# What is heat and how does it affect an atom?

What happens in an atom when it gets heated or cooled and turns into gas, liquid or solid?

I just want to know how does heat affect an atom.

• This is an interesting question. It turns out that heat is a purely macroscopic phenomena and atoms are absolutely microscopic. It's not easy to define heat if you only have a single atom!
– Ali
Commented Jul 22, 2013 at 18:21
• I think of heat as molecular vibration.
– Dale
Commented Jul 22, 2013 at 18:50

Let's define temperature to be a measure the kinetic energy of the atom.

A single atom has limited numbers of ways it can store energy. It can translate in X, Y or Z. It can't really rotate (well it does rotate, but it takes so little energy to make it rotate that we can ignore it). It can't vibrate. It does have electronic modes where adding energy can increase the orbits of the electrons.

So let's define a ground state. This is the point at which there is no energy stored in the translational or electronic modes. Since there is zero energy and we defined temperature to be a measure of energy, this state is at 0K or absolute zero.

As we add heat to the atom, the atom begins to translate around. The more heat that is added, the faster the translation. We keep adding heat, it translates faster and faster. There is more kinetic energy, so the temperature is higher. Eventually we add so much energy that now it can go into the electronic modes. Electrons start to move to higher orbits. Eventually we could add so much that they are set free and the atom loses some, or all, of it's electrons.

We can still call the energy stored in the electronic modes as a temperature, but it's not the same as what we are used to. You would now have two temperatures, one translational and one electronic.

The same principles apply to molecules, except those can rotate and vibrate as well.

## States of matter

Ultimately, it doesn't make sense to look at an atom as a solid, liquid or gas. It's just an atom. A collection of atoms becomes a solid, liquid or gas. How the atoms move about is . This theory has differences for solids, liquids and gases and each differs in various ways.

But let's look at how the states change. In a solid, you have a bunch of atoms that can be thought of as masses connected by springs. As heat is added to the system, the atoms begin to vibrate in the lattice of springs. As more heat is added, they vibrate enough to break the springs. This is when the solid begins to melt and turn to a liquid.

Now you have a liquid where the atoms are all moving around but they aren't free to move wherever they want. More heat is added to the system and the atoms begin to translate faster and faster. Eventually they translate fast enough to overcome the forces that are holding them together in a liquid. Now they fly free and are a gas.

So ultimately, heat is energy that makes atoms and molecules move in some way. They may translate, rotate, vibrate, or the electrons may begin moving around depending on how much heat is there and what configuration the molecule has.

• What does it mean that atoms translate? Commented Jul 22, 2013 at 20:19
• @zoran404 We started by defining temperature to be a measure of the kinetic energy of an atom. As we add energy to the system (heat), it causes the atom to move around, rotate (if it were a molecule), vibrate (if it were a molecule), or the electrons to change orbits. Commented Jul 22, 2013 at 20:23
• @zoran404 Infrared cameras detect energy emitted in the infrared spectrum, not "heat" per se. What you see is radiation caused by the motion of atoms/molecules and it just happens that the radiation is in the infrared range of the spectrum. A full discussion of how the cameras work, or why the emission is in infrared or caused by thermal motion, should be their own stand-alone questions. Commented Jul 22, 2013 at 20:29
• @Demis Ah, that definitely needs clarified. Energy can only transfer modes through collisions with other molecules (or I guess absorption/emission of photons). I can't think of any way of adding energy that doesn't involve colliding or hitting with photons, so I guess it is implicit but it should be clarified. Thanks! Commented Aug 9, 2016 at 21:00
• @Demis No -- if there are no other particles, then it does not make sense to talk of a temperature. Temperature is an ensemble average of a system of particles, and that may explain the misunderstanding here. Because it is an ensemble average, it is the relative motion and energy within the group of particles that defines its temperature, not the absolute (bulk) velocity of translation. Commented Aug 10, 2016 at 15:45