What makes up an atom? An atom is composed of a heavy nucleus of protons (positively charged particles, written as p⁺) and neutrons (neutral particles, written as n^o), around which orbit a cloud of extremely light electrons (negatively charged particles, written as e^-) .

What defines an element? The number of protons in the nucleus of each atom.

• Hydrogen (H) atoms have 1 proton.
• Hydrogen atoms with 1 proton and 1 electron are neutral hydrogen (¹H₁).
• Hydrogen atoms with 1 proton, 1 electron, and 1 neutron are a heavy isotope of hydrogen called deuterium (²H₁).
• If a proton is added to hydrogen, we then have a different element - helium (⁴He₂).

• Nomenclature: For each element, the superscript denotes the number of protons and neutrons, and the subscript the number of protons.
• How many neutrons are there in neutral carbon (¹²C₆)?
  There are a total of 12 nucleons (neutrons and protons), and 6 protons, so there must be 12 - 6 = 6 neutrons.
• How many neutrons are there in the radioactive isotope called carbon-14 (¹⁴C₆)?
  There are a total of 14 nucleons (neutrons and protons), and 6 protons, so there must be 12 - 6 = 8 neutrons.

How does the make-up of the atom or element tell us what its spectrum will look like?

• Electrons exist in stationary states within atoms, each defined by a discrete, unique level of energy. Only certain energy levels, like orbits with certain radii, are allowed.

• Light, or radiation, emitted or absorbed by atoms as electrons move from one energy level to another can be thought of as a stream of quanta called photons. Each photon carries an energy E = h × v. We define these energy levels as follows, saying that the electron is in an excited state when it has extra energy (think of a child bouncing off the walls with excitement).

  
  

• The ground state, the lowest energy level possible
• The 1^st excited state, the next highest allowed energy level
• The 2^nd excited state, the next highest allowed energy level
• The 3^rd excited state, the next highest allowed energy level
• ...
• Till the point at which the electron is no longer bound to the atom

• An atom usually has the same number of protons and electrons. Because protons have a positive charge and electrons have a negative charge, it carries no charge in this state. When the atom loses (or gains) an electron we say that it is ionized, and it then carries an electrical charge.

Entropy tells us that all things are naturally drawn to the lowest possible energy state:

• Logs and water roll downhill.
• Bouncing balls slow to a halt.
• People collapse into bed at night and find it hard to get up in the morning.
• In the same fashion, hydrogen atoms tend to be in the ground state.

• Most of the photons zip right past without interacting with the atom.
• But photons with just the right energy get absorbed by the atom.
• In this case, right means that the energy of the photon corresponds to the energy level difference between allowed orbits in the hydrogen atom, and absorbed means that the energy of the photon will be taken into the atom (leaving the atom in a higher energy state).

  
  

• A photon with frequency v will be absorbed by an atom if the energy of the photon corresponds to an energy level difference between allowed states in the atom.

What happens next?

• Remember that entropy seeks the lowest available energy level for all things, so the electron which has been raised to an excited orbit will eventually drop back to the ground state.

• Conservation of Energy, tells us that the energy difference between the excited state and ground state must appear somewhere when the electron makes the transition. It is emitted by the atom as a photon, with the same energy of the original one which was absorbed.

  
  

Here is a schematic diagram of the allowed orbits in a hydrogen atom. If you can answer the questions listed below, you've got the right idea!

• Which transition(s) correspond(s) to the absorption of a photon? A & D
• Which transition corresponds to the highest energy photon emitted ? C
• Which transition corresponds to the shortest wavelength photon emitted? C
• Which transition corresponds to the highest frequency photon emitted? C