The nuclear model of the atom

At the heart of chemistry and physics is the atom, the basic building block of all matter. The modern model describes the atom as being composed of three fundamental subatomic particles. At the centre is a tiny, incredibly dense nucleus. The nucleus contains positively charged protons and uncharged neutrons. Protons and neutrons are collectively called nucleons, and they account for almost all of the atom's mass. Orbiting the nucleus in regions of space called shells or energy levels are negatively charged electrons. Electrons are vastly lighter than nucleons. In a neutral atom, the number of electrons is equal to the number of protons, so the overall charge is zero. The powerful electrostatic force of attraction between the positive nucleus and negative electrons keeps the electrons in their orbits, while an even stronger force, the strong nuclear force, binds the protons and neutrons together within the nucleus, overcoming the electrostatic repulsion between the protons.

To distinguish between different atoms, we use two key numbers: the atomic number and the mass number. The atomic number (symbol Z), also called the proton number, is the number of protons in an atom's nucleus. This number is fundamental because it defines the element. For example, any atom with 6 protons is a carbon atom, and any atom with 8 protons is an oxygen atom. The mass number (symbol A), also called the nucleon number, is the total number of protons and neutrons in the nucleus. Since we know the number of protons (Z) and the total number of nucleons (A), we can easily find the number of neutrons (N) by subtracting the atomic number from the mass number: N = A - Z. These numbers are written in a standard format called nuclide notation: $^{A}_{Z}X$, where X is the chemical symbol for the element.

While the number of protons defines an element, the number of neutrons can vary. Atoms of the same element that have the same number of protons but different numbers of neutrons are called isotopes. For example, every carbon atom has 6 protons. However, some carbon atoms have 6 neutrons (giving a mass number of 12), while others might have 7 neutrons (mass number 13) or 8 neutrons (mass number 14). These are all isotopes of carbon, written as carbon-12 ($^{12}_{6}$C), carbon-13 ($^{13}_{6}$C), and carbon-14 ($^{14}_{6}$C). Because isotopes of an element have the same number of protons, they also have the same number of electrons in their neutral state. Since chemical reactions are determined by the electron configuration, isotopes have identical chemical properties. However, their different masses and nuclear structures mean they can have different physical properties, such as density, rate of diffusion, and nuclear stability (i.e., whether they are radioactive).

Before 1911, the accepted model of the atom was J.J. Thomson's 'plum pudding' model, which suggested a sphere of positive charge with negative electrons embedded within it. To test this, Ernest Rutherford's team conducted the famous alpha particle scattering experiment. They fired a narrow beam of fast-moving, positively charged alpha particles at a very thin sheet of gold foil. Based on the plum pudding model, they expected the particles to pass straight through with only minor deflections. The results were astonishing and led to three crucial conclusions:

1. Observation: Most alpha particles passed straight through the foil undeflected. Conclusion: The atom is mostly empty space.
2. Observation: A small number of alpha particles were deflected by small angles. Conclusion: There is a concentration of positive charge in the atom that repels the positive alpha particles. This was named the nucleus.
3. Observation: A very tiny fraction (about 1 in 8000) were deflected by more than 90°, some even bouncing straight back. Conclusion: The nucleus must be extremely small, dense, and contain most of the atom's mass to cause such a strong repulsion and recoil.

These results destroyed the plum pudding model and gave birth to the nuclear model of the atom: a tiny, massive, positive nucleus surrounded by a large volume of empty space in which electrons orbit.