2.2 Chemical Bonds Link Atoms

2.2 Chemical Bonds Link Atoms

• Organisms are mainly composed of carbon, hydrogen, oxygen, and nitrogen atoms.

• Molecules: Two or more chemically joined atoms.

  • Example: Hydrogen (H2), Oxygen (O2), Nitrogen (N2) consist of two atoms of the same element.

  • Compounds: Molecules consisting of two or more different elements.

    • Example: Carbon monoxide (CO), Water (H2O).

• Characteristics of Compounds vs. Elements:

  • Compounds can have properties drastically different from their constituent elements.

  • Example: Sodium chloride (NaCl) - Sodium: reactive metal, Chlorine: corrosive gas; when combined, form stable table salt crystals.

  • Methane (CH4) comprises carbon (black solid) and hydrogen (combustible gas).

• Molecular Formula: Represents the atoms in a compound with symbols and subscripts.

  • Examples: Methane (CH4), Table salt (NaCl), Water (H2O), Carbon dioxide (CO2).

A. Electrons Determine Bonding

• Electrons occupy energetic regions called orbitals, constantly in motion.

  • Orbitals: Most likely location for electrons; each can hold up to two electrons.

• Energy Shells: Concentric shells with increasing energy levels.

  • 1st shell: 1 orbital (2 electrons); 2nd & 3rd shells: 4 orbitals (8 electrons each).

• An atom’s valence shell is its outermost shell, contributing to its stability.

  • Inert gases like Helium (He) and Neon (Ne) have full valence shells, making them chemically unreactive.

• Atoms with partially filled valence shells achieve stability by donating, stealing, or sharing electrons.

  • Electronegativity: Atom's ability to attract electrons (scale: 0 to 4).

  • Example: Oxygen has higher electronegativity than sodium.

B. In an Ionic Bond, One Atom Transfers Electrons to Another Atom

• Ions: Atoms that lose or gain electrons, resulting in positive or negative charges.

• Ionic Bonds: Form from electrical attraction between oppositely charged ions (e.g., NaCl).

• Characteristics of Ionic Bonds:

  • Strong in solids, break easily in water (salts dissolve).

C. In a Covalent Bond, Atoms Share Electrons

• Covalent Bond: Forms when two atoms share electrons, a common type in biological molecules.

  • Example:

    • Methane (CH4): Carbon shares with four hydrogen atoms.

    • Water (H2O): Oxygen shares with two hydrogen atoms.

    • Oxygen gas (O2): Two oxygen atoms form by sharing electrons.

• Representation of Covalent Bonds:

  • Bonds depicted as lines between atoms; single bond = one shared pair, double bond = two pairs, triple bond = three pairs.

D. Polar and Nonpolar Covalent Bonds

• Polar Covalent Bonds: Unequal sharing of electrons creates partial charges.

  • Highly electronegative atoms form polar bonds, creating regions of positive and negative charge.

  • Example: Oxygen in water: H2O - makes it vital for biological functions.

• Nonpolar Covalent Bonds: Equal sharing of electrons between similar atoms (e.g., O2, H2).

E. Partial Charges and Hydrogen Bonds

• Hydrogen bonds form due to partial charges in polar covalent molecules.

  • Example: Water molecules have hydrogen bonds between partial positive (H) and partial negative (O) charges.

• Hydrogen bonds are weak but stabilize larger molecules like proteins and DNA.

• Water's unique properties arise from hydrogen bond dynamics affecting its interactions.