Fundamentals of Chemical Elements and Bonding
The Periodic Table of Chemical Elements
All chemical elements are arranged in order of increasing atomic number (Z) in a table called the Periodic Table, in a way that reflects their atomic structure.
The elements are arranged in:
- Seven horizontal rows called periods.
- Eighteen vertical columns called groups.
The periods are classified by length:
- Short periods: The first period (containing two elements: hydrogen and helium) and the two following periods (each with eight elements).
- Long periods: The remaining periods. Periods 4 and 5 contain 18 elements. Period 6 contains 32 elements.
The long period 7 includes the actinide group, which contains fully synthesized radioactive nuclei beyond element 92, uranium.
Understanding Chemical Reactions
Chemical reactions are phenomena involving the chemical transformation of substances into different ones. They involve:
- Reagents: The reactive substances.
- Reaction products: The substances that are obtained.
State of Oxidation (Oxidation Number)
The oxidation number or oxidation state of an element in a chemical species is the number of electrons that an atom of that element loses or gains (according to its electronegativity) during chemical bonding to form the species or substance.
Lewis Electronic Formula
Substances can be represented by the formula known as the Lewis Electronic Formula.
For example, in the case of sodium chloride (NaCl), the representation is:
Na+ + Cl–
The symbol Na+ represents the sodium atom, which has lost one electron, resulting in complete electron shells. Similarly, the symbol Cl– represents the chloride anion, which has gained an electron.
In the Lewis structure for Cl–, seven electrons from its valence shell are typically drawn using dots, and the captured electron from sodium is often represented with an ‘x’. Thus, both ions complete their octet, which implies stability.
Types of Chemical Bonds
Ionic Bonds
This bond is characteristic of the union between metals and nonmetals. Atoms become ions, as seen with NaCl, LiF (lithium fluoride), and KI (potassium iodide). The union is produced by electrostatic attraction between anions and cations. One atom gains and the other loses electrons to form the bond.
Covalent Bonds
Covalent bonding usually occurs between atoms of nonmetal elements. It occurs when two atoms share one or more pairs of electrons.
For example, the chlorine molecule (Cl2):
It is also commonly represented as:
Metallic Bonds
This bond is characteristic of metals. It explains how the atoms that constitute a metallic body are united among themselves. Atoms can be of the same element (as in a copper wire) or different elements (as in alloys, for example, bronze).
Hydrogen Bonds
Hydrogen bonding is characteristic of certain substances and occurs due to the presence of hydrogen atoms (H) bonded to small, highly electronegative atoms (O, N, or F).
It is a relatively common bond, dominantly electrostatic. When these highly electronegative atoms attract electrons, a negative charge density develops on them. Consequently, a positive charge density develops on the H atoms, which then attracts the negative charge density of a neighboring molecule.