Chemistry: Materials, elements and the Periodic Table

All matter can be classified into three main types. An 'element' is a pure substance consisting of only one type of atom, which cannot be broken down into anything simpler by chemical means (e.g., Iron, Fe). A 'compound' is a pure substance formed when two or more different elements are chemically bonded together in a fixed ratio. The properties of a compound are completely different from its constituent elements (e.g., Water, H₂O, is very different from hydrogen and oxygen). A 'mixture' consists of two or more substances (elements or compounds) that are not chemically bonded. They can be separated by physical means, and the substances in a mixture largely keep their original properties (e.g., air is a mixture of nitrogen, oxygen, and other gases).

Atoms are the building blocks of elements. They are made of three subatomic particles: protons, neutrons, and electrons. Protons (positive charge, +1) and neutrons (no charge, 0) are found in the center of the atom, in the nucleus. Electrons (negative charge, -1) orbit the nucleus in shells. The 'atomic number' (Z) of an element is the number of protons in its nucleus, and this number uniquely defines the element. The 'mass number' (A) is the total number of protons and neutrons in the nucleus. In a neutral atom, the number of electrons is equal to the number of protons, balancing the overall charge to zero.

The periodic table is a masterpiece of organization, arranging all known elements in order of increasing atomic number. The horizontal rows are called 'Periods'. Elements in the same period have the same number of electron shells. The vertical columns are called 'Groups'. Elements in the same group have the same number of electrons in their outermost shell, which gives them similar chemical properties. For example, all elements in Group 1 are highly reactive metals. A zig-zag line, sometimes called the 'staircase', separates the metals (on the left) from the non-metals (on the right).

Elements can be broadly classified into two main categories: metals and non-metals. They have very different sets of typical physical properties. Metals (like iron, copper, gold) are typically strong, shiny (lustrous), malleable (can be hammered into shape), ductile (can be drawn into wires), and are excellent conductors of heat and electricity. Most are solid at room temperature (mercury is a key exception). Non-metals (like sulfur, oxygen, chlorine) are typically dull, brittle when solid, and are poor conductors of heat and electricity (insulators). They exist as solids, liquids, or gases at room temperature.

A pure metal consists of atoms of the same size arranged in regular layers. These layers can slide over each other easily, which makes pure metals relatively soft. An 'alloy' is a mixture created by adding another element (often another metal or carbon) to a pure metal. This introduces atoms of a different size into the metal's structure. These different-sized atoms distort the regular layers, making it much more difficult for them to slide. As a result, alloys are harder, stronger, and often more resistant to corrosion than their parent metals. Examples include steel (iron and carbon), brass (copper and zinc), and bronze (copper and tin).

Particle diagrams are simple drawings used to represent the arrangement of atoms and molecules in elements, compounds, and mixtures. A single circle represents one atom. For an 'element', all the circles are identical and separate (e.g., Argon, Ar). For a 'diatomic element', the circles are identical but joined in pairs (e.g., Oxygen, O₂). For a 'compound', different circles (representing different elements) are joined together in fixed groups or molecules (e.g., water, H₂O, would be one large circle joined to two smaller ones). For a 'mixture', different types of particles (which could be single atoms, diatomic molecules, or compound molecules) are shown in the same container but are not joined to each other.