Basic Concepts about Chemistry
Chemistry is the science that studies the matter and its changes. Those changes are called chemichal changes, and they are mostly involved in making and braking what we know as chemichal bonds. When this thing occurs, we are talking about a chemichal reaction, where the reactants or reagents react each other to yield products. Those reactions can be classifed as: reversibles (most of them) and irreversibles.
But, if chemistry studies matter, we have to define what matter is. Matter is any substance present in Universe, which are divided into two major groups: pure substances and mixtures. Pure substances are sub-divided into two groups: elements (formed by a unique kind of atoms) and compounds (formed by two or more kinds of atoms). The former are present in an atomic way and are able to join each other to form molecules. A molecule is a substance that is composed by two or more atoms, it doesn't matter if they are different or not. Compunds are only present in a molecular way.
On mixtures, we can also find two sub-divisions: heterogeneous mixtures and homogeneous mixtures. The hetegeneous mixtures are those that their components are distinguishable to the nacked eye and there are many types, just as colloidals and emulsions. On the other hand, the homogeneous mixtures are those which their components are not distinguishable to the nacked eye, but they seem to be just only one substance. This category is also called dissolutions.
The chemichal elements are ordered in the periodic table of chemichal elements, which was arranged following an increasing order of number of protons of each atom (it's also called atomic number or Z). How it was said before, the elements are atoms, which are formed by 3 sub-atomic species: protons, neutrons (both of which are situated into the nucleus) and electrons (which are situated into the electronic cloud, atomic crust or electrosphere). The protons and electrons are charged particles (positively and negatively, respectively) with same magnitud, but neutrons have no charge (they are neutral). In a "normal" atom, there are same number of protons and electrons, that's why it's' said that an atom in electrically neutral. Atoms are characterized by some properties, just as the atomic number (defined previosly), the atomic number (A) (it's the sum between protons and neutrons), the chemichal symbol (every element has one, which represents its name in one or two letters), its atomic mass (sometimes it's also called molar mass) and its electronic configuration. We can stablish relations between two atoms following the properties described before. These relations created the following concepts: isotope (atoms that have same number of protons (same element), but have different number of neutrons), isobar (two different elements that have same atomic number), isotone (two different elements that have same number of neutrons) and isolelectronic elements (two different elements, with different number of neutrons, that have same number of electrons).
All these concepts and many others are going to be seen more deeply throughout this web site, this is only juast an approaching to some basics notions about some fundamental concepts, which will help you to understand Chemistry. Now, we are going to start our first chapter, revising some discoveries, which begin from the spheric notion that Dalton has about the atom to the actual atomic model, which rules the world nowadays.