Metals Answer Lab

Metallic bonding is the attraction between positive metal ions and a shared pool of electrons. When metal atoms pack together, each atom gives up its outer (valence) electrons. Those electrons are delocalized: instead of staying with one atom, they are free to move anywhere in the piece of metal, like water flowing around pebbles.

In the “sea of electrons” model, the metal is a regular arrangement of positive ions (cations) sitting in a sea of mobile, negatively charged electrons. The electrostatic attraction between the positive ions and the negative sea is what holds the metal together. The bonds have no fixed direction, because every ion is attracted to the electrons all around it, not to one particular neighbor.

Key words: cations · valence electrons · delocalized · electrostatic attraction · non-directional

Electricity. An electric current is a flow of charge. In a metal, the delocalized electrons are already free to move. When a voltage is applied across the metal, it pushes those electrons so that, on top of their random motion, they all drift in the same direction. That organized drift is the current. Because there are enormous numbers of mobile electrons and nothing holding them to a particular atom, metals conduct very well. In an ionic solid the charged particles are locked in place, so it cannot conduct unless it is melted or dissolved.

Heat. Heat is energy of motion. When one end of a metal is heated, the electrons there move faster. Because they are free to travel, these fast electrons quickly carry that extra kinetic energy through the metal and share it by colliding with ions and other electrons far away. The vibrating ions pass heat along too, but the mobile electron sea is why metals feel cold to the touch and conduct heat so much better than plastics or ceramics.

Key words: current = flow of charge · drift · mobile electrons · kinetic energy

Malleable means a material can be hammered into sheets; ductile means it can be drawn into wires. Both need layers of atoms to slide past each other without the solid breaking.

In a metal the bonding is non-directional: every positive ion is attracted to the electron sea all around it. If a force pushes one layer of ions sideways, the ions move to new positions but are still surrounded by delocalized electrons, so the attraction is the same as before. The bonds rearrange instead of breaking, and the metal changes shape.

In an ionic compound such as sodium chloride, positive and negative ions alternate, and the attraction is between specific opposite charges. If a layer is pushed just half a step, positive ions end up next to positive ions and negative next to negative. Like charges repel strongly, the layers are forced apart, and the crystal shatters along that plane. That is why salt crumbles when you hit it, while copper flattens.

Key words: malleable · ductile · layers slide · non-directional bonds · like charges repel

A crystal structure is the repeating 3D pattern that atoms make in a solid; the smallest repeating block is the unit cell.

• Face-centred cubic (FCC). A cube with an atom at each of the 8 corners and one in the centre of each of the 6 faces. Corner atoms are shared by 8 cells and face atoms by 2, so each cell owns 8×⅛ + 6×½ = 4 atoms. Each atom touches 12 neighbours. Examples: aluminum, copper, gold, silver, nickel, lead.
• Body-centred cubic (BCC). A cube with an atom at each corner and one atom in the very centre of the cube. Each cell owns 8×⅛ + 1 = 2 atoms, and each atom touches 8 neighbours. Examples: iron (at room temperature), chromium, tungsten, molybdenum, sodium.
• Hexagonal close-packed (HCP). Flat hexagonal layers of atoms stacked so that the third layer sits directly above the first (an ABAB… pattern). The unit cell is a hexagonal prism with 6 atoms, and each atom touches 12 neighbours. Examples: magnesium, zinc, titanium, cobalt, cadmium.

Key words: unit cell · corners / faces / body centre · ABAB stacking · coordination number

“Close-packed” means the atoms are packed together as tightly as identical spheres can possibly be, with the least empty space between them. In a close-packed structure every atom touches 12 neighbours (6 in its own layer, 3 above and 3 below), which is the maximum possible. The atomic packing factor (packing efficiency) is the fraction of the unit cell’s volume that is filled by atoms.

FCC and HCP are both close-packed: about 74% of their volume is atom and 26% is empty space. They differ only in how the hexagonal layers are stacked (ABCABC for FCC, ABAB for HCP). BCC is more open: each atom touches only 8 neighbours, and about 68% of the volume is filled. The atoms in BCC do not form flat, touching hexagonal layers, which is why it is not close-packed.

Key words: atomic packing factor · 74% vs 68% · coordination number 12 vs 8 · ABC vs AB stacking

Metals deform when whole planes of atoms slide over each other. A plane and a direction along which this sliding happens easily is called a slip system. Sliding is easiest on the most densely packed planes, because the atoms in those planes are close together and the surface they slide over is smooth, with only small “bumps” to climb.

FCC has four sets of truly close-packed planes, each with three easy directions, giving 12 well-defined, low-resistance slip systems. That is why FCC metals such as gold, copper and aluminum bend, roll and stretch so readily. BCC has no close-packed planes at all. Its layers are rougher, so more force is needed to make them slide, and at low temperatures this gets even harder, which is why BCC metals like iron and chromium are stronger but less ductile and can become brittle in the cold. (HCP metals have close-packed planes too, but only in one orientation, so they are usually less ductile than FCC.)

