Elements, Atomic Structure, and Chemical Bonding

Atoms, Bonding, and Molecular Crystals

Ancient history knew elements such as gold, silver, etc. Over time, more elements were discovered; currently, over a hundred are known. A basic classification of elements, based on their appearance and physical properties, allows us to divide them into two main groups: metals and non-metals.

Metals and Non-Metals

To distinguish these groups, we examine their differing characteristic features.

1. Metallic Elements

• They have a characteristic luster.
• They are opaque and good conductors of heat and electricity.
• They are solid at ambient temperature and usually tend to have high melting points.

2. Non-Metallic Elements

• They do not have metallic luster.
• They are poor conductors of heat and electricity.
• At ambient temperature, they can be solid, liquid, or gas. Solids are typically fragile.
• The solids generally have low melting points, and liquids have low boiling points.

The Current Periodic Table

At present, the elements are organized in a table or periodic system, ordered by the values of their atomic numbers (Z). That is, the position depends on the number of protons. According to the current periodic table, elements are distributed in vertical columns called groups (there are 18). Groups contain elements with the same number of electrons in the outermost layer, resulting in similar chemical properties.

Elements that have the same number of electron shells (layers) in their electronic configuration are located in the same horizontal row or period (there are seven).

1. Trends in Periodic Properties

1. Metallic Character: Across the periodic table, metallic character generally decreases from left to right (as the number of protons increases). Within a group, metallic character increases from top to bottom (as the number of electron shells increases).
2. Other Features:
   • The number of protons and electrons increases across the table, from left to right.
   • The number of electron shells (orbits) increases from top to bottom.
   • Some elements exhibit intermediate characteristics between the two main groups; these are called semimetals or metalloids.

The Association of Atoms in Matter

Atoms that form matter can appear alone (isolated) or together to form molecules or crystals.

1. Isolated Atoms (Noble Gases)

While atoms generally combine to form substances, some elements exist naturally as isolated atoms. This occurs because their electronic configurations are the most stable. If we examine elements in Group 18 (helium, neon, argon, etc.), we see that they all have a full outermost electron shell. This group of elements possesses a very stable structure, which is why they are isolated in nature and exist as gases at ambient temperature. Because they are so stable, they do not tend to combine with themselves or other elements; this group is known as noble gases or inert gases.

Note: All other elements seek stability by combining with themselves or other elements to achieve a full outermost electron shell, which gives rise to the various molecules and crystals found in nature.

2. Molecular Structure

All elements seek stability in their outermost electron shells. To achieve this, atoms combine with others, often by sharing electrons, thereby consolidating and forming a molecule.

Characteristics of Molecules

1. Molecules can be formed by the same atoms (simple substances) or different atoms (compounds).
2. Molecules are normally isolated or linked by very weak forces. Due to this, molecular substances often exist as gases at ambient temperature, or as solids that are quite fragile.