Atoms and Atomic Structure

Definition and Structure of Atoms

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 central part of the atom, containing protons and neutrons.

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

• Neutron: A particle found in the nucleus with no charge (neutral).

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

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

Energy Levels and Electron Configuration

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

• First energy level: can hold up to 2 electrons

• Second energy level: can hold up to 8 electrons

• Third energy level: can hold up to 18 electrons

All energy levels being used must be filled for an element to be stable.

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

Electron Configuration: The arrangement of electrons in an atom's energy levels. For example, for an atom with 10 electrons: 2 in the first level, 8 in the second level (stable configuration).

Elements and the Periodic Table

Definition of Elements

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 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

The atomic number is the number of protons in an atom and is found above the element symbol on the periodic table. The atomic mass is the number found below the element symbol and represents the sum of protons and neutrons.

• Protons: Equal to the atomic number.

• Electrons: Also equal to the atomic number (in a neutral atom).

• Neutrons: Calculated as atomic mass minus atomic number.

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

Ions and Isotopes

Ions

Ions are charged particles formed when an atom gains or loses 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.

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

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: Salt (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 (like oxygen) in another molecule. These bonds are important in 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 has a stronger pull on electrons than hydrogen, making the oxygen side slightly negative.

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

• Adhesion: Attraction between molecules of different substances (water sticks to glass, causing a meniscus).

• 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

A solution consists of a solute (substance dissolved) and a solvent (substance doing the dissolving). Water is known as the "universal solvent."

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

Acids, Bases, and pH

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: Substance with more H+ ions, pH below 7 (e.g., soda, pH 3).

• 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. Coefficients indicate the number of molecules, and subscripts indicate the number of atoms in each molecule.

• Example: Photosynthesis equation:

• Reactants: Substances that combine (CO2 and H2O)

• Products: Substances formed (C6H12O6 and O2)

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

• Subscripts: Numbers within formulas (e.g., H2O has 2 hydrogens per molecule)

Summary Table: Atomic Particles

Summary Table: Types of Chemical Bonds

Additional info: Some details, such as the exact electron capacity of higher energy levels and the biological relevance of water, have been expanded for academic completeness.