Fundamental Atomic Structure, Subatomic Particles, and Chemical Species Classification

Atom: The smallest particle of an element that can exist and still retain all the characteristic physical and chemical properties of that element.

Atoms possess an internal structure and are composed of smaller building blocks termed subatomic particles.
Subatomic Particle: A very small particle that serves as a structural building block for atoms.
The three fundamental types of subatomic particles are:
- Electron:
- A subatomic particle that possesses a negative electrical charge.
- It is the smallest of the three subatomic particle types in terms of mass.
- Proton:
- A subatomic particle that possesses a positive electrical charge.
- Protons and electrons carry the exact same magnitude (amount) of charge, but their charges are opposite in sign (positive versus negative).
- Neutron:
- A subatomic particle that has no electrical charge associated with it; it is electrically neutral.

The Nucleus:
- The central region of an atom, which is small, dense, and positively charged.
- A nucleus is always positively charged because it contains positively charged protons.
- Almost all (more than $99.9\%$ ) of the total mass of an atom is concentrated within its nucleus because it houses the heavy subatomic particles (protons and neutrons).
The Extranuclear Region (Electron Cloud):
- The outer region surrounding the nucleus contains all of the atom's electrons.
- This extranuclear region accounts for most of the volume of an atom.
- Electrons move rapidly around the nucleus within this outer region.
- Electrons are attracted to the positively charged protons of the nucleus by electrostatic forces existing between particles of opposite charge.
- Fan Blade Analogy: The rapid motion of the electrons in the extranuclear region determines the dynamic volume (size) of the atom in the same manner that the circular motion of a spinning fan blade determines a volume.
- Electron Cloud: The volume occupied by the rapidly moving electrons is referred to as the electron cloud.
- Because electrons are negatively charged, the electron cloud as a whole is negatively charged.

Nucleon: Any subatomic particle that is found inside the nucleus of an atom.
Both protons and neutrons are categorized as nucleons.
Consequently, the atomic nucleus can be regarded as containing a localized collection of nucleons.

Atomic Number ( $Z$ ):
- Defined as the number of protons contained in the nucleus of an atom.
- Because a neutral atom has an equal number of electrons and protons, the atomic number also specifies the total number of extranuclear electrons present.
Mass Number ( $A$ ):
- Defined as the sum of the number of protons and the number of neutrons in the nucleus of an atom.
- The mass number gives the total count of subatomic particles (nucleons) present in the nucleus.
Subatomic Particle Formulas:
- $\text{Number of Protons} = Z$
- $\text{Number of Electrons} = Z$
- $\text{Number of Neutrons} = A - Z$

The mass number and atomic number of a given atom are specified using complete chemical symbol notation.
Rules for Notation:
- The chemical symbol of the element is written as the central identifier (e.g., $\text{X}$ ).
- The atomic number ( $Z$ ) is placed as a subscript in front of the chemical symbol.
- The mass number ( $A$ ) is placed as a superscript in front of the chemical symbol.
- General Format: ${}_{Z}^{A}\text{X}$
Example:
- Notation: ${}_{9}^{19}\text{F}$
- Interpretation: Represents a fluorine ( $\text{F}$ ) atom that has an atomic number $Z = 9$ (containing $9$ protons and $9$ electrons) and a mass number $A = 19$ (containing $19$ total nucleons).

Problem Statement: An atom has an atomic number of $9$ and a mass number of $19$ .
- a. Determine the number of protons present.
- b. Determine the number of neutrons present.
- c. Determine the number of electrons present.
Step-by-Step Solution:
- Given: Atomic Number $Z = 9$ , Mass Number $A = 19$ .
- a. Number of Protons:
- $\text{Protons} = Z = 9$
- b. Number of Neutrons:
- $\text{Neutrons} = A - Z = 19 - 9 = 10$
- c. Number of Electrons:
- $\text{Electrons} = Z = 9$

Isotopes:
- Similarity: Same number of protons (identical atomic number $Z$ , belonging to the same element).
- Difference: Different number of neutrons (and consequently different mass numbers $A$ ).
Isotones:
- Similarity: Same number of neutrons ( $A - Z$ is constant).
- Difference: Different number of protons (different atomic numbers $Z$ , representing different elements).
Isobars:
- Similarity: Same mass number ( $A$ ).
- Difference: Different elements (different atomic numbers $Z$ and different proton/neutron ratios).
Isomers:
- Similarity: Same chemical formula (identical composition of atoms).
- Difference: Different chemical structure (different arrangement or spatial connectivity of atoms).