Protons, neutrons and electrons Knowledge-Based Study

Study notes

1. The Basic Architecture of the Atom

   All matter is composed of atoms, which were once thought to be indivisible. Modern atomic theory demonstrates that atoms consist of three fundamental subatomic particles: protons, neutrons, and electrons.

   An atom is structured into two main regions:

   • The Nucleus: A dense, positively charged core at the center of the atom that contains both protons and neutrons. Despite occupying a tiny fraction of the atom's total volume, the nucleus contains virtually all of its mass.
   • The Electron Cloud: The vast, mostly empty space surrounding the nucleus where electrons move. This outer region determines the atomic volume and governs chemical bonding and reactivity.

   Comparing Subatomic Properties

   Subatomic particles are distinguished by their relative electric charges and relative masses:

   • Protons: Possess a relative electrical charge of +1. Each proton has a mass of approximately 1 unified atomic mass unit (amu).
   • Neutrons: Carry no electrical charge (electrically neutral, charge of 0). The mass of a neutron is approximately 1 amu, essentially identical to that of a proton for practical chemical calculations.
   • Electrons: Possess a relative electrical charge of -1. An electron is extremely light, having a mass of approximately 1/1836 amu. Because this mass is so small, electrons contribute negligibly to the overall mass of an atom.

   Because opposite charges attract, the positively charged nucleus holds the negatively charged electrons within the atom via electrostatic attraction.

   Defining Atomic Identity and Mass

   The numbers of subatomic particles dictate the identity, mass, and charge state of an atom:

   • Atomic Number ($Z$): The number of protons located in the nucleus. The atomic number uniquely identifies a chemical element. For example, any atom containing 6 protons is invariably carbon.
   • Mass Number ($A$): The total count of nucleons (protons and neutrons) in the nucleus:
     Mass Number = Protons + Neutrons, or $A = Z + N$, where $N$ is the number of neutrons.
   • Neutron Count ($N$): Can be calculated directly by rearranging the relationship: $N = A - Z$.
   • Electrical Neutrality: In a neutral, uncharged atom, the total positive charge equals the total negative charge. Therefore, the number of electrons is exactly equal to the number of protons.

   Calculating Subatomic Composition

   Standard nuclide notation displays the mass number as a superscript and the atomic number as a subscript to the left of the chemical symbol.

   Consider an atom of aluminum represented as $^{27}_{13}\text{Al}$:

   1. Protons: Equal to the atomic number ($Z = 13$). The atom contains 13 protons.
   2. Electrons: Since the atom is neutral, the number of electrons equals the number of protons. There are 13 electrons.
   3. Neutrons: Calculated by subtracting the atomic number from the mass number: $N = 27 - 13 = 14$. The nucleus contains 14 neutrons.

   If an atom gains or loses electrons, it forms an ion, altering the balance between protons and electrons, while the number of protons in the nucleus remains unchanged.