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 dense central core of the atom, containing protons and neutrons.

• Proton (p+): A positively charged particle located inside the nucleus.

• Neutron (n0): A particle with no charge, also found in the nucleus.

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

Energy Levels (Electron Shells)

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

Electron Configuration Examples

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

• 16 electrons: 2 in the first, 8 in the second, 6 in the third (not a full third shell, so less 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 also equals the number of electrons in a neutral atom.

• Atomic Mass: The sum of protons and neutrons in the nucleus.

Determining Subatomic Particles:

• Protons = Atomic Number

• Electrons = Atomic Number (for neutral atoms)

• Neutrons = Atomic Mass - Atomic Number

Example: Carbon has atomic number 6 (6 protons), atomic mass 12 (6 neutrons).

Ions and Isotopes

• Ion: An atom or molecule with a net electric charge due to the loss or gain of electrons.

• Protons do not change in ions; only electrons do.

• Isotope: Atoms of the same element with different numbers of neutrons (e.g., Carbon-12, Carbon-13, Carbon-14).

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

A covalent bond forms 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

An ionic bond forms when atoms transfer electrons, 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) and another electronegative atom. These bonds are important in holding water molecules together and stabilizing large biological molecules.

Properties of Water

Polarity and Hydrogen Bonding

Water is a polar molecule due to the unequal sharing of electrons between oxygen and hydrogen. This results in a partial negative charge on oxygen and a partial positive charge on hydrogen, allowing water molecules to form hydrogen bonds with each other.

Key Properties of Water

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

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

• 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

• Solution: 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 called the "universal solvent" due to its ability to dissolve many substances.

Acids, Bases, and pH

pH Scale

The pH scale measures the concentration of hydrogen ions (H+) in a solution, ranging from 0 (most acidic) to 14 (most basic).

• Acid: Substance with more H+ ions; pH less than 7 (e.g., soda, pH 3).

• Base: Substance with more OH- ions; pH greater than 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. The general format is:

$\text{Reactants} \rightarrow \text{Products}$

• Chemical Equation Example: $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$

• Coefficients: Numbers before compounds/elements indicating the number of molecules or atoms involved (e.g., 6CO2 means 6 molecules of CO2).

• Subscripts: Numbers within chemical formulas indicating the number of atoms of each element in a molecule (e.g., H2O has 2 hydrogen atoms and 1 oxygen atom).

Table: Comparison of Bonds