Chemical Bonds and Atomic Structure: Understanding Stability

Chemical Bonds and Atomic Structure

Most atoms are present together with other peers, forming elements or different ones to form compounds. The physical states evolve toward states of minimum potential energy, which correspond with maximum stability.

Chemical Bonding

Two or more atoms are held together if the energy of the system is less when they are together than when separated.

The Octet Rule

The noble gases have very stable atoms that do not combine with other atoms and do not alter their electronic structure. All of them have 8 electrons except helium. It follows that the electronic provision of a noble gas property is a particularly stable arrangement called the octet.

Octet Rule

In forming compounds, atoms exchange electrons to acquire 8 electrons in their last layer. There are 2 types of atomic clusters: molecules and crystals.

Molecules

Molecules consist of an indefinite, generally small, number of atoms. For example, diatomic molecules have 2 atoms, and triatomic molecules have 3 atoms. There are:

• Molecules of elements: made of equal atoms, such as H₂
• Molecules of compounds: formed by different atoms, such as H₂O

A mole is the amount of a substance that contains as many units as there are atoms in 12 grams of carbon-12. A mole of atoms of anything contains 6.02 x 10²³ atoms of the same, and its mass, called molar mass, is given by atomic mass expressed in grams.

Crystal Lattices

Crystal lattices or crystals are formed by a variable number of atoms, ions, or molecules, generally very large, that are arranged as a regular three-dimensional structure. They can be:

• Crystals of elements: consisting of equal atoms
• Compound crystals: formed by different atoms

Types of Chemical Bonds

Ionic Bond

This occurs between metal atoms, which form positive ions, and atoms of a nonmetal, which form negative ions. These ions come together to form an ionic crystal.

Covalent Bond

This occurs between atoms of nonmetals, which share their valence electrons. Each shared pair of electrons forms a covalent bond. They form covalent molecules and crystals.

Metallic Bond

In this bond, metal atoms donate electrons and form positive ions. These positive ions are arranged neatly in a network: the transferred electrons form an electron cloud throughout the network. They form metallic crystals.

Properties of Different Crystal Types

Ionic Crystals

Solid at room temperature. High melting points. Hard. Soluble in water. Not conductive in solid state but conductive when fused.

Molecular Crystals

Gases or volatile liquids at room temperature. Slightly soluble in water. Not conductive.

Covalent Crystals

Solid at room temperature. Very hard. Insoluble. Not conductive.

Metallic Crystals

Solid at room temperature. They have metallic brightness. Ductile and malleable. Good conductors.

Molecular and Empirical Formulas

Molecular Formula

Used to describe groups of atoms that form molecules.

Empirical Formula

Used to describe crystal lattices.

Molecular Mass

Molecular mass is the sum of the atomic masses of the atoms composing the formula unit. It is a relative magnitude.

Percentage Composition

The percentage composition of a compound indicates the percentage of its molecular mass corresponding to each of the constituent elements.