Water is one of the most important molecules in human life, not only because makes up more than two thirds of human body weight, but also because all the cell and organ functions depend on water for their functioning. Something similar occurs in animals and plants. This means that without water most kind of life on earth would disappear in a few days.

Water is composed of one Oxygen atom and two Hydrogen atoms. Oxygen has six electrons in its valence shell, and needs two electrons to acquire great stability, those two electrons are taken from two Hydrogen atoms.

The molecule of H2O is a three dimensional structure, Oxygen is the centre of the molecule, around him there are two pairs of electrons, and two Hydrogen atoms with angles around 107º among them.


Oxygen and Hydrogen have electronegativity values of  3.5 and 2.1 respectively, when they are bonded the shared electrons are attracted to the Oxygen side, leaving the Hydrogen atoms with a slightly positive charge, and producing a final polar molecule.

When a water molecule is surrounded by another water molecule, they will interact each other through those slight charges. This interaction is called “Hydrogen Bonding”, which is a weak electrostatic attraction between slight positive and negative charges of different molecules.


In a Non polar Covalent Bond, atoms share their electrons equally. That happens when, they have the same electronegativity or the same electron affinity. The best examples for this  kind of bonds are molecules created through the union of atoms of the same element, as in H2, N2, O2, Cl2  

When non-polar molecules interact among them, there are negligible forces of attraction, and they remain as individual molecules, that means these substances are gases.

In our environment we have these substances in the Air, which has approximately the following composition, 78% of N2, 21% of O2, 0.9% Ar (noble gas)




Covalent bonds involve the sharing of electrons by two atoms, in contrast to the transfer of electrons in Ionic Bonds.

Covalent Bond is another way to create a noble gas configuration for each atom. For example, Hydrogen has 1 electron, and needs another to have the same electronic configuration as gas noble Helium. Chloride, has 7 electrons, and with one more acquire the distribution of Argon.
However, when H-Cl bond is formed, there is still an unequal sharing of the electrons, because the electrons spend more time around the more nonmetallic atom, and more electronegative, in this case the Chloride, giving us a Polar Covalent Bond.
In a Polar Covalent Bond there is an atom being slightly more positive (H) than the other (Cl), i.e., the bond will produce a dipole moment, which can be evident when many H-Cl molecules interact among them, because the positive extreme of one molecule will be attracted by the negative part of the other. producing, in most of the cases, liquid substances. The most popular compound with Polar Covalent Bond is Water, where the Oxygen is the slightly negative extreme of the molecule.



Atoms in our planet are not by themselves, they are joined at least to another atom through a chemical bond. Bonds allow atoms achieve a stable electron configuration, similar as the noble gases, which are mostly not joined to other atoms.

The more important kind of bonds are Ionic and Covalent.

Ionic Bond

Is formed between atoms from the groups IA, IIA (metals) and VIA, VIIA (non metals). In this bond,  metals donate one or more electrons to the non-metals leading to form ions, one of them positively charged (cation) and the other with negative charge (anion).

For example, common table salt is Sodium Chloride. When Sodium (Na) and Chlorine (Cl) are combined, the sodium atoms lose an electron, forming cations (Na+), and the chlorine atoms gain an electron to form anions (Cl).

Na + Cl =  Na+ + Cl = NaCl 

Most of the Ionic compounds in our planet are in the solid state and form lattice structures. The two principal factors in determining the form of the lattice are the charge of the ions and their sizes.



The atomic radius is the distance between the nucleus to the boundary of the surrounding cloud of electrons.

Atomic radii vary in a predictable and explicable manner across the periodic table. The radius increases moving down a group due to the addition of a new energy level or shell.

The atomic radius generally decrease along each period of the table, because although more electrons are being added to atoms, they are at similar distances to the nucleus (same shell), and the increasing nuclear charge “pulls” the electron clouds inwards, making the atomic radii smaller.

The noble gases have full valence electron shells, corresponding to an electron configuration s2 p6, making them very stable and not following the exact behavior than other atoms in the neighborhood.



The Periodic Table shows us all the known elements in our planet. In the Periodic Table, the elements are presented in increasing atomic number.

Every vertical column represents a Group or Family. Groups are considered the most important method of classifying the elements, because the elements have very similar properties and exhibit a clear trend in properties down the group. Under the International naming system, the groups are numbered numerically 1 through 18 from the left most column (the alkali metals) to the right most column (the noble gases).

The rows, in the periodic table, are called Periods, there are 7 periods, the first one contains only two elements, hydrogen and helium, they are filling orbital 1s. The second and third periods have 8 elements, because involve elements with orbitals s and p and follow the octet rule. Elements in period 4 start with d orbitals (d-block), and makes the periodic table more extensive, including 10 more elements than the previous periods, these elements are the Transition Elements. When elements include orbitals f (f-block) the periodic table needs 14 more spaces, they are covered for Lanthanides and Actinides.