Understanding Atoms and Bonds

Posted on Mar 6, 2025 in Chemistry

The Atom’s Composition

The atom consists of:

  • Nucleus: Located at the center, it has a positive charge and contains almost all of the atom’s mass.
  • Cortex (Electron Cloud): Contains electrons, which are negatively charged and orbit the nucleus, contributing to the atom’s small mass.

Atomic Masses

Atomic masses are very small, so units are used to make calculations.

Isotopes

Isotopes are atoms of the same element with the same atomic number but different mass numbers. Each isotope has a different mass-dependent percentage.

Atomic Cortex and Valence Electrons

The atomic cortex is the region where electrons orbit. Electrons remaining incomplete on the last layer are called valence electrons.

Ions

Ions are atoms that have lost or gained an electron in their outer shell.

  • Cations: Atoms that have lost an electron (positive charge).
  • Anions: Atoms that have gained an electron (negative charge).

Periodic Table

The periodic table is composed of 7 periods and 18 groups.

Bohr Model

Postulates

  1. Electrons move in circular orbits around the nucleus without emitting energy.
  2. An electron in its orbit has a specific energy value. The closer the orbit is to the nucleus, the lower the energy.
  3. Atoms emit energy when electrons jump from a higher to a lower orbit.

Conclusions of the Bohr Model

The electrons in an atom are in orbits grouped into layers or energy levels. Each orbit can hold a specific number of electrons.

  • 1st layer – 1 orbit – 2 electrons
  • 2nd layer – 4 orbits – 8 electrons
  • 3rd layer – 9 orbits – 18 electrons

Dalton’s Postulates

  1. Matter is made of atoms that are indivisible and indestructible.
  2. Atoms of the same element are identical in mass and properties.
  3. Atoms of different elements differ in mass and properties.
  4. Atoms of different elements combine in simple whole-number ratios to form compounds.

Chemical Bonds

A chemical bond is a force that holds atoms, ions, or molecules together. It is produced when atoms combine to become more stable and occupy a minimum energy state.

Valence Electrons

Valence electrons are the electrons in the outermost valence shell, the last energy level of an atom.

Octet Rule

The octet rule indicates the formation of molecules and compounds, produced by the tendency of atoms to acquire the electronic configuration of a noble gas, completing 8 electrons in their last layer.

Duet Rule

Hydrogen, Lithium, and Beryllium, when bonding, complete their last level with 2 single energetic electrons, reaching the electron configuration of helium.

Types of Bonds

  • Ionic
  • Covalent
  • Metallic

Ionic Bond

The force of attraction between positive and negative ions. It is produced by “electron transfer.” It occurs between a metallic element that tends to yield electrons, forming positive ions, and a nonmetallic element that tends to gain electrons, forming negative ions.

Properties of Ionic Compounds

  • Are solid at room temperature
  • Have high melting and boiling points
  • Are usually soluble in water and other polar solvents
  • Conduct electricity only when in solution or molten
  • Form highly stable crystal lattices
  • Dissociate when dissolved in water
  • Are hard and brittle

Covalent Bond

The force of attraction between non-metal atoms of the same or different electronegativity, when sharing their valence electrons to form molecules.

Nonpolar Covalent Bond

Occurs between atoms of equal electronegativity. Electrons are placed symmetrically on both nuclei.

Polar Covalent Bond

Occurs between atoms of different electronegativity. The distribution of charges is not symmetrical.