Key words: slip system · close-packed planes · 12 slip systems in FCC · smoother sliding = more ductile

When one element can exist in more than one crystal structure it is called allotropy, and the different forms are allotropes. The structure an element adopts is the one with the lowest energy, and that depends on temperature. At low temperature the arrangement that gives the strongest bonding wins; at higher temperature the atoms vibrate more, and a structure that gives them more freedom to vibrate can become more stable. For iron, BCC (called α-iron or ferrite) is stable up to 912 °C, then the atoms rearrange into FCC (γ-iron or austenite). Above 1394 °C it flips back to BCC (δ-iron) before melting at 1538 °C.

This matters enormously. FCC iron has close-packed slip planes, so hot iron is far easier to bend and hammer than cold iron: this is why a blacksmith heats a bar until it glows before shaping it. FCC iron also has larger gaps between atoms and can dissolve up to about 2% carbon, while BCC iron dissolves almost none (about 0.02%). Steelmakers heat steel into the FCC range so the carbon spreads through the metal, then control how fast it cools. Cooling slowly or quenching quickly traps the carbon in different ways and produces steels of very different hardness. All heat treatment of steel depends on this change of structure.

Key words: allotropy · ferrite (BCC) → austenite (FCC) at 912 °C · hot iron is easier to shape · FCC dissolves more carbon · heat treatment

An alloy is a metal made by mixing a metal with one or more other elements (other metals or non-metals such as carbon). The mixture is still held together by metallic bonding and still conducts and shines like a metal, but the added atoms change its properties. At the atomic level, a pure metal has identical atoms in a perfectly regular lattice; an alloy has foreign atoms scattered through that lattice.

Substitutional alloy. The added atoms are about the same size as the host atoms (within roughly 15%), so they take the place of host atoms in the lattice, sitting on the normal lattice sites. Example: brass, where zinc atoms replace some copper atoms; bronze (copper and tin) and sterling silver (silver and copper) are also substitutional.

Interstitial alloy. The added atoms are much smaller than the host atoms, so they squeeze into the gaps (interstices) between host atoms rather than replacing them. Example: steel, where tiny carbon atoms fit into the spaces between iron atoms.

Key words: mixture that keeps metallic properties · similar size → substitutes · much smaller → fills gaps · brass · steel

A pure metal is soft because its planes of identical atoms are flat and regular, so they slide over each other easily (Question 6). Strength and hardness are about how much force it takes to make those planes slip.

When atoms of a different size are added, they distort the lattice around them: a larger substitutional atom pushes its neighbours outward, a smaller one lets them sag inward, and an interstitial atom wedges the host atoms apart. The planes are no longer smooth. When a force tries to slide one plane over another, the plane catches on these distorted regions, so a much larger force is needed to move it. The foreign atoms act like pebbles jammed between the pages of a book that someone is trying to shear.

Because the planes cannot slip easily, the alloy is stronger and harder than the pure metal. But slipping is exactly what lets a metal bend and stretch without breaking, so blocking it also makes the alloy less ductile and, if taken too far, brittle. Bronze is harder than copper and steel is much stronger than iron for this reason.

Key words: lattice distortion · planes catch on foreign atoms · more force to slip → stronger, harder · less slip → less ductile

Gold’s workability comes from both levels of its structure.

Bonding. Gold is held together by metallic bonding: gold cations in a sea of delocalized electrons. The bonds are non-directional, so when layers of atoms are forced to move, the cations are still surrounded by electrons and the bonding simply rearranges (Question 3). Nothing breaks, so gold can be squashed into leaf or drawn into wire without cracking.

Crystal structure. Gold is FCC, the structure with the most close-packed slip systems. Its atomic planes are smooth and slide over one another with very little force (Question 6). On top of that, gold is very pure in practice and its bonds are relatively soft, so there is little to pin the planes in place. Because both the bonding and the geometry favour sliding, a single ounce can be hammered to a leaf only a few hundred atoms thick.

Key words: non-directional metallic bonds rearrange · FCC → many easy slip systems · planes slide, nothing breaks

Pure iron is soft because its planes of identical atoms slip easily. In steel, small carbon atoms sit in the gaps between iron atoms (an interstitial alloy, Question 8). A carbon atom is small, but it is still bigger than the gap it squeezes into, so it pushes the surrounding iron atoms apart and warps the lattice around it. These distorted spots catch the atomic planes as they try to slide (Question 9), so far more force is needed to deform the metal. That is why steel with even a fraction of a percent of carbon is several times stronger than pure iron.

Adding more carbon adds more of these obstacles, and when steel is heated into the FCC range and cooled (Question 7) the carbon also forms hard iron-carbide particles that block slip even more. So hardness and strength keep rising with carbon content. But slipping is what lets a metal absorb energy by bending instead of cracking. With the planes pinned, a high-carbon steel cannot deform to relieve stress, so it snaps instead. Low-carbon “mild” steel (≈0.1–0.3% C) bends and welds easily; high-carbon steel (≈0.6–1.0% C) holds a sharp edge but chips; above about 2% carbon the material is cast iron, very hard and quite brittle.

Key words: carbon in interstitial gaps · lattice distortion pins slip planes · more carbon → harder · no slip → brittle