Classification of Matter

AtomsJohn Dalton proposed in 1805 that elements were composed of small indivisible particles called atoms.He developed this idea in order to explain how elements always reacted in ratios of whole numbers. John Dalton proposed that atoms are simply rearranged in a chemical reaction. Nowadays, it is known that atoms are composed of smaller particles called protons, electrons and neutrons.

The nucleus is the centre of the atom and is composed of smaller particles called protons which are positively charged (1.6022 × 10–19 Coulombs) and neutrons which have no charge. These particles are collectively referred to as nucleons, as they are present in the nucleus of the atom. The mass of the proton is 1.6726 × 10–27 kilograms, while the neutron is similar in size with a mass of 1.6749 × 10–27 kilograms.

Electrons (e–) are small negatively charged particles which orbit the central positive charged nucleus of an atom. They have a mass significantly smaller than the proton (9.1094 × 10–31 kg), and have a charge equal to the proton. They orbit in energy levels called shells, which are different energy levels, which the electrons occupy when they orbit the nucleus.

Molecules A molecule is made of two or more atoms joined with covalent bonds (bonds between elements due to the sharing of electrons to obtain a stable octet in the outermost shell). In other terms, molecules consist of the smallest unit of a substance (covalently bonded substance) that can move independently.

All covalently-bonded compounds (two or more atoms bonded together) are molecules, even though some exist not as individual molecules but as a lattice, such as SiO2 (silicon dioxide – sand) or C (carbon – diamond). Other molecules include when the same element is bonded with itself e.g. H2, O2 or N2.

Monatomic molecules are molecules with only one atom include the noble gases, which exist as uncombined elements, and therefore as single elements. Some examples include: He (helium), Ne (neon) and Ar (argon).

Diatomic molecules include some elements such as O2 (oxygen), H2 (hydrogen), N2 (nitrogen), and some compounds such as hydrogen iodine HI and hydrogen fluoride HF.

Diatomic molecules can also be compounds such as HCl (hydrochloric acid) but not ionic compounds such as NaCl (sodium chloride – table salt)

Some triatomic molecules include ozone O3, water H2O and sulfur dioxide SO2

Some tetratomic molecules include white phosphorus P4 and ammonia NH3

CompoundsA compound consists of two or more elements joined chemically. In compounds, the atoms form chemical bonds between the electrons in the outer shell of the atom. Unlike elements, compounds can be chemically broken down into their elemental components. Compounds are chemical bonded with fixed ratios. If they are not chemically bonded they may be amixture or alloy. For example in HCl, there is one hydrogen atom for every chlorine atom.

Ionic compounds are compounds formed between usually a metal and a non-metal, where the metal transfers its excess electrons to the non-metals forming an ionic compound.

Ionic compounds are lattices, and the formula used (empirical formula) represents the ratio of atoms in the lattice.

Covalent compounds are compounds formed between two or more non-metals, through the sharing of electrons to reach a stable octet in the outer shell of each element.

Covalent compounds are molecules. The formula used is a direct proportion to the structure of the molecule.

Ions

Ions are charged particles. Positive ions are formed when metal atoms lose electrons, e.g. Na+, Mg2+ or Al3+. Negative ions are formed when non-metal ions gain electrons e.g. Cl–, S2– or N3–

A polyatomic ion or radical is a charged particle made up of more than one type of atom, e.g. NH4+, or CO32–

Pure substances and impure substances Matter can either be classified by their state of matter at room temperature, either as a solid, liquid

or gas. Matter can also be classified into two categories, pure substances and impure substances.

Homogeneous and heterogeneous substances

Pure substances include elements and compounds. These substances have a fixed composition and properties and therefore are unable to be separated using simple physical separation methods.

Impure substances, also referred to as mixtures, have variable composition and properties and are able to be separated easily using various methods such as sieving, sedimentation and evaporation.

Homogeneous refers to a substance that is composed of a single substance for example elements or a mixture that has roughly a uniform composition or structure.

Heterogeneous refers to a mixture that is composed of different substances without a uniform composition or structure.