Atoms and Atomic Structure

Definition of an 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.

Subatomic Particles and Atomic Arrangement

• 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 found in the nucleus with no electric charge.

• Electron (e-): A negatively charged particle that orbits the nucleus in energy levels.

Energy Levels (Electron Shells)

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: up to 2 electrons

• Second energy level: up to 8 electrons

• Third energy level: 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 electrons in the first energy level and 6 in the second. This configuration is stable for oxygen.

Electron Configuration Examples

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

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

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 total number 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 Subatomic Particles

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

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

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

Example: For carbon (atomic number 6, atomic mass 12): Protons = 6, Electrons = 6, Neutrons = 12 - 6 = 6

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)

Chemical Bonds and Compounds

How Elements Combine

• Compound: A substance made of two or more different elements bonded together (e.g., NaCl, H2O).

• Molecule: A group of atoms held together by covalent bonds (e.g., O2).

Covalent Bonds

Covalent bonds form when two atoms share electrons.

• Polar Covalent Bond: Electrons are shared unequally, resulting in partial charges (e.g., H2O).

• Nonpolar Covalent Bond: Electrons are shared equally (e.g., H2, ethane).

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) and another electronegative atom. They are important in holding water molecules together and stabilizing large molecules like proteins.

Properties of Water

Polarity of Water

Water (H2O) is a polar molecule due to the uneven distribution of electrons. Oxygen has a stronger pull on electrons, making the oxygen side slightly negative and the hydrogen side slightly positive.

Hydrogen Bonding in Water

Hydrogen bonds cause water molecules to attract each other, leading to unique properties:

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

• Adhesion: Attraction between molecules of different substances (e.g., water climbing up a plant stem).

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

• Evaporative Cooling: As water evaporates, it removes heat (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), with 7 being neutral.

• Acid: More H+ ions, pH less than 7 (e.g., soda, pH 3.0).

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

• Neutral: Equal H+ and OH- ions (e.g., pure water, pH 7.0).

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 indicate the number of atoms in a molecule.

Example:

Photosynthesis equation:

$$ 6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2 $$

• Reactants: Substances that start the reaction (CO2 and H2O).

• Products: Substances formed by the reaction (C6H12O6 and O2).

• Chemical coefficients: The numbers before compounds (e.g., 6CO2) show how many molecules are involved.

• Subscripts: The small numbers in formulas (e.g., H2O) show the number of atoms of each element in a molecule.

Summary Table: Subatomic Particles

Key Point: The number of protons defines the element, while the number of neutrons determines the isotope, and the number of electrons determines the charge (ion).

Additional info: Some explanations and examples have been expanded for clarity and completeness, such as the electron configuration and the summary table of subatomic particles.