Atomic Structure, Chemical Bonding, and Ionic Interactions

Subatomic Particle Properties:
- Protons: positively charged subatomic particles ( $+$ ).
- Neutrons: uncharged, neutral subatomic particles.
- Electrons: negatively charged subatomic particles ( $-$ ).
Atomic Number:
- Each chemical element contains a unique, specific number of protons; this count is defined as the atomic number.
- Atoms are electrically neutral overall because the number of protons equals the number of electrons.
Electron Shell Distribution and the Octet Rule:
- First electron shell: holds a maximum capacity of $2$ electrons.
- Subsequent electron shells: every shell after the first fits up to $8$ electrons ( $8\,e^-$ ).
- Chemical reactivity principle: atoms react with other atoms to fill their outer electron shell (valence shell) and achieve stability.
Structural Example — Nitrogen Atom:
- Total atomic content: $7$ electrons, $7$ protons, and $7$ neutrons.
- Electron shell distribution: $2$ electrons in the first electron shell and $5$ electrons in the second electron shell.
- Orbital distribution details:
- First electron shell: filled with $2$ electrons.
- Second electron shell $s$ orbital: filled with $2$ electrons (a pair of electrons).
- Second electron shell $p$ orbitals: contains three distinct $p$ orbitals, each holding $1$ single electron.

Atomic Mass Calculation:
- Under standard conditions, the number of protons equals the number of neutrons in an atom.
- Formula: $\text{Atomic mass} = \text{protons} + \text{neutrons}$
Isotopes and Radioisotopes:
- Isotopes are structural variations of chemical elements that share the same number of protons but differ in the number of neutrons.
- Radioisotopes: unstable isotopes that undergo spontaneous decay and emit radiation/energy.
Gram Atomic Mass:
- Defined as the exact amount of an element in grams that equals the numerical value of its atomic weight.

Ion Formation:
- Ions are created when neutral atoms gain or lose electrons, thereby acquiring a net electrical charge.
- Cations: positively charged ions formed when an atom loses electrons (e.g., sodium losing an electron: $Na \rightarrow Na^+ + e^-$ ).
- Anions: negatively charged ions formed when an atom gains electrons (e.g., chlorine gaining an electron: $Cl + e^- \rightarrow Cl^-$ ).
Primary Ions Encountered in Human Physiology:
- Hydrogen ( $H$ ): forms Hydrogen ion ( $H^+$ ) by losing $1$ electron.
- Sodium ( $Na$ ): forms Sodium ion ( $Na^+$ ) by losing $1$ electron.
- Potassium ( $K$ ): forms Potassium ion ( $K^+$ ) by losing $1$ electron.
- Chlorine ( $Cl$ ): forms Chloride ion ( $Cl^-$ ) by gaining $1$ electron.
- Magnesium ( $Mg$ ): forms Magnesium ion ( $Mg^{2+}$ ) by losing $2$ electrons.
- Calcium ( $Ca$ ): forms Calcium ion ( $Ca^{2+}$ ) by losing $2$ electrons.

Molecules:
- Formed when two or more atoms are joined together by chemical bonds.
- Represented chemical formulas denote their constituent atoms and exact atomic ratios (e.g., glucose is represented as $C_6H_{12}O_6$ ).
Chemical Bond Classification:
- Covalent Bonds: represent the strongest type of chemical bond, formed when one or more pairs of electrons in outer orbitals are shared between atoms.
- Polar Covalent Bonds:
- Characterized by unequal electron sharing between bonded atoms due to differences in electronegativity.
- Produces a dipole moment, denoted by the symbol $\delta$ (slight charge).
- The more electronegative atom acquires a slight negative charge ( $\delta^-$ ), while the less electronegative atom acquires a slight positive charge ( $\delta^+$ ) (e.g., hydroxyl groups where oxygen holds $\delta^-$ relative to hydrogen holding $\delta^+$ ).
- Non-Polar Covalent Bonds:
- Characterized by completely equal sharing of electrons between identical or similarly electronegative atoms, resulting in no charge separation.