Atoms and Atomic Structure

Definition and Structure of the Atom

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.

Energy Levels

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

• The first energy level can hold 2 electrons.

• The second energy level can hold 8 electrons.

• The third energy level can hold 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 energy level and 6 in the second energy 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, 6 in the third (not fully stable).

Elements and Atomic Properties

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

• Atomic Number: The number above an element symbol on the periodic table; equals the number of protons (and electrons in a neutral atom).

• Atomic Mass: The number below the element symbol; equals the sum of protons and neutrons.

Determining Subatomic Particles

• Protons: Equal to the atomic number.

• Electrons: Equal to the atomic number (unless the atom is an ion).

• Neutrons: Atomic mass minus atomic number.

• Example: Carbon has atomic number 6, so 6 protons and 6 electrons; neutrons = atomic mass (usually 12) - 6 = 6.

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 (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).

How Elements Combine: Compounds and Molecules

Compounds

A compound is a substance made of two or more different elements bonded together.

• Example: Sodium chloride (NaCl), Water (H2O).

Molecules

A molecule is a group of atoms held together by covalent bonds.

• Example: Oxygen (O2).

Chemical Bonds

Covalent Bonds

Covalent bonds form when two atoms share electrons.

• Example: Water (H2O).

• Found in organic compounds.

Polar Covalent Bonds

• Electrons are not shared equally.

• Results in partial charges on atoms.

• Example: Water (H2O) is polar.

Nonpolar Covalent Bonds

• Electrons are shared equally.

• Example: Hydrogen gas (H2), Ethane (C2H6).

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 or nitrogen) in another molecule. They are important in holding water molecules and large biological molecules together.

• Provide a place for chemical reactions.

• Help maintain the structure of proteins and DNA.

Properties of Water

Polarity

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 the oxygen atom and a partial positive charge on the hydrogen atoms.

Cohesion and Adhesion

• 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).

• Capillary action is a result of adhesion.

High Specific Heat Capacity

• Water can absorb large amounts of heat due to hydrogen bonding.

• Helps regulate temperature in organisms and environments.

Evaporative Cooling

• As water evaporates, it removes heat (e.g., sweating).

Versatile Solvent

• Water dissolves many substances due to its polarity.

• Called the "universal solvent" in biological systems.

Solutions and pH

Solutions

A solution is a homogeneous mixture of two or more substances.

• Solute: The substance dissolved (e.g., iced tea mix).

• Solvent: The substance doing the dissolving (e.g., water).

• Water is often the solvent in biological systems.

pH: Acids and Bases

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).

Chemical Equations

Structure of Chemical Equations

Chemical equations represent the reactants and products in a chemical reaction.

• Reactants: Substances that combine or change during the reaction.

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

• Coefficients: Numbers before compounds/elements indicating the number of molecules or atoms involved.

• Subscripts: Numbers written below and to the right of element symbols indicating the number of atoms in a molecule.

Example Equation:

• 6CO2: 6 molecules of carbon dioxide (6 C atoms, 12 O atoms).

• 6H2O: 6 molecules of water (12 H atoms, 6 O atoms).

Table: Comparison of Bond Types

Additional info: Energy levels for electrons are filled in order of increasing energy, and the stability of an atom is often determined by the octet rule (having 8 electrons in the outermost shell).