charge of the cations equals the total negative charge of the anions. Ionic bonds: the electrostatic forces that hold ions together in ionic compounds. One atom loses and electron and one gains. Ex: The formation of sodium chloride Na Cl  Na + Cl - 1s 2 2s 2 2p 6 3s 1 1s 2 2s 2 2p 6 3s 2 3p 5 1s 2 2s 2 2p 6 1s 2 2s 2 2p 6 3s 2 3p 6

Formation of Ionic Compounds Formula Units: A chemical formula shows the kinds and numbers of atoms in the smallest representative unit of a substance. A formula unit is the lowest whole-number ratio of ions in an ionic compounds. Ex: NaCl (Na +, Cl - ) Ratio 1:1 MgCl 2 (Mg 2+, Cl - ) Ratio 1:2 AlBr 3 (Al 3+, Br - ) Ratio 1:3 Example Problem: Predict the formulas for the compounds a. Potassium and oxygen b. Magnesium and nitrogen

Formation of Ionic Compounds Example Problem: Predict the formulas for the compounds a. Potassium and oxygen b. Magnesium and nitrogen Solution: a. Potassium and oxygen Remember: Atoms lose their valence electrons when forming compounds K has 1 valence electron and O has 6. Therefore, O is the closest to an octet needing to gain 2 electrons. So, for every O there will be 2 K. Formula = K 2 O

Formation of Ionic Compounds Example Problem: Predict the formulas for the compounds a. Potassium and oxygen b. Magnesium and nitrogen Solution: b. Magnesium and nitrogen Mg has 2 valence electrons and N has 5. Therefore, N is the closest to an octet needing to gain 3 electrons. So, for every 2 N there will be 3 Mg. Formula = Mg 3 N 2

Properties of Ionic Compounds Most ionic compounds are crystalline solids at room temperature. The component ions in these crystals are arranged in repeating three-dimensional patterns. Ionic compounds generally have high melting points. The coordination number of an ion is the number of ions of opposite charge that surround the ion in a crystal. Ionic compounds can conduct an electric current when melted or dissolved in water.

BONDING IN METALS Section 7.3

Metallic Bonds and Metallic Properties Metals are made up of closely packed cations rather than neutral atoms. The valence electrons of metal atoms can modeled as a sea of electrons, meaning they 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. Remember metals are good conductors, ductile, malleable all of which are due to the fluid nature of the valence electrons.

Crystalline Structure of Metals Metal atoms are arranged in very compact and orderly patterns. Popular arrangements include body-centered cubic, face- centered cubic, and hexagonal close-packed arrangements. Body-centered cubic: Every atom has eight neighbors (sodium, potassium, iron). Face-centered cubic: Every atom has twelve neighbors (copper, silver, gold). Hexagonal close-packed: Every atom has twelve neighbors, but in a hexagonal shape (magnesium, zinc, cadmium).

Crystalline Structure of Metals

Alloys Alloys are mixtures composed of two or more elements, at least one of which is a metal (ex. Brass is made of copper and zinc). Alloys are important because their properties are often superior to those of their component elements. For example, sterling silver is made up of silver and copper and is more durable than pure silver, but still soft enough to be made into jewelry. The most important alloys are steels which are resistant to corrosion, ductile, and tough. Two types of alloys are substitutional (atoms are about same size and can replace each other in crystal) and interstitial (atoms are different sizes and smaller ones fit inside bigger ones).