The atom consists of a central nucleus and surrounding 'cloud' of orbiting electrons.
Nucleus
The nucleus is the tightly packed central part of the atom. It contains two types of hadrons:
- Protons are positively charged hadrons, consisting of two up and one down quark $2(\frac23) + \frac{-1}{3} = 1$
- Neutrons are neutrally charged hadrons, consisting of one up and two down quarks $\frac23 + 2(\frac{-1}{3}) = 0$
The nucleus is held together by the residual strong force, which occurs between quarks of neighbouring nuclei. This force is very strong at incredibly short distances, and overcomes the repulsive electromagnetic force between the positively charged protons. The residual strong force is created by converting part of the mass of the nucleons into energy, known as the mass defect.
The number of protons in the nucleus determines the element as well as the atomic number of the atom. The number of neutrons and protons determines the atomic weight of the atom.
Orbital electrons
Normal atoms contain negatively charged leptons called electrons in equal number to their protons. If the number of electrons is different to the number of protons the atom is said to be ionised and will generally react strongly with other atoms.
In the atom, electrons are conceptually thought of as orbiting the nucleus in shells. Each shell may contain a set number of electrons. Once a shell is filled, electrons begin filling the next shell. The innermost shell is the K shell, and subsequent shells are lettered in sequence (L, M, N etc).
Binding energy of electrons
Electrons are attracted to the nucleus by the electromagnetic force. The strength of this force is dependent on the shell in which the electron is located. The energy required to overcome this force is the binding energy of the electron. If energy over this level is supplied to the electron, it may be ejected from the atom, causing ionisation. Smaller amounts of energy may be sufficient to lift the electron from one shell to a more distant shell. This is known as excitation of the nucleus. This excitation leaves a 'hole' in the shell, and an outer electron may fall back into this 'hole'. As it does so, energy is released as characteristic radiation.
Binding energy of the nucleus
The binding energy of the nucleus is equal (via $E=mc^2$) to the mass defect. It is the energy which is required to split the nucleons into individual individual nucleons.
