Atoms and Atomic Structure

Definition and Components of the Atom

An atom is the smallest unit of an element that retains the chemical properties of that element. Atoms are composed of subatomic particles arranged in a specific structure.

• Nucleus: The central region of the atom containing protons and neutrons.

• Proton: A positively charged particle located inside the nucleus.

• Neutron: A particle with no charge, also found in the nucleus.

• Electron: A negatively charged particle that orbits the nucleus in defined energy levels.

Example: A carbon atom contains 6 protons, 6 neutrons, and 6 electrons.

Energy Levels and Electron Arrangement

Electrons occupy specific energy levels around the nucleus. Each energy level can hold a limited number of electrons:

• The first energy level can hold up to 2 electrons.

• The second energy level can hold up to 8 electrons.

• The third energy level can hold up to 18 electrons.

All energy levels in use must be filled for an atom to be stable.

Example: An atom with 8 electrons will have 2 in the first level and 6 in the second level.

Electron Configuration Examples

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

• 16 electrons: 2 in the first level, 8 in the second level, 6 in the third level (not fully stable).

Elements and the Periodic Table

Definition of Elements

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

• Only 25 elements are essential for living organisms.

• 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; also equals the number of electrons in a neutral atom.

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

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

Determining Subatomic Particles

• Protons: Equal to atomic number.

• Electrons: Equal to atomic number in a neutral atom; differs in ions.

• Neutrons: Atomic mass minus atomic number.

Ions and Isotopes

Ions

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

• Cation: Positively charged ion (loss of electrons).

• Anion: Negatively charged ion (gain of electrons).

Example: Na+ has 11 protons and 10 electrons.

Isotopes

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

• Carbon-12: 6 protons, 6 neutrons

• Carbon-13: 6 protons, 7 neutrons

• Carbon-14: 6 protons, 8 neutrons

Chemical Bonds and Compounds

Compounds and Molecules

A compound is a substance made of two or more different elements bonded together. A molecule is a group of atoms held together by covalent bonds.

• Example of Compound: Sodium chloride (NaCl), water (H2O)

• Example of Molecule: Oxygen (O2)

Covalent Bonds

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

• Polar Covalent Bond: Electrons are shared unequally, resulting in partial charges (e.g., water).

• Nonpolar Covalent Bond: Electrons are shared equally (e.g., hydrogen gas).

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 in one molecule and a highly electronegative atom (such as oxygen) in another molecule. These bonds are important for holding water molecules together and stabilizing large biological molecules.

Properties of Water

Polarity and Hydrogen Bonding

Water is a polar molecule due to the uneven distribution of charge; oxygen attracts electrons more strongly than hydrogen, resulting in a partial negative charge on oxygen and a partial positive charge on hydrogen.

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).

High Specific Heat Capacity

Water can absorb large amounts of heat due to its hydrogen bonds, helping regulate temperature in organisms and environments.

Versatile Solvent

Water dissolves many substances due to its polarity, making it an excellent solvent for biological reactions.

Solutions and pH

Solutions

A solution consists of a solute (substance dissolved) and a solvent (substance doing the dissolving). Water is commonly the solvent in biological systems.

• Example: Iced tea mix (solute) dissolved in water (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 (acidic) to 14 (basic), with 7 being neutral.

• Acid: More H+ ions, pH below 7 (e.g., soda, pH 3).

• Base: More OH- ions, pH above 7 (e.g., hair remover, pH 13).

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

Chemical Equations

Structure of Chemical Equations

Chemical equations represent the reactants and products in a chemical reaction. Coefficients indicate the number of molecules, while subscripts show the number of atoms in each molecule.

• Example Equation:

• Reactants: Substances that undergo change (left side).

• Products: Substances formed (right side).

• Chemical coefficients: Numbers before compounds (e.g., 6CO2 means 6 molecules of CO2).

• Subscripts: Numbers within formulas (e.g., H2O has 2 hydrogen atoms).

Table: Comparison of Bonds

Table: pH Examples

Additional info: Some details about energy levels and electron configurations were inferred based on standard chemistry knowledge.