Atoms and Atomic Structure

Definition and Components of the Atom

An atom is the smallest particle of an element that retains the properties of that element. Atoms are the fundamental building blocks of matter.

• Nucleus: The dense central core of the atom, containing protons and neutrons.

• Proton (p+): A positively charged particle found inside the nucleus.

• Neutron (n0): A particle with no charge, also located in the nucleus.

Electrons travel around the nucleus in specific regions called energy levels or shells. Each energy level can hold a certain maximum number of electrons:

• The first energy level can hold 2 electrons.

• The second energy level can hold 8 electrons.

• The third energy level can hold 8 electrons (for main group elements).

All energy levels being used must be filled for an element to be stable (octet rule for main group elements).

Example: An atom with 8 electrons will have 2 electrons in the first energy level and 6 in the second. This is the electron configuration for oxygen.

Electron Configuration Examples

• 10 electrons: 2 in the first level, 8 in the second (Neon, stable noble gas configuration).

• 16 electrons: 2 in the first, 8 in the second, 6 in the third (Sulfur, not a full outer shell, so not as stable as noble gases).

Elements and the Periodic Table

Definition of an Element

An element is a substance that cannot be broken down into simpler substances by chemical means. There are about 90 naturally occurring elements, all listed on the Periodic Table.

• Only about 25 elements are essential for living things.

• 96% of the mass of a human is composed of carbon (C), hydrogen (H), nitrogen (N), and oxygen (O).

Atomic Number and Atomic Mass

• Atomic Number: The number of protons in the nucleus of an atom. It is unique for each element and also equals the number of electrons in a neutral atom.

• Atomic Mass (Mass Number): The sum of protons and neutrons in the nucleus.

Example: Carbon has an atomic number of 6 (6 protons) and a typical atomic mass of 12 (6 protons + 6 neutrons).

Determining Protons, Neutrons, and Electrons

• Protons (p+): Equal to the atomic number.

• Electrons (e-): Equal to the atomic number in a neutral atom; different in ions.

• Neutrons (n0): Atomic mass minus atomic number.

Ions

Ions are charged particles formed when atoms gain or lose electrons. The number of protons does not change.

• Cation: Positively charged ion (lost electrons).

• Anion: Negatively charged ion (gained electrons).

Example: Na+ has 11 protons and 10 electrons.

Isotopes

Isotopes are atoms of the same element with different numbers of neutrons.

• Example: Carbon-12 (6p/6n), Carbon-13 (6p/7n), Carbon-14 (6p/8n).

How Elements Combine: Compounds and Molecules

Compounds

A compound is a substance made of two or more different elements bonded together in fixed proportions.

• Example: Sodium chloride (NaCl), Water (H2O)

Molecules

A molecule is a group of atoms held together by covalent bonds.

• Example: Oxygen molecule (O2), Water (H2O)

Chemical Bonds

Covalent Bonds

Covalent bonds form when two atoms share electrons. These are common in organic compounds.

• Example: Water (H2O)

Polar Covalent Bonds

• Electrons are shared unequally, resulting in partial charges on atoms.

• Example: Water (H2O) is polar; oxygen is slightly negative, hydrogens are slightly positive.

Nonpolar Covalent Bonds

• Electrons are shared equally.

• Example: Hydrogen gas (H2), Ethane (C2H6).

Ionic Bonds

Ionic bonds form when atoms transfer electrons, resulting in oppositely charged ions that attract each other.

• Example: Na+ + Cl- → NaCl

Hydrogen Bonds

Hydrogen bonds are weak attractions between a hydrogen atom (already covalently bonded to a highly electronegative atom like oxygen or nitrogen) and another electronegative atom. These are important in holding water molecules together and stabilizing large biological molecules.

• Example: Hydrogen bonds between water molecules.

Properties of Water

Polarity of Water

Water is a polar molecule due to the uneven distribution of electrons between oxygen and hydrogen atoms. Oxygen has a stronger pull on electrons, making it slightly negative, while hydrogens are slightly positive.

Cohesion and Adhesion

• Cohesion: Attraction between molecules of the same substance (e.g., water molecules stick together).

• Adhesion: Attraction between molecules of different substances (e.g., water and glass, causing a meniscus in a graduated cylinder).

• Capillary Action: The ability of water to flow in narrow spaces against gravity, due to cohesion and adhesion.

High Specific Heat Capacity

Water can absorb large amounts of heat with little temperature change, due to hydrogen bonding. This property helps regulate temperature in organisms and environments.

Evaporative Cooling

As water evaporates, it removes heat from surfaces, providing a cooling effect (e.g., sweating).

Versatile Solvent

Water's polarity allows it to dissolve many substances, making it an excellent solvent for biological reactions.

Solutions, Acids, and Bases

Solutions

A solution is a homogeneous mixture of two or more substances.

• Solute: The substance that is dissolved (e.g., iced tea mix).

• Solvent: The substance that does the dissolving (e.g., water).

• In biological systems, water is almost always the solvent.

pH Scale (Acids & Bases)

The pH scale measures the concentration of hydrogen ions (H+) versus hydroxide ions (OH-) in a solution. The scale ranges from 0 (most acidic) to 14 (most basic), with 7 being neutral.

• Acid: Substance with more H+ ions; pH below 7 (e.g., soda, pH 3.0).

• Base: Substance with more OH- ions; pH above 7 (e.g., Nair, pH 13).

• Neutral: Equal amounts of H+ and OH- ions; pH 7 (e.g., pure water).

Chemical Equations

Structure of Chemical Equations

Chemical equations represent the reactants and products in a chemical reaction. The general format is:

• Coefficients: Numbers before compounds/elements, indicating the number of molecules or atoms involved.

• Subscripts: Small numbers within formulas, indicating the number of atoms of each element in a molecule.

Example:

• 6CO2 means 6 molecules of carbon dioxide, each with 1 carbon and 2 oxygen atoms.

• Subscripts: In O2, the subscript 2 means each molecule has 2 oxygen atoms.

Table: Comparison of Bond Types

Additional info: Some explanations and examples have been expanded for clarity and completeness, following standard General Chemistry curriculum.