Atomic Structure
The Structure of the Atom
An atom is the smallest part of an element that can exist. Atoms are incredibly small — a typical atomic radius is about \(1 \times 10^{-10}\) m. Each atom has a tiny, dense nucleus at its centre. The nucleus contains two types of subatomic particle: protons (positive charge) and neutrons (no charge). Electrons orbit the nucleus in energy levels (shells) and carry a negative charge.
Relative charges and approximate relative masses: Proton — charge +1, mass 1. Neutron — charge 0, mass 1. Electron — charge −1, mass approximately \(\frac{1}{1836}\) (negligible). The nuclear radius is much smaller than the radius of the atom — most of the atom is empty space.
Atomic Number and Mass Number
The atomic number tells you the number of protons in the nucleus. It defines what element an atom is — all atoms of the same element have the same atomic number. In a neutral atom, the number of protons equals the number of electrons, so the atom has no overall charge. The mass number is the total number of protons plus neutrons.
To calculate the number of neutrons:
$$\text{number of neutrons} = \text{mass number} - \text{atomic number}$$
For example, sodium (Na) has atomic number 11 and mass number 23, so it has \(23 - 11 = 12\) neutrons, 11 protons and 11 electrons.
Isotopes and Ions
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. This means they have different mass numbers but identical chemical properties. For example, carbon-12 has 6 protons and 6 neutrons, while carbon-14 has 6 protons and 8 neutrons.
An ion is an atom that has gained or lost electrons. Losing electrons forms a positive ion (cation) because there are now more protons than electrons. Gaining electrons forms a negative ion (anion). The nucleus does not change when an ion forms — only the number of electrons changes.
Standard notation shows mass number (top left), atomic number (bottom left) and chemical symbol. For example, \(^{23}_{11}\text{Na}\) tells you sodium has mass number 23 and atomic number 11. For ions, the charge is shown as a superscript: \(\text{Na}^+\) has lost one electron, \(\text{Cl}^-\) has gained one electron.
The Development of the Atomic Model
Our understanding of the atom has changed over time as new experimental evidence emerged.
Dalton’s Model
John Dalton proposed atoms were tiny, solid, indivisible spheres. He thought all atoms of the same element were identical.
Thomson’s Plum Pudding Model
J.J. Thomson discovered the electron and proposed a ball of positive charge with negative electrons embedded in it.
Rutherford, Geiger & Marsden
They fired alpha particles at thin gold foil. Most passed straight through (the atom is mostly empty space), but some deflected at large angles and a few bounced back. This disproved the plum pudding model. Rutherford proposed the nuclear model: a small, dense, positively charged nucleus with electrons orbiting around it.
Bohr’s Model
Niels Bohr refined the model by placing electrons in fixed energy levels at specific distances from the nucleus, rather than orbiting randomly.
The alpha scattering experiment disproved the plum pudding model. Most alpha particles passed through (mostly empty space), some bounced back (small, dense, positive nucleus).
Electronic Configuration
Electrons occupy energy levels (shells) around the nucleus. The first shell holds a maximum of 2 electrons. The second and third shells each hold a maximum of 8. Electrons fill the lowest available energy level first.
Find the element’s atomic number (= number of electrons in a neutral atom). Fill the first shell with up to 2, the second with up to 8, and the third with up to 8. For example: oxygen (8 electrons) is 2, 6. Sodium (11 electrons) is 2, 8, 1. Calcium (20 electrons) is 2, 8, 8, 2. The number of outer-shell electrons matches the group number for Groups 1–7, explaining why elements in the same group react similarly.
The electronic configuration determines how an element reacts. Group 1 elements have one outer electron and lose it easily. Group 7 elements have seven outer electrons and tend to gain one. Group 0 elements have full outer shells and are very unreactive.