Atoms and Atomic Structure

Definition and Structure of Atoms

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

• 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.

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

Energy Levels and Electron Configuration

Electrons occupy specific regions called energy levels around the nucleus. Each energy level can hold a limited 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

For an atom to be stable, its energy levels must be filled according to the number of electrons present.

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

Additional info: The stability of an atom is often associated with a filled outermost energy level (octet rule).

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 about 90 naturally occurring elements, all listed on the Periodic Table.

• Only 25 elements are essential for living things.

• Four elements—carbon (C), hydrogen (H), nitrogen (N), and oxygen (O)—make up 96% of the mass of a human body.

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 sum of protons and neutrons and is found below the element symbol.

• Number of protons = atomic number

• Number of electrons = atomic number (in a neutral atom)

• Number of neutrons = atomic mass - 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

An ion is a charged particle 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.

• 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).

Example: Water (H2O) is a polar molecule.

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 in holding water molecules together and stabilizing large biological molecules.

• Hydrogen bonds provide a place for chemical reactions and contribute to the structure of 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. Oxygen attracts electrons more strongly, 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, causing a meniscus).

These properties allow for phenomena such as capillary action.

Specific Heat Capacity and Evaporative Cooling

• Water has a high specific heat capacity due to hydrogen bonding, allowing it to absorb large amounts of heat.

• Evaporative cooling: As water evaporates (e.g., sweating), it removes heat from surfaces.

Water as a Versatile Solvent

Water's polarity makes it an excellent solvent, capable of dissolving many substances.

• Solution: A mixture of a solute (e.g., iced tea mix) dissolved in a solvent (e.g., water).

• Water is often called the "universal solvent" because it dissolves more substances than any other liquid.

Acids, Bases, and pH

pH Scale and Ion Concentration

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).

• pH < 7: Acidic (more H+ ions)

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

• pH > 7: Basic (more OH- ions)

Examples:

• Pure water: pH 7.0

• Soda: pH 3.0

• Hair remover (Nair): pH 13.0

Additional info: The formula for pH is .

Chemical Equations

Reactants, Products, and Coefficients

Chemical equations represent the transformation of reactants into products. Coefficients indicate the number of molecules involved.

• Reactants: Substances that undergo change

• Products: Substances formed as a result

• Chemical equation example:

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

• Subscripts: Numbers within formulas indicating the number of atoms (e.g., O2 means 2 oxygen atoms)

Table: Comparison of Bond Types