Atoms and Their Structure

Definition and Components of the Atom

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: A positively charged particle located inside the nucleus.

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

• Electron: A negatively charged particle that orbits the nucleus in defined 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 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 element to be stable.

• 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 stable when its outer shell is full.

Electron Configuration Examples

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

• 16 electrons: 2 in the first shell, 8 in the second, 6 in the third (Sulfur, not a full outer shell, so not as stable as noble gases).

Elements and the Periodic Table

Definition of an Element

An element is a pure substance that cannot be broken down into simpler substances by chemical means. Each element is made up of only one kind of atom.

• 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 determines the element's identity.

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

For a neutral atom:

• Number of protons = atomic number

• Number of electrons = atomic number

• Number of neutrons = atomic mass - atomic number

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

Ions and Isotopes

Ions

Ions are atoms or molecules that have gained or lost electrons, resulting in a net charge.

• Loss of electrons produces a cation (positive ion).

• Gain of electrons produces an anion (negative ion).

• The number of protons does not change in ion formation.

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

Isotopes

Isotopes are atoms of the same element with different numbers of neutrons.

• Example: Carbon-12 (6 protons, 6 neutrons), Carbon-13 (6 protons, 7 neutrons), Carbon-14 (6 protons, 8 neutrons).

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).

Chemical Bonds

Covalent Bonds

Covalent bonds form when two atoms share electrons.

• Found in many organic compounds.

• Example: Water (H2O).

Polar Covalent Bonds

• Electrons are shared unequally, resulting in partial charges on atoms.

• Example: Water (H2O) is polar; oxygen is slightly negative, hydrogens are slightly positive.

Nonpolar Covalent Bonds

• Electrons are shared equally.

• Example: Hydrogen gas (H2), ethane (C2H6).

Ionic Bonds

Ionic bonds form when electrons are transferred from one atom to another, creating 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.

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

Properties of Water

Polarity of Water

Water is a polar molecule due to the uneven distribution of electrons between oxygen and hydrogen atoms. Oxygen has a stronger pull on electrons, making it slightly negative, while hydrogens are 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 to glass, causing a meniscus).

• These properties enable capillary action.

High Specific Heat Capacity

• Water can absorb large amounts of heat with little temperature change due to hydrogen bonding.

• This property helps regulate temperature in organisms and environments.

Evaporative Cooling

• As water evaporates, it removes heat, cooling the surface (e.g., sweating).

Versatile Solvent

• Water's polarity allows it to dissolve many substances, making it an excellent solvent for biological reactions.

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 biological systems, water is almost always the 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 less than 7 (e.g., soda, pH 3.0).

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

• Neutral: Equal amounts of 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:

• Coefficients: Numbers in front of compounds/elements indicating the number of molecules or atoms involved.

• Subscripts: Numbers written below and to the right of element symbols, indicating the number of atoms in a molecule.

• Example: Photosynthesis equation:

• 6CO2: 6 molecules of carbon dioxide

• 6H2O: 6 molecules of water

• C6H12O6: 1 molecule of glucose

• 6O2: 6 molecules of oxygen

Coefficients multiply the number of each atom in the formula. For example, in 6CO2, there are 6 carbon atoms and 12 oxygen atoms.

Summary Table: Key Atomic Particles