Irish Leaving Certificate Chemistry

Chemistry is a way of finding out how matter behaves and then explaining that behaviour with models. The facts in the subject were established by people who measured carefully, checked each other's work and changed their ideas when the evidence required it, so the methods matter as much as the results. ## Understanding about chemistry: models and their limits A **model** is a simplified representation of a system or phenomenon, built on stated assumptions. A model is useful when it explains observations and predicts what will happen in a new situation. Every model also has limits, and knowing where it stops working is part of knowing the model. The kinetic theory treats particles as tiny hard spheres in constant motion. That explains pressure, diffusion and changes of state, but it ignores the forces between particles and the space the particles occupy, so it fails for real gases at high pressure and low temperature. The Bohr model of the atom explains the line spectrum of hydrogen very well and fails for atoms with more than one electron. Ball-and-stick models show the shape of a molecule and hide the fact that electrons spread over regions of space rather than sitting on sticks. Models change when new data arrives. Dalton's solid atom gave way to Thomson's atom with electrons in it, then to Rutherford's nuclear atom, Bohr's energy levels and finally the orbital model. Each step kept what the earlier model explained and added what it could not. Older models remain in use…