Understanding Atoms: Models, Properties, and Bonds
Historical Atomic Models
Dalton’s Atomic Theory
It states that matter is made up of atoms. These atoms are described as solid, spherical, and indivisible particles. Subsequent scientists built upon Dalton’s model, developing increasingly precise descriptions of the atom.
Thomson’s Atomic Model
This model proposes that the atom is a positively charged solid sphere with electrons embedded within it, often likened to a “plum pudding.”
Rutherford’s Atomic Model
Rutherford’s model suggests that the atom is mostly empty space. It consists of a dense, positively charged nucleus where protons reside, with electrons revolving in circular orbits around this nucleus.
Bohr’s Atomic Model
Improving upon Rutherford’s model, Bohr proposed that electrons orbiting the nucleus can only exist in specific circular orbits, each possessing a certain energy level.
Modern Atomic Structure
The modern atomic model incorporates the existence of neutrons in the nucleus. Neutrons have almost the same mass as protons but carry no electric charge.
Parts of an Atom
The Nucleus
Formed by protons, which have a positive charge and a mass of approximately 1.67 x 10-27 kg. It also contains neutrons, which have no electric charge and a mass nearly equal to that of protons.
The Electron Shell (Crust)
Formed by electrons that orbit the nucleus, appearing and disappearing at specific orbitals with certain energy levels. Electrons have a mass approximately 2000 times smaller than protons, meaning the vast majority of an atom’s mass is concentrated in its nucleus.
Atomic Volume
The volume of an atom can be 100,000 times greater than the volume of its nucleus, indicating that the atom is mostly empty space.
Key Atomic Definitions
Atomic Number (Z)
The atomic number (Z) defines an element by representing the number of protons in the nucleus of its atoms. Since an atom is typically neutral, the number of protons equals the number of electrons.
Mass Number (A)
The mass number (A) is the total number of nucleons (protons and neutrons) present in the nucleus of an atom.
Isotopes
Isotopes are atoms of the same element that have the same atomic number (Z) but different mass numbers (A). This difference arises from varying numbers of neutrons.
Atomic Mass
While atomic mass can be expressed in kilograms, it is more commonly measured using the atomic mass unit (u). One atomic mass unit (1 u) is defined as one-twelfth (1/12) of the mass of a carbon-12 atom. All matter we observe is formed by discrete atoms.
Chemical Bonds
A chemical bond is the union that can occur between atoms. There are three main types of chemical bonds:
Ionic Bond
An ionic bond forms when one atom loses an electron and another atom gains an electron, resulting in the formation of ions.
- If an atom loses an electron, it becomes a positive ion (cation).
- If an atom gains an electron, it becomes a negative ion (anion).
The bond is formed by the electrostatic attraction between a positive ion and a negative ion.
Metallic Bond
A metallic bond is the type of bond found between metal atoms. In this bond, metal atoms convert into positive ions, which are then arranged in an orderly crystal lattice, surrounded by a “sea” of delocalized electrons.
Summary of Key Concepts
This section provides a concise summary of fundamental atomic concepts:
Modern Atomic Model Update
The atom is fundamentally divided into two parts:
- Nucleus: Contains protons (positively charged) and neutrons (no charge).
- Electron Shell (Crust): Contains electrons (negatively charged) distributed in different energy levels and orbitals.
Atomic Number (Z)
The number of protons in the nucleus of an atom of a specific element.
Mass Number (A)
The total number of nucleons (protons and neutrons) in the nucleus of an atom.
Isotope
Atomic forms of the same element that differ in their mass number due to a varying number of neutrons.
Atomic Mass or Relative Isotopic Mass
The number of times the mass of an atom of a specific isotope is greater than the atomic mass unit (u), where 1 u is defined as one-twelfth of the mass of an atom of carbon-12.