In Ionic, Molecular, Network and Metallic Solids you sorted solids into ionic, molecular, network and metallic. Metals deserve a closer look, because their bonding explains everyday properties and how alloys such as steel and brass are designed.
Key idea
A metal is a lattice of positive ions in a "sea" of delocalized valence electrons that belong to the whole crystal. An alloy is a mixture of a metal with other elements. In a substitutional alloy (brass: copper and zinc) atoms of similar size replace some host atoms. In an interstitial alloy (steel: iron and carbon) much smaller atoms fit into the gaps between host atoms.
Why it works
The valence electrons are free to move through the whole crystal, so a small voltage pushes them along: metals conduct electricity, and the moving electrons also carry heat. The ions are held by the electron sea in every direction, not by bonds to particular neighbors, so layers of ions can slide past one another without breaking the solid apart: metals bend, hammer flat (malleable) and draw into wire (ductile). An alloy's foreign atoms disturb the even layers. They make sliding harder, so most alloys are harder and stronger than the pure metal; interstitial atoms such as carbon in iron are especially good at pinning the layers.
Worked example: Which kind of alloy?
Carbon atoms (radius about 77 pm) dissolve in iron (radius about 126 pm). Zinc atoms (about 134 pm) dissolve in copper (about 128 pm). Classify each alloy.
- Carbon is much smaller than iron (about 60% of its radius), small enough to sit in the gaps: interstitial.
- Zinc and copper are within about 5% in size, so zinc atoms can take copper's places: substitutional.
Answer: Steel is interstitial; brass is substitutional.
Worked example: Alloy composition
A sterling silver spoon of mass $40.0\ \text{g}$ is 92.5% silver and 7.5% copper by mass. How many grams of copper does it contain, and how many moles of each metal (Ag $107.87$, Cu $63.55$)?
- Copper: $40.0 \times 0.075 = 3.0\ \text{g}$; silver: $40.0 \times 0.925 = 37.0\ \text{g}$.
- Moles: Ag $37.0/107.87 = 0.343\ \text{mol}$; Cu $3.0/63.55 = 0.047\ \text{mol}$.
- The mole ratio is about 7 : 1, not a simple formula; another spoon could be 90% silver. An alloy is a mixture.
Answer: $3.0\ \text{g}$ of Cu; $0.343\ \text{mol}$ Ag and $0.047\ \text{mol}$ Cu.
Common mistake
Two common errors. One is treating an alloy as a compound with a fixed formula; its composition can vary continuously, like any solution. The other is saying a metal conducts because its cations move. The cations stay in their lattice sites; the delocalized electrons carry the current. Check: a solid metal conducts while its ions are locked in place, but solid NaCl, whose ions are also locked in place and which has no free electrons, does not.