Atoms and molecules are the building blocks of life. They combine through various chemical bonds to form the complex structures that make up living organisms. Understanding these fundamental components is crucial for grasping biochemical processes.

Chemical bonding plays a vital role in shaping molecular properties. From covalent and ionic bonds to weaker interactions like hydrogen bonds, these forces determine how molecules behave and interact, influencing everything from water's unique properties to protein folding.

Atomic and Molecular Structure

Atoms and Elements

• Atoms consist of protons, neutrons, and electrons
• Protons have a positive charge, neutrons are neutral, and electrons have a negative charge
• Atomic number represents the number of protons in an atom's nucleus and determines the element
• Elements are pure substances that cannot be broken down into simpler substances by chemical means
• Elements are organized in the periodic table based on their atomic number and chemical properties
• Examples of elements include carbon (C), oxygen (O), and hydrogen (H)

Molecules and Compounds

• Molecules are formed when two or more atoms bond together through chemical bonds
• Compounds are substances composed of two or more elements chemically combined in a fixed ratio
• Examples of molecules include water (H2O) and carbon dioxide (CO2)
• Molecular formula represents the number and type of atoms in a molecule (CH4 for methane)
• Structural formula shows the arrangement of atoms and bonds in a molecule (H-O-H for water)

Electronegativity and Polarity

• Electronegativity is the ability of an atom to attract electrons in a chemical bond
• Atoms with higher electronegativity tend to pull electrons towards themselves, creating polarity in bonds
• Polarity arises from unequal sharing of electrons between atoms in a molecule
• Polar molecules have an uneven distribution of charge due to differences in electronegativity (H2O is polar)
• Nonpolar molecules have an even distribution of charge and symmetric arrangement of atoms (CO2 is nonpolar)

Chemical Bonding

Covalent and Ionic Bonds

• Chemical bonds hold atoms together in molecules and compounds
• Covalent bonds involve the sharing of electrons between atoms
• Single, double, and triple covalent bonds differ in the number of shared electron pairs (H2, O2, N2)
• Ionic bonds form when electrons are completely transferred from one atom to another, creating ions
• Ionic compounds are held together by the electrostatic attraction between positive and negative ions (NaCl)

Hydrogen Bonds and Van der Waals Forces

• Hydrogen bonds are attractive forces between a hydrogen atom bonded to an electronegative atom (N, O, F) and another electronegative atom
• Hydrogen bonds are stronger than other intermolecular forces but weaker than covalent and ionic bonds
• Hydrogen bonding is responsible for the unique properties of water, such as high boiling point and surface tension
• Van der Waals forces are weak attractive forces between molecules, arising from temporary fluctuations in electron distribution
• Van der Waals forces include dispersion forces (London forces), dipole-dipole interactions, and dipole-induced dipole interactions
• These forces contribute to the properties of gases and the formation of molecular crystals (iodine, I2)

Molecular Properties

Polarity and Its Effects

• Molecular polarity depends on the arrangement of polar and nonpolar bonds within a molecule
• Molecules with polar bonds can be nonpolar if the bond polarities cancel out due to symmetric arrangement (CO2)
• Polar molecules interact with other polar molecules through dipole-dipole interactions (acetone)
• Polarity affects solubility, as polar substances dissolve in polar solvents and nonpolar substances dissolve in nonpolar solvents ("like dissolves like")
• Polarity also influences melting and boiling points, as polar molecules have stronger intermolecular forces and require more energy to overcome them
• Surface tension and capillary action are phenomena related to the polarity of water and its ability to form hydrogen bonds