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.

Energy Levels

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 2 electrons.

• The second energy level can hold 8 electrons.

• The third energy level can hold 18 electrons.

All energy levels in use must be filled for an atom to be stable.

Example: An atom with 8 electrons will have 2 in the first energy level and 6 in the second. This configuration is stable for oxygen.

Additional info: The maximum number of electrons in each energy level is given by , where is the energy level number.

Electron Configuration Examples

• 10 electrons: 2 in the first level, 8 in the second. Stable configuration (Neon).

• 16 electrons: 2 in the first, 8 in the second, 6 in the third. Not a full third level, so 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 the element symbol on the periodic table; equals the number of protons and, in a neutral atom, electrons.

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

Formula:

• 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

• Ion: A charged particle formed when an atom gains or loses electrons. The number of protons does not change.

• Isotope: Atoms of the same element with different numbers of neutrons. Example: Carbon-12 (6p/6n), Carbon-13 (6p/7n), Carbon-14 (6p/8n).

Example: Na+ has 11 protons and 10 electrons.

Chemical Bonds and Compounds

How Elements Combine

• Compound: A substance made of two or more different elements bonded together. Example: Sodium chloride (NaCl), Water (H2O).

• Molecule: A group of atoms held together by covalent bonds. Example: Oxygen (O2).

Covalent Bonds

A covalent bond forms when two atoms share electrons.

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

• Nonpolar Covalent Bond: Electrons are shared equally. Example: Hydrogen gas (H2), Ethane (C2H6).

Ionic Bonds

An ionic bond forms 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 an electronegative atom (like oxygen or nitrogen) in another molecule. These bonds are important for holding water molecules together and stabilizing large biological molecules.

• Hydrogen bonds provide a place for chemical reactions and help maintain the structure of proteins and DNA.

Properties of Water

Polarity and Hydrogen Bonding

Water is a polar molecule due to the uneven distribution of charge; oxygen attracts electrons more strongly than hydrogen, giving the oxygen side a slight negative charge.

• Polarity allows water molecules to form up to four hydrogen bonds with other water molecules.

Cohesion and Adhesion

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

• Adhesion: Attraction between molecules of different substances (water to glass), causing phenomena like the meniscus and capillary action.

Other Properties

• High Specific Heat Capacity: Water absorbs 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, Acids, and Bases

Solutions

A solution consists of a solute (substance dissolved) and a solvent (substance doing the dissolving). Water is a common solvent.

• 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 (acidic) to 14 (basic).

• pH 7: Neutral (pure water)

• pH < 7: Acidic (more H+ ions)

• pH > 7: Basic (more OH- ions)

Examples:

• Pure water: pH 7

• Soda: pH 3

• Hair remover (Nair): pH 13

Formula:

Chemical Equations

Reactants, Products, and Coefficients

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

• Example:

• Reactants: C6H12O6, O2

• Products: CO2, H2O, energy

• Coefficients multiply the number of atoms: 6CO2 means 6 carbon atoms and 12 oxygen atoms.

Subscripts indicate the number of atoms in a molecule: O2 means 2 oxygen atoms.

Summary Table: Atomic Particles

Summary Table: Types of Chemical Bonds