Atoms & Elements

Structure of the Atom

An atom is the smallest particle 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 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 travel around the nucleus in specific regions called energy levels or shells. Each energy level can hold a certain maximum number of electrons:

• The first energy level can hold up to 2 electrons.

• The second energy level can hold up to 8 electrons.

• The third energy level can hold up to 18 electrons.

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

• Example: An atom with 8 electrons: 2 in the first level, 6 in the second. The second level is not full, so the atom is not fully stable.

Electron Configuration Examples

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

• 16 electrons: 2 in the first, 8 in the second, 6 in the third (not fully stable).

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 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 (Z): The number of protons in the nucleus of an atom. It also equals the number of electrons in a neutral atom.

• Atomic Mass (A): The total number 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 (Z = 6, A = 12): 6 protons, 6 electrons, 6 neutrons

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)

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 one atom transfers electrons to another, 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 molecules like proteins.

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.

Hydrogen Bonding in Water

• Hydrogen bonds form between the positively charged hydrogen of one water molecule and the negatively charged oxygen of another.

• These bonds are responsible for many of water's unique properties.

Key Properties of Water

• Cohesion: Attraction between molecules of the same substance (e.g., water droplets).

• Adhesion: Attraction between molecules of different substances (e.g., water climbing up a plant stem).

• 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

A solution consists of a solute (substance dissolved) and a solvent (substance doing the dissolving). Water is known as the "universal solvent." For example, in iced tea, the mix is the solute and water is the solvent.

Acids, Bases, and pH

pH Scale

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: More H+ ions, pH below 7 (e.g., HCl in water forms H+ and Cl-).

• Base: More OH- ions, pH above 7 (e.g., NaOH in water forms Na+ and OH-).

• Neutral: Equal amounts of H+ and OH- (e.g., pure water, pH 7).

Chemical Reactions & Equations

Chemical Equations

Chemical equations represent the reactants and products in a chemical reaction. Reactants are substances that combine, and products are substances formed.

• Chemical Equation Example:

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

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

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