The structure of the atom (protons, neutrons and electrons), relative charges and masses, atomic number and mass number, isotopes, electronic structure, and the development of the model of the atom from Dalton to the nuclear model.

What this topic is asking

OCR wants you to describe the sub-atomic particles and their relative charges and masses, define atomic number and mass number, explain isotopes, write electronic structures, and outline how the model of the atom developed.

Inside the atom

The three sub-atomic particles have these relative charges and masses: a proton has a charge of $+1$ and a relative mass of $1$; a neutron has no charge ($0$) and a relative mass of $1$; an electron has a charge of $-1$ and a relative mass of about $\dfrac{1}{1840}$ (so small it is usually taken as negligible). Because an atom has equal numbers of protons and electrons, the positive and negative charges cancel and the atom is neutral overall. A typical atom has a radius of about $1 \times 10^{-10}$ m, and the nucleus is around $10{,}000$ times smaller still.

Atomic number, mass number and isotopes

For example, chlorine has two common isotopes: chlorine-35 ($17$ protons, $18$ neutrons) and chlorine-37 ($17$ protons, $20$ neutrons). Isotopes of an element have the same chemical properties because chemistry depends on the electrons, and isotopes have the same number and arrangement of electrons. Their physical properties (such as density) differ slightly because of the different masses.

Electronic structure and the developing model

Electrons occupy shells around the nucleus, filling the lowest (innermost) shell first. The first shell holds up to 2 electrons, the second up to 8, and the third up to 8 at this level. So sodium (11 electrons) is written $2, 8, 1$, and chlorine (17 electrons) is $2, 8, 7$. The number of electrons in the outer shell links directly to an element's group in the periodic table and its reactivity.

The model of the atom changed as new evidence appeared. John Dalton pictured atoms as tiny solid spheres. J.J. Thomson discovered the electron and proposed the "plum pudding" model (a ball of positive charge with electrons embedded in it). Rutherford's alpha-particle scattering experiment showed that most of the atom is empty space with a small, dense, positive nucleus, giving the nuclear model. Bohr then refined it by placing electrons in fixed shells, and later the neutron was discovered. This shows how scientific models are revised when experiments give new evidence.