Particles Revision Notes

Subatomic Particles and Their Location

Atoms are the fundamental building blocks of matter, and they are composed of even smaller particles called subatomic particles. These include protons, neutrons, and electrons. Protons and neutrons are found together in the central part of the atom, which is known as the nucleus. The nucleus is tiny but contains almost all of the atom's mass. Electrons, on the other hand, are much lighter and orbit the nucleus in specific energy levels or shells. The arrangement of these electrons determines the atom's chemical properties and how it interacts with other atoms.

Relative Charges of Subatomic Particles

Each subatomic particle carries an electrical charge, or is neutral. Protons have a positive charge, conventionally assigned a relative charge of +1. Neutrons are electrically neutral, meaning they have no charge; their relative charge is 0. Electrons possess a negative charge, assigned a relative charge of -1. In a neutral atom, the number of positively charged protons is equal to the number of negatively charged electrons, meaning their charges cancel out, resulting in an overall charge of zero for the atom.

Approximate Relative Masses of Subatomic Particles

The mass of subatomic particles varies significantly. Protons and neutrons have approximately the same mass. For convenience in chemistry, they are both assigned an approximate relative mass of 1. Electrons are drastically lighter. Their mass is so small that it is often considered negligible when calculating the overall mass of an atom. The approximate relative mass of an electron is about 1/1836 (or sometimes simplified to 0 for most calculations) compared to a proton or neutron. This explains why an atom's mass is almost entirely contained within its nucleus.

The Basic Particle Model (Higher)

The particle model visualises matter as tiny, discrete particles. These particles are often represented as solid, inelastic spheres, similar to bowling balls. This simple model is fundamental for understanding basic properties of solids, liquids, and gases, such as their relative densities, ability to flow, and response to compression. It successfully explains phenomena like diffusion, where particles spread out, and gas pressure, caused by particles colliding with container walls.

Limitations for Explaining Changes of State (Higher)

While useful, the basic particle model, particularly when using inelastic spheres, has significant limitations when explaining changes of state. It fails to adequately represent the complex interactions occurring at a molecular level. One key limitation is that inelastic spheres cannot accurately show the nature of the forces between particles. In reality, strong intermolecular forces or electrostatic forces hold particles together in solids and liquids, requiring energy input to overcome them during melting or boiling. The 'inelastic sphere' model doesn't provide a way to visualise these forces or their disruption.

Energy Changes and Particle Representation (Higher)

Changes of state involve significant energy changes (latent heat) absorbed or released without a change in temperature. The particle model, using inelastic spheres, struggles to explain where this energy goes. It cannot demonstrate how absorbed energy increases the potential energy of particles by overcoming intermolecular forces, rather than increasing their kinetic energy (which would lead to a temperature rise). The spheres also cannot represent the different forms of internal energy (vibrational, rotational, translational kinetic energy) that particles possess and how these change during phase transitions.

Inadequate Representation of Particle Structure and Interactions (Higher)

The model's simplification of particles as identical spheres overlooks the diverse nature of actual particles (atoms, molecules, ions). Different substances have different types and strengths of intermolecular forces or electrostatic bonds, which determine their unique melting and boiling points. The inelastic sphere model cannot account for these distinctions, nor can it illustrate how specific bonds are broken or formed during changes of state. It simplifies interactions to 'bumping' rather than complex forces, making it hard to explain why specific energy inputs are required for different substances to change state.

Defining Physical Changes

Physical changes involve alterations to a substance's appearance or state, but not its chemical composition. In terms of the particle model, this means the individual particles (atoms or molecules) of the substance remain unchanged. Their arrangement, spacing, or movement might change, but their internal structure and identity are preserved. Examples include melting, freezing, boiling, condensing, dissolving, and grinding. These changes are often easily reversible.

Defining Chemical Changes

Chemical changes, also known as chemical reactions, result in the formation of one or more new substances with different chemical properties from the original substances. According to the particle model, a chemical change involves the breaking of existing chemical bonds and the formation of new chemical bonds between atoms. The atoms rearrange to form new molecules or ionic compounds. The original particles are consumed, and new particles are produced. These changes are often difficult to reverse.

Particle Model Application to Physical Changes

When a physical change occurs, such as water boiling, the H₂O molecules themselves do not change. They simply gain enough energy to overcome intermolecular forces and move further apart, transitioning from a liquid to a gas. If sugar dissolves in water, the sugar molecules disperse among the water molecules but retain their identity as sugar molecules. No new chemical bonds are formed, and no existing chemical bonds within the sugar or water molecules are broken. The particles are merely rearranged spatially.

Particle Model Application to Chemical Changes

During a chemical change, like the burning of methane (CH₄), the methane molecules react with oxygen molecules (O₂). The chemical bonds within CH₄ and O₂ molecules break, and the atoms (carbon, hydrogen, oxygen) rearrange and form new chemical bonds to produce carbon dioxide (CO₂) and water (H₂O) molecules. The original particles (methane and oxygen) are no longer present, having been transformed into entirely new particles (carbon dioxide and water). This breaking and forming of bonds is the key distinction.

The Central Nucleus

Every atom contains a central part called the nucleus. This nucleus is positively charged. It is made up of protons, which are positively charged, and neutrons, which have no charge. These subatomic particles are significantly heavier than electrons. The nucleus is extremely small in comparison to the overall size of the atom, but it contains almost all of the atom's mass.

Electrons and Their Location

Surrounding the positively charged nucleus are negatively charged particles called electrons. These electrons orbit the nucleus in specific energy levels or shells. They are much lighter than protons and neutrons. The electrons determine the chemical properties of an atom and occupy the vast majority of the atom's volume, making the atom mostly empty space.

Size and Mass Distribution in an Atom

The radius of the nucleus is considerably smaller than the radius of the entire atom. This means that if an atom were expanded to a macroscopic size, the nucleus would be an almost infinitesimally small dot at its centre. Despite its tiny size, the nucleus holds more than 99.9% of the atom's total mass, indicating that mass is highly concentrated within this central region. The electrons, while defining the atom's size, contribute very little to its overall mass.

Charge Balance in a Neutral Atom

The nucleus is positively charged due to its protons. The electrons are negatively charged. In a neutral atom, the number of positive charges (protons) in the nucleus is exactly equal to the number of negative charges (electrons) orbiting it. This balance results in a net electrical charge of zero for the atom as a whole. An imbalance in these charges results in an ion.