Atoms and subatomic particles Knowledge-Based Study

Study notes

1. What is an Atom?

   All matter in the universe is composed of extremely small building blocks called atoms. An atom is the smallest unit of an element that retains all the characteristic properties of that element.

   Although the word atom originally meant "indivisible," experiments in modern physics and chemistry have demonstrated that atoms are composed of even smaller fundamental constituents known as subatomic particles.

   Basic Architecture of an Atom

   An atom consists of two distinct regions:

   • The Nucleus: A dense, positively charged region located at the very center of the atom, containing almost all of its mass.
   • The Electron Cloud: A vast, largely empty space surrounding the nucleus where electrons move rapidly.

   Protons, Neutrons, and Electrons

   The three primary subatomic particles that make up atoms are the proton, the neutron, and the electron. Each has distinct properties regarding electric charge, mass, and location.

   1. Protons

   • Location: Inside the nucleus.
   • Relative Electric Charge: $+1$ elementary charge ($+1.602 \\times 10^{-19}\\text{ C}$).
   • Relative Mass: Approximately $1\\text{ u}$ (unified atomic mass unit).

   2. Neutrons

   • Location: Inside the nucleus.
   • Relative Electric Charge: $0$ (electrically neutral).
   • Relative Mass: Approximately $1\\text{ u}$ (slightly heavier than a proton).

   3. Electrons

   • Location: In orbitals or energy levels surrounding the nucleus.
   • Relative Electric Charge: $-1$ elementary charge ($-1.602 \\times 10^{-19}\\text{ C}$).
   • Relative Mass: Approximately $\\frac{1}{1836}\\text{ u}$ (negligible mass compared to nucleons).

   Defining Atomic Properties

   The behavior and identity of an atom depend on the exact number and balance of its subatomic particles.

   Atomic Number ($Z$)

   The atomic number is the number of protons found in the nucleus of an atom. The atomic number defines the identity of the chemical element; for example, every carbon atom contains exactly 6 protons.

   Mass Number ($A$)

   The mass number is the total number of protons and neutrons (collectively called nucleons) in an atom's nucleus:

   $$A = Z + N$$

   where $N$ represents the number of neutrons. Electrons contribute negligibly to total atomic mass and are not included in the mass number.

   Electrical Neutrality and Net Charge

   In a neutral atom, the number of negatively charged electrons is exactly equal to the number of positively charged protons:

   • Neutral Atom: $\\text{Number of protons} = \\text{Number of electrons}$
   • Cation (Positive Ion): An atom that has lost one or more electrons.
   • Anion (Negative Ion): An atom that has gained one or more electrons.

   Key Takeaways

   • The nucleus contains protons and neutrons, contributing almost all the atom's mass in a compact central volume.
   • Electrons occupy the space surrounding the nucleus and determine the chemical bonding and electrical behavior of the atom.
   • Protons have a positive charge ($+1$), neutrons have zero charge ($0$), and electrons have a negative charge ($-1$).
   • The number of protons determines the element's identity, while the balance between protons and electrons determines the atom's net charge.