What affects the resistivity/conductivity of metals?

[Crushgear64]

Homework Statement

Why does copper conduct electricity better than other metals like nickel and alloys like steel?

Also other metals like Zinc, Tin, Aluminium, Iron and Lead.

The Attempt at a Solution

When I tried researching it seemed related to the amount of electrons to carry charge (Like gold has more so conducts better), and also the lattice vibrations interrupting charge carriers, however lots of answers varied.

 

[frogjg2003]

Could you provide a bit more information about your physics, math, and chemistry background so we can better answer your question?

 

[Crushgear64]

This is high school science, but I'm not looking for anything over simplified.

 

[aralbrec]

Solids are an arrangement of atoms in a lattice -- ie the atoms are positioned around each other in a regular pattern.

Think about alloys for a moment. Alloys are a solid solution made up of a dominant material (eg iron) with impurities added (eg carbon). If the two atoms are not the same size, what does that do to the atomic structure of a pure material (eg pure iron)?

Why some pure metals in a perfect lattice** conduct better than others is more complicated. It has to do with how many free electrons are available at a given temperature. In metals, some of the electrons of each atom share energy levels with neighbours so it is easy for them to move from one atom to the next. In your chemistry class you might have called this metallic bonding.

You may want to look at what makes some materials conductors (low resistance), insulators (high resistance) and semiconductors (in the middle) too. Maybe you would like to speculate why metals tend to have growing resistance with temperature while semiconductors have lower resistance with increasing temperature (around room temperature).** There is no such thing as a perfect lattice in a pure material. There will always be spots where an atom should be but is vacant or where an atom shouldn't be and managed to squeeze between some neighbours. This could happen because the surrounding atoms didn't solidify perfectly. Stresses (eg twisting or pulling on the material) can cause the atoms to slip past one another too. This can cause permanent changes in the lattice with some atoms switching bonds with their neighbours. The result is more imperfections in the lattice, which will give a rougher ride to moving electrons. So even a pure material the resistance will vary depending on how quickly the material was cooled (was there time for the atoms to form into a regular lattice?) and on whether mechanical deformations were applied (one method of making steels stronger is called cold working where they purposely stretch the material to introduce problems in the lattice in order to make it stronger).

 

[frogjg2003]

Some elements' electrons are more tightly bound to the atom than others. Oxygen, fluorine, chlorine, and other elements in the top right of the periodic table hold on to their electrons very tightly. Elements on the bottom left, like francium, cesium, and barium are almost giving away their electrons. The more loosely the electrons are bound to the atom, the more conductive the element is.

At the same time, the alkali metals can only give away one electron and the alkaline Earth metals can only give away two because the remaining electrons are in what is called a closed electron shell. Elements like in the copper group (copper, silver, and gold) have the most electrons to give away. This means that even though one electron isn't as bound as the the first two groups, they make up for this in sheer number.

The freer the electrons are, the more they behave like a gas (or liquid, as in "electron sea") inside the metal, and therefore the better they can flow through material. The more electrons, the greater that flow can be.

So far, we've only looked at the chemistry of conductivity. There is also a lot of physics involved as well. This is where that lattice you were talking about comes in. The atoms in a pure metal are uniform and packed really tightly. Atoms of different elements are different sizes. This causes irregularities in the lattice, and disrupts the smooth flow of electrons. There are also phenomena that lead to an increase of conductivity, but this is an upper level undergraduate/graduate topic, not very easily comprehended by even professional researchers, let alone high schoolers. This leads to some counter intuitive phenomena like superconductivity and semiconductivity.

 

[Crushgear64]

Some elements' electrons are more tightly bound to the atom than others. Oxygen, fluorine, chlorine, and other elements in the top right of the periodic table hold on to their electrons very tightly. Elements on the bottom left, like francium, cesium, and barium are almost giving away their electrons. The more loosely the electrons are bound to the atom, the more conductive the element is.

Why do they 'cling' more?

 

[frogjg2003]

The far right column of elements are called the noble gasses. They are highly non-reactive and even the heaviest are naturally found as a gas instead of a solid. They behave this way because of the quantum mechanics make the electron configuration of these elements to have really low energy compared to their neighboring elements. To create the electron configurations of the other elements, you just take the configuration of the last noble gas, and add extra electrons. These extra electrons are called valence electrons and are not as tightly bound as the core electrons in the closed shells. When there are only a few extra electrons, it takes less energy to remove those electrons and create a positively charged, but closed shelled, ion and free electrons than to add electrons. This is where the electrons in the "electron sea" come from when talking about conductivity in metals. When there are almost a full shell, it takes less energy to add electrons and create a closed shelled and negative ion. This is the cause of ionic bonding which you should have learned or will learn about in high school chemistry. This is complicated by the presence of sub-shells, which partially explains why the copper group elements are such good conductors but the zinc group elements are not.