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.



Isotopes are variants of atoms of a particular chemical element. They still have the same number of protons (and belong to the same element), but they have different number of neutrons.

The Hydrogen has three isotopes:  {}_{1}^{1}H,\text{ }{}_{1}^{2}\text{H, }{}_{1}^{3}\text{H}

Protium,  {}_{1}^{1}H

Is the most common hydrogen isotope with an abundance of more than 99.98%. This is the only isotope without neutron.

Deuterium, {}_{1}^{2}\text{H}

The other stable hydrogen isotope, 0.015%, is not radioactive and has insignificant toxicity hazard. When is part of the water instead of the normal hydrogen, forms the heavy water.

Tritium, {}_{1}^{3}\text{H}

Contains one proton and two neutrons in its nucleus. It is radioactive, but exist because of the interaction of cosmic rays with atmospheric gases.

There are more hydrogen isotopes, but they are synthesized in the laboratory, and they are highly unstable.

Carbon Isotopes are   {}_{6}^{12}C,\text{ }{}_{6}^{13}\text{C, }{}_{6}^{14}\text{C}

With that information you should know:

  1. The atomic number of the Carbon
  2. The number of protons, and electrons
  3. Which one is the heaviest one?, and How many neutrons has?


The Mass Number (A), is the total number of protons and neutrons in an atomic nucleus.

The Mass Number gives the approximate weight of the atom, because it counts the weight of the protons and neutrons, does not include the electrons weight because they are negligible. (See Atom Description for the values)

The Mass Number, does not identify an atom or element, in fact any element has different values of Mass Number. For example, the Hydrogen (Atomic Number, Z=1) has three values, A= 1, 2, 3.

{}_{1}^{1}H,\text{ }{}_{1}^{2}H\text{, }{}_{1}^{3}H


Mathematically, the Mass Number is expressed as, A = Z + N

Where, N= number of neutrons

Thus, the Hydrogen has three Mass Numbers because some of them have zero neutrons, others have one neutron, or they could also have two neutrons.

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