Atoms and Atomic Structure

Definition and Structure of the 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.

• 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 with no charge, also located in the nucleus.

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

Energy Levels and Electron Configuration

Electrons occupy specific energy levels (also called shells) around the nucleus. 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

For an atom to be stable, its outermost energy level (valence shell) should be full or follow the octet rule (8 electrons).

• Example: An atom with 8 electrons will have 2 in the first level and 6 in the second. It is not fully stable (octet not complete).

• Example: Draw the electron configuration for an atom with 10 electrons: 2 in the first level, 8 in the second (stable configuration).

Atomic Number, Atomic Mass, and Isotopes

• Atomic Number (Z): The number of protons in the nucleus of an atom. It defines the element and is equal to the number of electrons in a neutral atom.

• Atomic Mass (A): The total number of protons and neutrons in the nucleus.

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

Calculating Subatomic Particles:

• Number of protons = atomic number

• Number of electrons = atomic number (for neutral atoms)

• Number of neutrons = atomic mass - atomic number

Ions

Ions are charged particles formed when atoms gain or lose 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.

Elements and Compounds

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 in 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).

Compounds and Molecules

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

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

Chemical Bonds

Covalent Bonds

A covalent bond forms when two atoms share electrons. Covalent bonds are common in organic compounds.

• 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

An ionic bond forms when one atom transfers electrons to another, resulting in oppositely charged ions that attract each other (e.g., Na+ + Cl- → NaCl).

Hydrogen Bonds

Hydrogen bonds are weak attractions between a hydrogen atom (already covalently bonded to a highly electronegative atom like oxygen or nitrogen) and another electronegative atom. These bonds are important in holding water molecules together and stabilizing large biological molecules like proteins.

Properties of Water

Polarity and Hydrogen Bonding

Water (H2O) is a polar molecule due to the uneven distribution of electrons. Oxygen has a stronger pull on electrons, making it slightly negative, while hydrogen atoms are slightly positive. This allows water molecules to form hydrogen bonds with each other.

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 and capillary action).

Other Properties of Water

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

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

• Versatile Solvent: Water dissolves many substances due to its polarity, making it an excellent solvent in biological systems.

Solutions, Acids, and Bases

Solutions

A solution is a homogeneous mixture of two or more substances. It consists of a solute (the substance dissolved) and a solvent (the substance doing the dissolving). Water is known as the "universal solvent."

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

pH Scale, 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), with 7 being neutral.

• Acid: Substance with more H+ ions (pH < 7). Example: Soda (pH 3.0).

• Base: Substance with more OH- ions (pH > 7). Example: Nair (pH 13).

• Neutral: Equal amounts of H+ and OH- ions (pH 7.0). Example: Pure water.

Chemical Equations

Structure of Chemical Equations

Chemical equations represent the reactants and products in a chemical reaction. The general format is:

• Coefficients: Numbers placed before compounds to indicate the number of molecules or atoms involved (e.g., 6CO2 means 6 molecules of CO2).

• Subscripts: Numbers written below and to the right of element symbols to indicate the number of atoms in a molecule (e.g., H2O has 2 hydrogen atoms).

Example:

• Reactants: C6H12O6 and O2

• Products: CO2, H2O, and energy

Coefficients and subscripts are used to balance chemical equations and show the conservation of mass.

Summary Table: Subatomic Particles

Key trend: Protons and neutrons have similar mass and are found in the nucleus, while electrons are much lighter and orbit the nucleus.

Key Formulas and Equations

• Number of neutrons:

• General chemical equation:

• Example (cellular respiration):