Elements, compounds and mixtures Study Guide

The position of an element in the Periodic Table is directly linked to the arrangement of electrons in its atoms and its atomic number. Each element is uniquely identified by its atomic number, which represents the number of protons in the nucleus of an atom. In a neutral atom, the number of electrons is equal to the number of protons, so the atomic number also tells us the total number of electrons.

Elements in the same group (vertical column) of the Periodic Table have similar chemical properties because their atoms have the same number of electrons in their outermost shell (valence electrons). For example, all elements in Group 1 have one valence electron, making them very reactive metals. Group 0 (or 18) elements, the noble gases, have a full outer shell of electrons (usually 8, except helium with 2), making them very unreactive.

The period (horizontal row) an element is in corresponds to the number of electron shells its atoms possess. For instance, elements in Period 1 have one electron shell, Period 2 elements have two electron shells, and so on. As you move across a period from left to right, the atomic number increases by one for each successive element, meaning each element has one more proton and, in a neutral atom, one more electron than the element before it. These additional electrons fill the existing electron shells until a new shell is started when moving to the next period.

The surface area to volume ratio (SA:V) describes the amount of surface exposed relative to the total volume of an object. This ratio is crucial in chemistry because it influences how quickly reactions occur and how effectively properties like heating or cooling manifest. For a given material, as the size of its particles decreases, its total surface area increases significantly while its total volume remains the same. Imagine a large cube. If you cut it into smaller cubes, the total volume of all the small cubes combined is the same as the original large cube. However, the sum of the surface areas of all the small cubes is much greater than the surface area of the original large cube. This means smaller particles have a much higher surface area to volume ratio than larger particles. A high surface area to volume ratio allows for more contact points with the surroundings. For example, in a chemical reaction, this means more particles are accessible to react at any given time, leading to a faster reaction rate. Catalysts often work by increasing the surface area available for reactions. Similarly, materials with a high SA:V ratio can absorb or release heat more quickly. For instance, crushed ice melts faster than an ice cube of the same mass because its larger surface area is exposed to the warmer surroundings.