I . Metallic Bonds

1. Bonding between Atoms I. Metallic Bonds Bonds created by charged metal cations surrounded by mobile valence electrons. (like cations floating around in a sea of electrons) Metals do not hold electrons strongly Valence electrons float away, leaving a positive ion behind (cation)

2. 1. Sea of Mobile electrons Each grey sphere is the positive ion of the metal The electrons are said to be Delocalized 2. Metallic Lattice Metal ions form a lattice which is more tightly packed and denser than the lattices in ionic compounds. They form crystals called grains. (Groups of metal atoms)

3. This model of metallic bonding explains the properties of metals 3. Conductivity Electricity = flow of electrons As one electron enters the metal, it is able to push other electrons Since e- are not bound to atoms, it creates a “current” of electrons e- + + + + + + + + + + e- e- e- e- e- e- e- e- e-

4. force 4. Ductility and malleability When a metal is hit, the layers of the lattice (grains) just slide over each other. The metallic bonds do not break because the electrons are free to move. The faster that molten metal is cooled, the smaller the grain size. The smaller the grains, the shorter the distance the atom layers can move. This means that metals with smaller grains are stronger and harder than metals with larger grains.

5. 5. Strength of Metallic Bonds The strength of a metallic bond in a pure metal depends on several factors Most easily seen is number of valence electrons More electrons contributed to the “sea of electrons”, more positive the ion Higher charge pulls on more electrons = stronger bond Lithium, Sodium, Potassium Calcium, Magnesium Cobalt, Iron About half way through the transition elements, bonds become weaker Due to difficulty in removing electrons Strongest metallic bonding is in the middle of the transition elements Cut with a knife Harder, can be cut Very hard to cut

6. II. Alloys • Mixtures of two or more elements, at least one of which is a metal. • Made by melting, mixing, then cooling the metals. • May contain non-metals like carbon. • Alloys usually still have a sea of mobile electrons, so conductivity is not affected. • The mixture of elements results in properties unlike pure metals • Examples • Stainless Steel • Fe 80.6%, Cr 18%, C 0.4%, Ni 1% • Cast Iron • Fe 96%, C 4%

7. 1. Types of Alloys a. Interstitial alloys form between atoms of different radius the smaller atoms fill the spaces between the larger atoms. The added atoms do not noticeably expand the lattice so the density is often increased. The smaller atoms make the lattice more rigid, decreasing malleability and ductility. Example - Steel Made of Iron and carbon Small carbon atoms fill in spaces between iron atoms Steel can be made stronger than pure iron.

8. b. Substitutional alloys form between atoms of comparable radius one atom substitutes for the other in the lattice. The density typically lies between those of the component metals, The alloy remains malleable and ductile. Example - Brass some copper atoms are substituted with a different element, usually zinc.