Introduction to Chemistry: Atomic Structure and Basic Bonding

Basic Concepts in Chemistry

For biological purposes, key chemical compounds to study include:
- Water (\text{H}_2\text{O})
- Salt (e.g., Sodium Chloride, \text{NaCl})
- Proteins
- Carbohydrates
Understanding their chemical composition is crucial.
Chemistry often focuses on important inorganic and organic compounds relevant to human biology.

Each element on the periodic table is identified by:
- Its atomic symbol (e.g., H for Hydrogen, Na for Sodium).
- Its atomic number (the number in the upper left-hand corner on periodic tables).
- The atomic number (Z) uniquely identifies an element and represents the number of protons in its nucleus.
Neutral Atoms:
- In a neutral atom, the number of protons equals the number of electrons.
- This balance results in an overall neutral charge for the atom (e.g., Hydrogen has 1 proton and 1 electron; Helium has 2 protons and 2 electrons; Carbon has 6 protons and 6 electrons).

Definition: Isotopes are different forms of the same element.
Characteristics:
- They have the same atomic number (same number of protons).
- They have different numbers of neutrons.
- This difference in neutrons leads to different atomic masses.
Example (Hydrogen): Different forms of hydrogen exist with varying numbers of neutrons, but all have 1 proton.

Concept: Most of the mass of an atom is located in its nucleus.
Calculation: Atomic mass is approximately equal to the sum of protons and neutrons.
- Atomic Mass = Number of Protons + Number of Neutrons.
Periodic Table Representation: The atomic mass listed on the periodic table is an average atomic mass, calculated by taking the average of the masses of all known isotopes of that element.

Electrons are distributed around the nucleus in specific energy levels or shells.
Rules for Electron Distribution (Simplified):
- First Shell: Can hold a maximum of 2 electrons.
- Second Shell: Can hold a maximum of 8 electrons.
- Third Shell: Can hold a maximum of 8 electrons for the purpose of bonding stability (octet rule).

Definition: Valence electrons are the electrons located in the outermost electron shell of an atom.
Importance: Only valence electrons are involved in atom-to-atom interactions and chemical bonding.

Atoms with incomplete valence shells are less stable.
Atoms tend to interact (bond) to achieve a complete (full) valence shell, which makes them more stable.
- For the first shell, a full shell has 2 electrons.
- For subsequent shells, a full shell typically has 8 electrons (the octet rule).

Definition: Electronegativity is the ability of an atom to attract electrons to itself (electron attracting capability).
Factors: Atoms with more electrons in their valence shell tend to be more electronegative.
- High Electronegativity: Stronger attraction for electrons.
- Low Electronegativity: Weaker attraction for electrons.

Process (Ionic Bonding): When atoms with significantly different electronegativities interact, valence electrons are transferred from the less electronegative atom to the more electronegative atom.
Ions: Atoms that gain or lose electrons are no longer electrically neutral; they become charged atoms called ions.
- Cation (+ charge): Formed when an atom loses one or more valence electrons. The number of protons becomes greater than the number of electrons (e.g., Na^+ after losing an electron).
- Anion (- charge): Formed when an atom gains one or more valence electrons. The number of electrons becomes greater than the number of protons (e.g., Cl^- after gaining an electron, originally chlorine, becomes chloride as an ion).
- Protons remain unchanged during electron transfer.

Ionic bonding is the strong electrostatic attraction between two oppositely charged ions (a cation and an anion).
Example: Sodium Chloride (\text{NaCl})
- Sodium (\text{Na}) has 1 valence electron (low electronegativity).
- Chlorine (\text{Cl}) has 7 valence electrons (high electronegativity).
- Sodium transfers its single valence electron to chlorine.
- Sodium becomes a positively charged cation (\text{Na}^+) with a full outer shell (the original second shell with 8 electrons).
- Chlorine becomes a negatively charged anion (\text{Cl}^-) with a full outer shell (8 valence electrons).
- The strong attraction between Na^+ and Cl^- forms an ionic bond.
Ionic Solids/Crystals: Ionic compounds often organize into solid, crystalline structures, like table salt (\text{NaCl} crystal), where ions are regularly arranged.

Alternative Interaction: Not all valence electron interactions involve transfer.
Covalent bonding involves the sharing of valence electrons between atoms.
This type of bonding occurs when atoms have similar electronegativities or when electron transfer is not favorable to achieve a full valence shell. (Further details will be discussed in upcoming lessons, potentially with examples like Hydrogen (\text{H}2), Oxygen (\text{O}2), and Carbon compounds).