3.1 Chemistry

Chemistry Foundations

Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and compounds made of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they undergo during a reaction with other substances. For the DCAT, mastering these fundamentals is crucial, as they form the basis for more advanced scientific reasoning on the exam.

Atomic Structure

At the core of chemistry is the atom, the fundamental building block of all matter. Atoms consist of a dense central nucleus containing positively charged protons and uncharged neutrons, surrounded by a cloud of negatively charged electrons. These subatomic particles define the characteristics of an atom.

• Protons: Positively charged particles. The number of protons determines the element's identity and is called the atomic number (Z).
• Neutrons: Neutral particles. Neutrons determine the isotope of the element. Together with protons, they make up almost all of the atom's mass, represented by the mass number (A).
• Electrons: Negatively charged particles that occupy energy levels or shells around the nucleus. They dictate the atom's chemical behavior and bonding capabilities.

Isotopes are atoms of the same element that have different numbers of neutrons. For example, Carbon-12 and Carbon-14 are both carbon, but Carbon-14 has two additional neutrons.

Ions are formed when atoms gain or lose electrons. Positively charged ions are cations (lost electrons), while negatively charged ions are anions (gained electrons). Electron configuration describes the distribution of electrons in atomic orbitals (s, p, d, f).

Exam Trap: Don't confuse atomic number with mass number. The atomic number is strictly the number of protons. The mass number is the sum of protons and neutrons.

Elements, Compounds, and Formulas

An element is a pure substance consisting of only one type of atom, such as oxygen (O) or iron (Fe). A compound is a substance formed when two or more different chemical elements are chemically bonded together in a fixed ratio, such as water ($H_2O$) or table salt (NaCl).

Chemical formulas express the types and numbers of atoms in a compound.

• Empirical Formula: The simplest whole-number ratio of atoms in a compound.
• Molecular Formula: The actual number of atoms of each element in a molecule.

Types of Chemical Bonds

Bonds form because atoms seek to achieve a stable electron configuration, typically a full valence shell (the octet rule).

• Ionic Bonds: Formed through the electrostatic attraction between oppositely charged ions. This occurs when electrons are completely transferred from a metal to a non-metal (e.g., NaCl).
• Covalent Bonds: Involve the sharing of electron pairs between atoms. This typically occurs between non-metals (e.g., $CO_2$). Covalent bonds can be polar (unequal sharing, leading to partial charges) or non-polar (equal sharing).
• Metallic Bonds: A lattice of positive ions in a "sea" of delocalized electrons, giving metals their conductivity and malleability.

The Periodic Table and Trends

The periodic table organizes elements by increasing atomic number. Elements with similar chemical properties fall into the same vertical columns, known as groups or families. Horizontal rows are called periods.

Periodic Trends are specific patterns in the periodic table:

• Atomic Radius: Decreases across a period (left to right) due to increased nuclear charge pulling electrons closer. Increases down a group due to the addition of electron shells.
• Ionization Energy: The energy required to remove an electron. Increases across a period and decreases down a group.
• Electronegativity: The tendency of an atom to attract electrons in a bond. Increases across a period and decreases down a group. Fluorine is the most electronegative element.

Chemical Equations and Stoichiometry

A chemical equation represents a chemical reaction. Reactants are on the left, and products are on the right. According to the Law of Conservation of Mass, matter cannot be created or destroyed, meaning equations must be balanced so that the number of atoms for each element is the same on both sides.

Types of reactions include:

1. Synthesis: $A + B \rightarrow AB$
2. Decomposition: $AB \rightarrow A + B$
3. Single Replacement: $A + BC \rightarrow AC + B$
4. Double Replacement: $AB + CD \rightarrow AD + CB$
5. Combustion: Hydrocarbon + $O_2 \rightarrow CO_2 + H_2O$

Stoichiometry is the quantitative relationship between reactants and products in a chemical reaction. It relies on the balanced chemical equation and the concept of the mole. Avogadro's number ($6.022 \times 10^{23}$) defines the number of particles in a mole.

To solve stoichiometry problems:

1. Ensure the chemical equation is balanced.
2. Convert the given quantity of a substance to moles using its molar mass.
3. Use the molar ratio from the balanced equation to find the moles of the desired substance.
4. Convert the moles of the desired substance back to the required units (grams, liters, etc.).

The limiting reactant is the reactant that is completely consumed first, limiting the amount of product formed. The theoretical yield is the maximum amount of product that can be produced, while percent yield is (actual yield / theoretical yield) $\times 100%$.

Acids, Bases, and the pH Scale

Acids and bases are critical classes of compounds with distinct properties.

• Arrhenius Theory: An acid produces $H^+$ ions in aqueous solution, while a base produces $OH^-$ ions.
• Brønsted-Lowry Theory: An acid is a proton ($H^+$) donor, and a base is a proton ($H^+$) acceptor.
• Lewis Theory: An acid is an electron-pair acceptor, and a base is an electron-pair donor.

Strong acids and bases dissociate completely in water (e.g., HCl, NaOH), whereas weak acids and bases only partially dissociate.

The pH scale measures the concentration of hydrogen ions ($[H^+]$) in a solution. It is a logarithmic scale, meaning each whole pH value below 7 is ten times more acidic than the next higher value.

• pH < 7: Acidic
• pH = 7: Neutral
• pH > 7: Basic (Alkaline)

Exam Tip: Remember the formula: $pH = -\log[H^+]$. If the $[H^+]$ is $10^{-4}$ M, the pH is 4. Conversely, $[H^+] = 10^{-pH}$. For bases, you can use $pOH = -\log[OH^-]$, and recall that at standard conditions, $pH + pOH = 14$.

A buffer solution is a mixture of a weak acid and its conjugate base (or a weak base and its conjugate acid) that resists changes in pH upon the addition of small amounts of strong acid or base. This is crucial in biological systems, such as maintaining human blood pH around 7.4.