Chemical Elements, Periodic Table, and Bonding Principles
Natural and Artificial Elements
There are approximately 90 naturally occurring elements. Necessary equipment in physics laboratories has allowed for the creation of additional, artificial elements. Few elements are found in their free state in nature; they are mainly found combined with other elements, forming compounds.
Element Distribution
Elements in the Earth’s Crust and Core
The element iron is abundant but primarily found in the Earth’s core. The Earth’s crust is mainly composed of oxygen and silicon.
Elements in Living Organisms
Approximately 99% of living organisms are composed of just six elements:
- Oxygen (O)
- Carbon (C)
- Hydrogen (H)
- Nitrogen (N)
- Phosphorus (P)
- Calcium (Ca)
The remaining 1% corresponds to a large diversity of elements known as trace elements (oligoelementos).
Most Abundant Elements in the Universe
The lightest and most abundant elements in the universe are hydrogen and helium.
The Periodic Table: Classification
In the 19th century, scientists sought to classify the elements. Dmitri Mendeleev and Lothar Meyer independently arranged the elements in order of increasing atomic mass, grouping those with similar properties into columns.
Groups and Periods
Alkali Metals (Group 1)
These are the metals in Group 1 of the periodic table (excluding hydrogen). They are soft, low-density metals (some less dense than water) and react energetically with water.
Halogens (Group 17)
These are the elements in Group 17. They are very corrosive and react with metals to form salts.
Noble Gases (Group 18)
These are the elements of Group 18. They are very stable and rarely combine with other elements.
General Properties of Element Types
Metals
Metals typically have a shiny appearance, are good conductors of heat and electricity (silver is the best conductor), and are malleable (can be hammered into thin sheets).
Non-metals
Many non-metals are gases at room temperature (like oxygen and nitrogen). Some, like carbon, have very high melting points. They are generally found on the right side of the periodic table.
Metalloids (Semi-metals)
These elements are difficult to classify definitively as metals or non-metals, having intermediate characteristics. Some metalloids, like silicon (Si) and germanium (Ge), are used in the manufacture of electronic components (semiconductors).
Understanding Chemical Bonds
A chemical bond is the union of two or more atoms, either of the same element or different elements. Atoms join together to achieve a more stable electron configuration. There are two main ways atoms group together:
- In discrete groups called molecules.
- In large, ordered structures of millions of atoms called crystals or crystalline networks.
Covalent Bonding: Molecular and Atomic Substances
Non-metallic elements typically bond with each other via covalent bonds, where electrons are shared.
- Atoms of the same element can join to form molecular substances (e.g., Oxygen – O2, Chlorine – Cl2, Ozone – O3, Nitrogen – N2) or atomic/network covalent substances, which are crystalline networks (e.g., Carbon as diamond or graphite).
- Atoms of different elements can join covalently to form molecular substances (e.g., Water – H2O, Methane – CH4, Ammonia – NH3) or atomic/network covalent substances formed by crystals (e.g., Silica – SiO2).
Ionic Bonding and Substances
An ionic bond typically forms between a metal and a non-metal. The metal atom, having few valence electrons, tends to lose them, forming a positively charged ion (cation). The non-metal atom, having many valence electrons, tends to gain electrons, forming a negatively charged ion (anion). The electrostatic attraction between these oppositely charged ions forms the ionic bond, resulting in ionic compounds.
Metallic Bonding and Substances
In metallic bonding, atoms of a metal are joined together. All the metal atoms share their valence electrons in a ‘sea’ of electrons that surrounds the positive metal ions. This sharing holds the atoms together in a metallic lattice.