2.2. Some chemical definitions

Next, I would like to explain some basic definitions that I consider you should know to understand some concepts that we will see later. You may already know them as they are basic notions about chemistry that we all study at school, but I would like to brush up on them just in case:

      Atomic number: indicates the number of protons in an element. It helps us to distinguish elements from one another

      Mass number: is the addition of the number of protons and number of neutrons (electrons are ignored).

      Isotopes: those atoms of the same element that contain equal number of protons but unequal number of protons.

      Atomic mass: calculated from mass numbers of several isotopes that are from the same element. I’ll give you an example to clarify:
Gallium is a chemical element with two isotopes 69Ga  and  71Ga whose relative abundances are 60.2% and 39.8%, so the atomic mass is:
Gallium atomic mass = [ (69*60.2) + (71*39.8) ] / 100 = 69.7 u (being u: atomic mass unit, which is approximately the mass of one nucleon, either a single proton or neutron).

      Radioisotopes: those isotopes with a more stable electronic configuration by means of the emission of protons, neutrons and/or electrons.

      Periodic Table: table where the elements are organised according to their atomic number and distributed in rows and columns conforming to shared physical and chemical properties.

      Ions: those atoms that are more stable when they gain or lose electrons. In the first case, they are named anions or negative ions (e.g.: fluoride (F-), sulfide (S2-), and in the second they are called cations or positive anions (e.g.: sodium (Na+), lead (Pb2+)).

      Molecule: two or more atoms joined by chemical bonds.

      Chemical reaction: it occurs when two or more atoms are joined by bonds to form a molecule or when joined atoms are broken down.

      Reactives: substances used at the beginning of a chemical reaction, normally on the left side of the reaction.

      Products: subtances resulting from the end of a reaction, usually on the right side of a reaction.

      Electronegativity: tendency of an atom nucleus to attract electrons. It increases as we move to the top and right along the periodic table.
Chemical reaction diagram

[1] Partículas que constituyen el núcleo atómico: protones y neutrones.

Source: OpenStax College, Biology. OpenStax College. 30 May 2013.

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