Atoms, Elements, and Isotopes

Atomic Structure and Notation

Atoms are the basic units of matter, consisting of protons, neutrons, and electrons. Elements are defined by their atomic number, which is the number of protons in the nucleus.

• Atomic Number (Z): Number of protons in the nucleus; determines the element.

• Atomic Mass/Weight (A): Sum of protons and neutrons in the nucleus.

• Symbol: One or two-letter abbreviation for the element (e.g., Li for lithium).

Example: Lithium (Li) has 3 protons (atomic number 3). Its most common isotope has 4 neutrons, giving it an atomic mass of 7.

Isotopes: Atoms of the same element with different numbers of neutrons. For example, lithium can have isotopes with 3 protons and either 3 or 4 neutrons.

• Normal Lithium: 3 protons, 4 neutrons (atomic mass 7)

• Isotope: 3 protons, 3 neutrons (atomic mass 6)

Molecules and Compounds

Definitions and Differences

A molecule is a group of two or more atoms held together by chemical bonds. A compound molecule is a molecule that contains atoms of different elements.

• Molecule: Can be made of the same or different elements (e.g., O2, H2O).

• Compound: Always made of different elements (e.g., H2O, NaCl).

Example: Water (H2O) is a compound molecule; oxygen gas (O2) is a molecule but not a compound.

Chemical Reactions

Parts of a Chemical Reaction

Chemical reactions involve the transformation of reactants into products.

• Reactants: Substances present before the reaction.

• Products: Substances formed as a result of the reaction.

General Equation:

Properties of Water

Unique Characteristics of Water

Water exhibits several unique properties that are essential for life.

• Ice Floats: Solid water is less dense than liquid water.

• Temperature Regulation: Water has a high specific heat, helping regulate temperature.

• Universal Solvent: Water can dissolve many substances due to its polarity.

• Surface Tension: Water molecules stick together (cohesion) and to other surfaces (adhesion).

Chemical Bonds

Types and Strengths of Chemical Bonds

Chemical bonds hold atoms together in molecules and compounds. Their strength varies:

• Covalent Bonds: Atoms share electrons; strongest type of bond.

• Ionic Bonds: Atoms transfer electrons, resulting in charged ions that attract each other.

• Hydrogen Bonds: Weak attractions between a hydrogen atom and an electronegative atom (e.g., oxygen or nitrogen).

Order of Strength: Covalent > Ionic > Hydrogen

Ionic Bonds Example

Formed when one atom donates an electron to another, resulting in positive and negative ions.

Example:

Polar and Nonpolar Molecules

A polar molecule has an unequal distribution of charge, leading to partial positive and negative regions.

• Polar: Unequal sharing of electrons (e.g., water, H2O).

• Nonpolar: Equal sharing of electrons (e.g., O2).

Example: In water, oxygen is partially negative, and hydrogens are partially positive.

Hydrogen Bonds

Hydrogen bonds occur between the hydrogen atom of one molecule and an electronegative atom of another molecule.

• Example: Hydrogen bonds between water molecules contribute to water's high surface tension.

Acidity and pH

Definition and Scale

Acidity refers to the concentration of hydrogen ions (H+) in a solution. The pH scale measures acidity and basicity.

• Acidic: pH 0 to 6.99

• Neutral: pH 7

• Basic: pH 7.01 to 14

Formula:

Biological Molecules

Major Classes of Biomolecules

Biological molecules are essential for life and include:

• Carbohydrates: Provide energy and structural support.

• Lipids: Store energy and make up cell membranes.

• Proteins: Perform a wide range of functions, including catalysis and structural roles.

• Nucleic Acids: Store and transmit genetic information (e.g., DNA, RNA).

Table: Comparison of Chemical Bonds

Additional info:

• Isotopes are important in biology for tracing biochemical pathways and in medical diagnostics.

• Water's polarity is crucial for its role as a universal solvent.

• pH affects enzyme activity and cellular processes.