Particles Study Guide

Atoms are the fundamental building blocks of all matter. Despite their tiny size, they have a specific internal structure. At the heart of every atom is a central core known as the nucleus. This nucleus carries a positive electrical charge, primarily due to the presence of protons. The nucleus is incredibly dense and contains almost all of the atom's mass. Orbiting this positively charged nucleus are much lighter, negatively charged particles called electrons. These electrons occupy a much larger region of space compared to the nucleus itself. Therefore, most of the volume of an atom is actually empty space. To put the scale into perspective, if an atom were the size of a football stadium, its nucleus would be roughly the size of a pea placed in the centre of the field. This highlights that the nuclear radius is vastly smaller than the overall radius of the atom. The opposing charges of the nucleus and the electrons keep the electrons bound to the atom, preventing them from flying off into space. The number of protons in the nucleus defines the element, and in a neutral atom, the number of electrons equals the number of protons, ensuring that the overall charge of the atom is zero.

The particle model helps us distinguish between physical and chemical changes by focusing on what happens to the particles (atoms, ions, or molecules) involved. In a physical change, the particles themselves remain unchanged. Their arrangement and spacing might alter, leading to changes in state (like melting ice or boiling water) or dissolution (like sugar dissolving in water), but the fundamental identity of each particle is preserved. For example, when ice melts, water molecules simply move more freely, but they are still H₂O molecules. No new substances are formed because the chemical bonds within the molecules are not broken or formed.

Conversely, a chemical change involves the breaking of existing chemical bonds and the formation of new chemical bonds. This rearrangement of atoms results in the creation of entirely new substances with different chemical properties. The original particles are chemically transformed into new types of particles. For instance, when wood burns, cellulose molecules (from the wood) react with oxygen molecules to form new molecules like carbon dioxide and water. The atoms are conserved, meaning no atoms are lost or gained, but they are re-bonded in new ways. This fundamental difference in how particles are affected – merely rearranged in physical changes versus chemically converted in chemical changes – is central to understanding these processes in terms of the particle model.