Dalton's Atomic Theory and Chemical Bonding Principles

Dalton's Atomic Theory and Chemical Composition

Dalton's theory states that elements consist of atoms, which are material particles that are separate and indestructible. Atoms of the same element are equal in mass and in all other qualities. The atoms of different elements have different mass properties. Compounds are formed by the joining of atoms of the corresponding elements in a simple numerical relationship. The "atoms" of a given compound are, in turn, identical in mass and all other properties.

Pure Substances and Elements

• A pure substance is a homogeneous system that has a defined and constant composition, possessing invariant physical properties, and cannot be separated by physical processes.
• A chemical element is a pure substance that cannot be decomposed into simpler ones by chemical means.
• A chemical compound is a substance that can be decomposed into simpler ones by chemical means.

Chemical Bonding Mechanisms

Ionic Bond

The ionic bond is formed by electrostatic forces exerted between ions of opposite signs. It forms between elements of very different electronegativity, typically between metallic and nonmetallic elements, or elements at the ends of the periodic table (s-block and p-block).

In ionic bonding, electrons are transferred from the more metallic element to the less metallic element, forming the corresponding ions.

Properties of Ionic Compounds

1. They are crystalline solids.
2. They exhibit high melting and boiling points.
3. They dissolve in polar solvents like water.
4. They are hard but brittle.
5. They are poor conductors of electricity in the solid state.
6. When dissolved in water, they become good conductors of electricity.

Covalent Bond

The covalent bond is formed by sharing electrons between the atoms that bind. This type of bond forms between elements of similar electronegativity.

Covalency is the term used for the number of covalent bonds an atom forms. Atoms bound by sharing electrons tend to acquire the noble gas structure (octet rule). Each atom tends to fill its valence shell, meaning it aims to be surrounded by 8 electrons (counting the shared electrons).

Intermolecular Forces

These are the forces that hold molecules together to one another. We recognize two main types:

• Links by Van der Waals forces.
• Links by Hydrogen Bridges (H-bonds).

Hydrogen Bonds

The hydrogen bond is a weak, electrostatic-type link. It appears when the molecule contains bonds between H and highly electronegative atoms such as F, O, or N. The bonding electron pair of the F, O, or N atom is strongly attracted to the H atom, creating a polar bond.