Key concepts in chemistry Revision Notes

Dalton's Atomic Model (Early 19th Century)

John Dalton posited that matter is composed of indivisible, solid spheres called atoms. He believed that all atoms of the same element are identical in mass and properties, and different elements have different atoms. Chemical reactions involved the rearrangement of these indivisible atoms.

Discovery of Electrons (Late 19th Century) and Thomson's Model

J.J. Thomson's experiments with cathode rays led to the discovery of electrons – negatively charged subatomic particles. This disproved Dalton's idea of indivisible atoms. Thomson proposed the 'plum pudding' model, where the atom was a sphere of uniformly distributed positive charge with negatively charged electrons embedded within it, like plums in a pudding.

Discovery of the Nucleus (Early 20th Century) and Rutherford's Model

Ernest Rutherford's gold foil experiment challenged the plum pudding model. Alpha particles were fired at a thin gold foil; most passed through, but some were deflected at large angles, and a very few bounced back. This indicated a small, dense, positively charged nucleus at the atom's centre, with electrons orbiting around it, largely in empty space. This is often called the nuclear model.

Bohr's Refinements (Early 20th Century)

Niels Bohr further refined Rutherford's model. He proposed that electrons orbit the nucleus in specific, fixed energy levels or 'shells'. Electrons can move between these shells by absorbing or emitting specific amounts of energy. This explained the stability of atoms and their emission spectra.

Discovery of Protons and Neutrons (Early 20th Century)

Further research showed that the positively charged nucleus is made up of smaller particles: protons (positively charged) and neutrons (no charge). This established the modern atomic model: a central nucleus containing protons and neutrons, surrounded by electrons in shells. Protons and neutrons are massive compared to electrons.

Basic Atomic Composition

Every atom consists of a central nucleus surrounded by electrons. The nucleus is where the vast majority of an atom's mass is concentrated. Electrons orbit the nucleus in specific energy levels.

Nucleus: Protons and Neutrons

The nucleus is made up of two types of subatomic particles: protons and neutrons. Protons have a relative charge of +1 and a relative mass of 1. Neutrons have no charge (0) and a relative mass of 1. The atomic number of an element is determined by the number of protons.

Electrons and Electron Shells

Electrons are negatively charged particles with a relative charge of -1 and a very small relative mass (approximately 1/1836th of a proton). They are arranged in electron shells, also known as energy levels, around the nucleus. In a neutral atom, the number of electrons is equal to the number of protons.

Definition of Isotopes

Isotopes are atoms of the same element that have the identical number of protons but distinct numbers of neutrons. This means they have the identical atomic number but different mass numbers.

Protons Determine the Element

The number of protons in an atom's nucleus is what defines a specific element. For example, all atoms with 6 protons are carbon. Changing the number of protons changes the element itself.

Neutrons Affect Mass, Not Element Identity

Varying the number of neutrons in an atom changes its overall mass, creating different isotopes of the same element. This variation does not alter chemical properties significantly because the number of electrons, which dictates chemical behaviour, remains tied to the constant number of protons.

Examples of Isotopes

Hydrogen has three common isotopes: protium (1 proton, 0 neutrons), deuterium (1 proton, 1 neutron), and tritium (1 proton, 2 neutrons). All are hydrogen because they all have one proton.

Subatomic Particles Overview

Atoms consist of protons, neutrons (both in the nucleus), and electrons (orbiting the nucleus). Understanding their charges and masses is fundamental to chemistry. Relative values are used for simplicity, comparing them to a standard (the mass of a proton or neutron).

Relative Charge Values

Protons have a relative charge of +1. Neutrons have a relative charge of 0, meaning they are neutral. Electrons have a relative charge of -1. These charges largely determine the electrostatic interactions within an atom and between atoms.

Relative Mass Values

Protons have a relative mass of 1. Neutrons also have a relative mass of 1. Electrons have a very small relative mass, approximately 1/1836. This means the mass of an atom is almost entirely concentrated in its nucleus.

Summary Table

It's helpful to remember these facts: Proton: relative charge +1, relative mass 1. Neutron: relative charge 0, relative mass 1. Electron: relative charge -1, relative mass 1/1836 (or effectively 0 for many calculations).