A pure substance (usually referred to simply as a substance) is matter that has distinct properties and a composition that does not vary from sample to sample. Water and table salt (sodium chloride) are examples of pure substances.
A chemical element is an atom characterized by a certain number of protons. A set of atoms of the same chemical element, taken individually have the same number and the same disposition of the electrons but can differ, without prejudice to the number of protons, for the atomic mass, or the number of neutrons.
Two or more atoms are said to belong to the same chemical element if they are characterized by the same atomic number (Z), that is by the same number of protons. Atoms of the same element may differ instead by the number of neutrons (mass); in particular atoms of the same chemical element but with different neutron numbers are called isotopes.
The origin of the chemical elements in the universe is usually derived from the astrophysical theory of the life cycle of the stars.
Large stars (supernovae) in fact end their existence by continuing to burn nuclear fuel: starting from the first nuclear fusion reactions that involve hydrogen to form helium, at the end of the fuel represented by hydrogen and the subsequent gravitational collapse of the star (due to lack of balance due to the absence of reactions) the consequent increase in temperature triggers successive nuclear reactions involving helium to form other heavier and more complex elements and so on in a long chain of nuclear reactions that lead to the formation of all other chemical elements. The final explosion of the supernova leads to the dispersion of various chemical elements in the universe.
A chemical compound is a substance that can be broken down into two or more simpler different chemically bonded components (because it has more than one element) with a fixed ratio determining the composition.
Chemical formulas indicate the nature and number of atoms present in a compound and, consequently, the relationships between these atoms. The name with which the compound itself is designated must be able to give the same indications as to the formula, defining, with one or more words, the main characteristics in a unique way.
Types of chemical compounds
There are four types of compounds, depending on how the constituent atoms are held together:
- molecules held together by covalent bonds;
- ionic compounds are composed of ions held together by electrostatic forces termed ionic bonding;
- intermetallic compounds held together by metallic bonds;
- certain complexes (coordination complexes) held together by coordinate covalent bonds.
Most molecular compounds derived from the combination of two or more non-metallic elements. Molecular compounds take the form of discrete molecules.
An ionic compound can be defined as a chemical compound formed by ions (monoatomic or polyatomic) having an overall zero charge. The compound is neutral overall, but consists of positively charged ions called cations and negatively charged ions called anions.
In a binary ionic compound, there are generally a metal and a non-metal, where the metal usually yields one or more electrons to the non-metal, positively charging. Then there are ternary ionic compounds, which are the salts of oxyacids, such as NaClO.
The force that unites the ions is of electrostatic type and is defined ionic bond, unlike molecular compounds where the electrons are partially shared between the atomic groups in a covalent bond.
Intermetallic compounds (also called an intermetallic alloy, ordered intermetallic alloy, and a long-range-ordered alloy) are a type of metallic alloy that forms a solid-state compound exhibiting defined stoichiometry and ordered crystal structure.
- Amount of substance (mole)