Atoms and Atomic Structure

Definition and Components of an Atom

An atom is the smallest unit of an element that retains the properties of that element. Atoms are the building blocks of matter and are composed of subatomic particles.

• Nucleus: The central part 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 energy levels.

Energy Levels and Electron Arrangement

Electrons occupy specific regions called 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.

For an atom to be stable, its outermost energy level must be filled according to the octet rule (for main group elements).

Example: An atom with 8 electrons will have 2 in the first energy level and 6 in the second, making it stable if the outer shell is full.

Additional info: The arrangement of electrons determines the chemical reactivity of the atom.

Elements and the Periodic Table

Definition and Properties of Elements

An element is a pure substance that cannot be broken down into simpler substances by chemical means. Each element is defined by its number of protons (atomic number).

• There are 90 naturally occurring elements.

• All elements are 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. It also equals the number of electrons in a neutral atom.

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

Formula:

Example: Carbon has atomic number 6 and atomic mass 12, so it has 6 neutrons.

Ions and Isotopes

Ions

Ions are atoms or molecules that have gained or lost electrons, resulting in a net charge.

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

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

• The number of protons does not change when forming ions.

Example: 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)

Chemical Bonds and Molecules

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, ethane).

Ionic Bonds

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

• Example:

Hydrogen Bonds

Hydrogen bonds are weak attractions between a hydrogen atom in one molecule and a highly electronegative atom (such as oxygen or nitrogen) in another molecule. These bonds are crucial for the structure of water and biological macromolecules.

• Hold water molecules together.

• Help stabilize large molecules like proteins and DNA.

Properties of Water

Polarity and Hydrogen Bonding

Water is a polar molecule due to the uneven distribution of electrons between oxygen and hydrogen atoms. Oxygen has a stronger pull on electrons, giving it a slight negative charge and hydrogen a slight positive charge.

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).

• Causes phenomena such as meniscus formation and capillary action.

Specific Heat Capacity and Solvent Properties

• High Specific Heat Capacity: Water absorbs large amounts of heat due to hydrogen bonding.

• Evaporative Cooling: Water cools surfaces as it evaporates (e.g., sweating).

• Versatile Solvent: Water dissolves many substances due to its polarity.

Solutions, Acids, and Bases

Solutions

A solution consists of a solute (substance dissolved) and a solvent (substance doing the dissolving). Water is known as the "universal solvent" due to its ability to dissolve many substances.

• Example: Iced tea mix (solute) dissolved in water (solvent).

pH Scale

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).

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

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

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

Example Equation:

Chemical Equations

Reactants, Products, Coefficients, and Subscripts

Chemical equations represent the transformation of reactants into products. Coefficients indicate the number of molecules, while subscripts show the number of atoms in each molecule.

• Reactants: Substances that undergo change.

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

• Chemical Equation Example:

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

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

Summary Table: Atomic Particles

Summary Table: Types of Chemical Bonds

Additional info: These foundational chemistry concepts are essential for understanding organic chemistry, biochemistry, and molecular biology. Mastery of atomic structure, bonding, and water properties is crucial for further study in organic reactions and biological systems.