The nuclear model of the atom

Before we understood the modern atom, scientists had different ideas. In the early 1800s, John Dalton suggested atoms were tiny, indivisible spheres. Later, in 1897, J.J. Thomson discovered the electron, a tiny, negatively charged particle. This meant atoms could be divided. To explain this, Thomson proposed the 'plum pudding model' in 1904. He imagined the atom as a sphere of uniform positive charge, with negative electrons dotted throughout it, like plums in a pudding. The total positive charge balanced the total negative charge, making the atom neutral overall.

In 1911, Ernest Rutherford's team, Geiger and Marsden, conducted a groundbreaking experiment. They fired a narrow beam of fast-moving, positively charged alpha particles at a very thin sheet of gold foil inside a vacuum. A circular detector screen around the foil would light up wherever an alpha particle hit it, allowing them to observe the particles' paths. The results were completely unexpected and could not be explained by the plum pudding model.

The surprising results of the scattering experiment led Rutherford to propose a new model for the atom. He reasoned that the experimental observations could only be explained if the atom's structure was radically different from the plum pudding idea.

1. Most particles passed straight through: This implies that the atom is mostly empty space.
2. Some particles were deflected: The positive alpha particles were being repelled by a concentration of positive charge within the atom.
3. A few particles bounced back: This meant the alpha particles had hit something very small, very massive, and with a strong positive charge.

Rutherford concluded that the atom has a tiny, dense, positively charged nucleus at its centre, containing almost all the mass. The much lighter, negatively charged electrons orbit this nucleus at a distance, much like planets orbiting the Sun. The volume of the atom is defined by these orbits, explaining why the atom is mostly empty space.

The nuclear model was further refined with the discovery of the particles that make up the atom. An atom consists of three main subatomic particles: protons, neutrons, and electrons. Protons and neutrons are found together in the nucleus and are collectively called nucleons. Electrons orbit the nucleus in shells. The number of protons defines the element. In a neutral atom, the number of electrons is equal to the number of protons, so the overall charge is zero.

Two important numbers describe the composition of a nucleus: the Atomic Number (Z) is the number of protons, and the Mass Number (A) is the total number of protons and neutrons.

While the number of protons (Atomic Number, Z) 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. Because they have the same number of protons and electrons, isotopes of an element have identical chemical properties (e.g., how they react). However, they have different mass numbers and thus different physical properties, such as density and rate of diffusion. For example, all carbon atoms have 6 protons. Carbon-12 has 6 neutrons, while the radioactive isotope Carbon-14 has 8 neutrons. We use nuclide notation to represent them: the mass number (A) is written as a superscript and the atomic number (Z) as a subscript, both to the left of the element symbol (e.g., ¹⁴₆C).