Particles Revision Notes

The Particle Model: Basic Concepts

The particle model describes all matter as being made up of tiny particles, which can be atoms, molecules, or ions. These particles are constantly in motion and have forces of attraction between them. The arrangement and movement of these particles determine the state of matter (solid, liquid, or gas). In solids, particles are tightly packed in a regular arrangement and vibrate in fixed positions. In liquids, particles are close together but randomly arranged and can move past each other. In gases, particles are far apart, randomly arranged, and move rapidly in all directions. Understanding this model is key to distinguishing between physical and chemical changes.

Physical Changes: No New Substances Formed

A physical change is a reversible process where the substance's identity remains the same, even though its physical properties might change. At the particle level, this means the chemical bonds within the particles are not broken, and no new chemical bonds are formed. The particles themselves do not change. Instead, physical changes involve changes in the arrangement, spacing, or movement of particles, such as during changes of state (melting, boiling, freezing, condensing) or dissolution. For example, when ice melts, its water molecules (H₂O) simply go from a fixed structure to a more mobile one; they are still H₂O molecules. Dissolving sugar in water is another physical change where sugar particles are dispersed among water particles but retain their chemical identity.

Chemical Changes: New Substances Formed

A chemical change, also known as a chemical reaction, results in the formation of one or more new substances with different chemical properties. This process is generally irreversible. In terms of the particle model, a chemical change involves the breaking of existing chemical bonds between atoms and the formation of new chemical bonds, leading to the rearrangement of atoms to create new particles (molecules or compounds). The original particles are transformed into new, chemically distinct particles. Evidence of a chemical change might include a change in colour, the production of gas, a change in temperature, or the formation of a precipitate. For instance, burning wood involves wood particles reacting with oxygen to form ash, carbon dioxide, and water vapor – entirely new substances.

Dalton's Model (Early 19th Century)

John Dalton proposed the first scientific atomic theory. He suggested that atoms were indivisible, solid spheres that could not be created or destroyed. He also stated that all atoms of an element are identical, and atoms of different elements are different. This model was a crucial step but lacked detail about the internal structure of an atom, as electricity had not yet been fully understood in terms of subatomic particles.

Thomson's 'Plum Pudding' Model (Late 19th Century)

J.J. Thomson discovered the electron through experiments with cathode rays. This led him to propose the 'plum pudding' model. In this model, the atom was depicted as a sphere of uniformly distributed positive charge, with negatively charged electrons embedded within it, much like plums in a pudding. This model explained the existence of subatomic particles and the overall neutrality of atoms, but it would later be disproven by experimental evidence.

Rutherford's Nuclear Model (Early 20th Century)

Ernest Rutherford's gold foil experiment radically altered the atomic model. By firing positively charged alpha particles at a thin gold foil, he observed that most passed through directly, but some were deflected significantly, and a few bounced back. This led to the conclusion that the atom consists primarily of empty space, with a tiny, dense, positively charged nucleus at its centre. Electrons were thought to orbit this nucleus. This model accounted for the deflection of alpha particles but didn't explain electron stability.

Bohr's Shell Model (Early 20th Century)

Niels Bohr refined Rutherford's model by proposing that electrons do not orbit randomly but exist in specific, fixed energy levels or 'shells' around the nucleus. Electrons can move between these shells by absorbing or emitting discrete packets of energy. This improved model explained why electrons do not spiral into the nucleus and the characteristic light spectra emitted by heated elements. Further discoveries led to the current understanding of the nucleus containing protons and neutrons, and electrons occupying more complex orbitals.

Introduction to Subatomic Particles

Atoms are the basic units of matter and are made up of three primary subatomic particles: protons, neutrons, and electrons. Understanding their properties is fundamental to chemistry. Protons and neutrons reside in the dense central nucleus, while electrons orbit the nucleus in electron shells. The number and arrangement of these particles determine an atom's identity and chemical properties.

Relative Charges of Subatomic Particles

Each subatomic particle possesses a specific electrical charge. Protons carry a positive charge, conventionally given a relative value of +1. Electrons carry a negative charge, with a relative value of -1. Neutrons, as their name suggests, are neutral and have a relative charge of 0. In a neutral atom, the number of protons equals the number of electrons, so the positive and negative charges balance out.

Approximate Relative Masses of Subatomic Particles

The mass of subatomic particles is typically expressed as approximate relative masses. Protons and neutrons have approximately the same mass, each assigned a relative mass of 1 atomic mass unit (amu). In contrast, electrons are significantly lighter. Their approximate relative mass is about 1/1836 amu. For many calculations, the mass of an electron is often considered negligible and approximated as 0 when determining the overall mass of an atom, as the nucleus accounts for almost all of the atom's mass.

Location of Subatomic Particles

Protons and neutrons are located in the nucleus, which is at the centre of the atom. The nucleus is extremely small but contains almost all of the atom's mass. Electrons are located outside the nucleus, moving in electron shells or energy levels. These shells determine the atom's volume and how it interacts with other atoms.

What is an Atom?

An atom is the smallest unit of an element that retains the chemical identity of that element. It is composed of a nucleus (containing protons and neutrons) surrounded by electrons. Atoms are incredibly small, far beyond the reach of standard optical microscopes. Their minute size is a fundamental aspect of chemistry.

Typical Size of an Atom

The typical diameter of an atom is approximately 0.1 nanometres (nm). To put this into perspective, a nanometre is one billionth of a metre (1 nm = 10⁻⁹ m). This means an atom's diameter is around 1 x 10⁻¹⁰ metres. This value represents an 'order of magnitude', meaning it's a general scale, rather than an exact fixed size for every single atom, as atomic sizes can vary slightly depending on the element.

Typical Size of Small Molecules

A small molecule consists of two or more atoms chemically bonded together. Because they are made of a few atoms, small molecules are slightly larger than individual atoms but are still within a similar order of magnitude. Their typical diameter ranges from about 0.1 nanometres to 1 nanometre. Examples of small molecules include water (H₂O) or carbon dioxide (CO₂). This again highlights the nanoscale nature of fundamental chemical building blocks.

Connecting Size to Measurement

The extremely small size of atoms and small molecules means they cannot be seen with ordinary light microscopes. The wavelength of visible light is much larger than these particles, making it impossible to resolve them. Advanced techniques like electron microscopy are required to visualise objects at the nanoscale, providing indirect images of atomic and molecular structures.