Atoms and Their Structure

Definition and Components of Atoms

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 (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 main group elements).

All energy levels being used must be filled for an element to be stable (octet rule for main group elements).

• Example: An atom with 8 electrons will have 2 in the first energy level and 6 in the second. This is the electron configuration for oxygen, which is not fully stable (needs 2 more electrons for a full octet).

Electron Configuration Examples

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

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

Elements and the Periodic Table

Definition of Elements

An element is a 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 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 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

• Protons (p+): Equal to the atomic number.

• Electrons (e-): Equal to the atomic number in a neutral atom; changes if the atom is an ion.

• Neutrons (n0): Calculated as atomic mass minus atomic number.

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

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 (loss of electrons).

• Anion: Negatively charged ion (gain of 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 chemically bonded together.

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

Molecules

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

• Example: Oxygen gas (O2).

Covalent Bonds

Covalent bonds form when two atoms share electrons.

• Example: Water (H2O).

• Found in most organic compounds.

Polar vs. Nonpolar Covalent Bonds

• Polar Bond: Electrons are shared unequally, resulting in partial charges (e.g., H2O).

• Nonpolar Bond: Electrons are shared equally (e.g., H2, ethane).

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 (already covalently bonded to a highly electronegative atom) and another electronegative atom. They are important in holding water molecules together and stabilizing large biological molecules like proteins and DNA.

Properties of Water

Polarity of Water

Water (H2O) is a polar molecule due to the uneven distribution of electrons. Oxygen has a stronger pull on electrons than hydrogen, making the oxygen side slightly negative and the hydrogen side slightly positive.

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 in a graduated cylinder).

High Specific Heat Capacity

Water can absorb large amounts of heat due to its many hydrogen bonds, helping to stabilize temperatures in organisms and environments.

Evaporative Cooling

As water evaporates, it removes heat from surfaces, providing a cooling effect (e.g., sweating).

Versatile Solvent

Water's polarity makes it an excellent solvent, able to dissolve many ionic and polar substances.

Solutions, Acids, and Bases

Solutions

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

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

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

• In aqueous solutions, water is always the solvent.

pH Scale (Acids & 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 below 7 (e.g., soda, pH 3.0).

• Base: Substance with more OH- ions, pH above 7 (e.g., Nair, pH 13).

• Neutral: Equal amounts of H+ and OH- ions (e.g., pure water, pH 7.0).

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 carbon dioxide).

• 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 and 1 oxygen atom).

Example Equation:

• Reactants: C6H12O6 (glucose) and O2 (oxygen)

• Products: CO2 (carbon dioxide), H2O (water), and energy

Counting Atoms in Equations

• Multiply the coefficient by the subscript to find the total number of atoms of each element.

• Example: In 6CO2, there are 6 × 1 = 6 carbon atoms and 6 × 2 = 12 oxygen atoms.

Summary Table: Key Subatomic Particles

Additional info:

• Some context and terminology were expanded for clarity and completeness, such as the octet rule and the distinction between cations and anions.

• Electron configuration examples were inferred based on standard chemistry knowledge.