Atoms and Their Structure

Definition and Components of Atoms

An atom is the smallest unit 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 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 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 8 electrons (for basic chemistry, though it can hold more in higher elements).

Atoms are most stable when their outermost energy level is full.

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

Electron Configuration Examples

• 10 electrons: 2 in the first shell, 8 in the second shell (full second shell, stable configuration).

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

Elements and Atomic Structure

Definition of Elements

An element is a pure substance that cannot be broken down into simpler substances by chemical means. Each element is defined by its number of protons.

• There are about 90 naturally occurring elements.

• All elements are listed in the Periodic Table.

• Only about 25 elements are essential for living things. Four elements (carbon, hydrogen, nitrogen, and oxygen) make up about 96% of the mass of a human body.

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 sum of protons and neutrons in the nucleus.

Determining Subatomic Particles:

• Number of protons = atomic number

• Number of electrons = atomic number (in a neutral atom)

• 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: 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 are gained or lost.

• Example: Na+ has 11 protons and 10 electrons.

• Isotope: Atoms of the same element with different numbers of neutrons.

• Examples: 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 chemically bonded together (e.g., NaCl, H2O).

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

Covalent Bonds

• Formed 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

• Formed when one atom donates an electron to another, resulting in oppositely charged ions that attract each other.

• Example: Na+ + Cl- → NaCl

Hydrogen Bonds

• Weak bonds that form between a hydrogen atom (already covalently bonded to a highly electronegative atom) and another electronegative atom.

• Important in holding water molecules together and stabilizing large biological molecules like proteins and DNA.

Properties of Water

Polarity and Hydrogen Bonding

• Water is a polar molecule due to the unequal sharing of electrons between oxygen and hydrogen.

• Oxygen has a stronger pull on electrons, making it slightly negative, while hydrogen becomes slightly positive.

• This polarity 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 sticking together).

• Adhesion: Attraction between molecules of different substances (e.g., water climbing up plant roots or forming a meniscus in a cylinder).

Other Properties of Water

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

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

• Versatile Solvent: Water dissolves many substances due to its polarity, making it the "universal solvent." In aqueous solutions, water is always the solvent.

Solutions, Acids, and Bases

Solutions

• A solution consists of a solute (substance being dissolved) and a solvent (substance doing the dissolving).

• Example: In iced tea, the mix is the solute and water is the solvent.

pH Scale

• The pH scale measures the concentration of hydrogen ions (H+) versus hydroxide ions (OH-) in a solution.

• Ranges from 0 (most acidic) to 14 (most basic), with 7 being neutral.

• Acids: Substances with more H+ ions (pH < 7).

• Bases: Substances with more OH- ions (pH > 7).

• Examples: Pure water (pH 7), soda (pH 3), hair remover (pH 13).

Chemical Equations

Structure of Chemical Equations

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

• Reactants: Substances that undergo change.

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

• Chemical Equation Example:

• Coefficients: Numbers in front of compounds/elements indicating 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, indicating the number of atoms in a molecule (e.g., H2O has 2 hydrogen atoms).

Table: Comparison of Bond Types