Chemistry is a science that studies the interaction between atoms and molecules, some of the most basic forms of matter. An atom is what we refer to as the basic building block of the materials and substances that make up the universe. An atom is composed of protons, neutrons, and electrons.
As can be seen from the picture, the protons and neutrons are clustered together in the center of the atom, forming what is referred to as the nucleus. This part of the atom contains the most mass and it takes a large amount of energy to remove a proton or neutron from the nucleus. Orbiting the nucleus are the electrons. Electrons possess very little mass relative to protons and neutrons, thus they are more easily moved.
Protons are positively charged particles, electrons are negatively charged, and neutrons, as their name may suggest, possess a neutral electrical charge. The charges are very important as they are the basis for chemical reaction.
Atoms are classified as elements. Each element has a specific number of protons, starting at one with hydrogen. The number of neutrons and electrons in an atom does not affect its status as a member of a given element, only the protons. The number of protons is referred to as the atomic number and is very important.
An atom with a specific number of neutrons is referred to as an isotope of its element. For example, a hydrogen atom with two neutrons is referred to deuterium, but it is still a hydrogen atom. A hydrogen atom with three neutrons is tritium, but is still a hydrogen atom. Interestingly enough, the number of neutrons has interesting effects on the properties of atoms, particularly when involved with nuclear reactions.
The mass number of an atom is essentially the number of protons and neutrons added together. The periodic table lists average mass numbers for an element since atoms have a number of possible isotopes, but we do know the relative presence of each isotope in the universe, thus we are able to calculate an average mass number. Take for example carbon. Most carbon atoms (98% or so) are isotopes with 6 neutrons, arriving at a mass number of 12. However, there are also other isotopes, such as carbon-14 and carbon-13, which raised the average mass number to approximately 12.01. The whole average mass number business is done for reasons of practicality; we cannot feasibly measure the mass number of each atom involved in a reaction, so we use an average, which is accurate enough for most purposes.
The reactivity of elements is dependent on the number of valence electrons available. Valence refers to the outermost shell of electrons on the atom. For clarification, as an atom gains electrons, they fill up shells in a specific order. A shell, or orbital, is simply an area in which the electron is most likely to be found at a given time. The first two shells filled are of the s-type, which is just a spherical orbit. Then p-orbitals start appearing and, finally, the d-orbitals. The s-orbital holds two electrons, the p-orbitals six, and the d-orbitals ten. The periodic table is arranged so that one can easily see which atoms normally have which orbitals. As one moves left to right and down in the periodic table, the number of shells and electrons generally increases. Looking at the periodic table, the first two columns make up the s- orbitals, while columns three through twelve make up the d-orbitals, and thirteen through 18 the p-orbitals. Remember, however, that the number of electrons per atom is not guaranteed, as atoms form ions.
An ion is basically an atom or molecule that does not possess a neutral charge. An anion has a negative charge, or a surplus of electrons, while a cation has a positive charge, or a deficiency of electrons. Since atoms and molecules react in an attempt to reach a more stable state, ions are unstable and very reactive.
The reason behind the reactions in chemistry is that atoms are attempting to fill their valence shells. The goal is 8 electrons, which is the exact number that the noble gases possess, explaining why they are not very reactive at all.