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 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 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

For an atom to be stable, its outermost energy level (valence shell) should be full or satisfy the octet rule (8 electrons for most atoms).

Example: An atom with 8 electrons will have 2 electrons in the first energy level and 6 in the second. This configuration is not fully stable, as the second shell can hold up to 8 electrons.

Electron Configuration Examples

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

• 16 electrons: 2 in the first shell, 8 in the second shell, 6 in the third shell (not fully stable, as the third shell can hold up to 18 electrons).

Elements and the Periodic Table

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 90 naturally occurring elements.

• All elements are listed on the Periodic Table.

• Only about 25 elements are essential for living things.

• Approximately 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 is unique for each element and is also equal to the number of electrons in a neutral atom.

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

Formula:

Example: Carbon has an atomic number of 6 (6 protons) and an atomic mass of 12 (6 protons + 6 neutrons).

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): Atomic mass minus atomic number.

Ions and Isotopes

Ions

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

• Cation: Positively charged ion (loss of electrons).

• Anion: Negatively charged ion (gain of electrons).

• The number of protons does not change when an atom becomes an ion.

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).

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

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.

• Example: Na+ + Cl- → NaCl

Hydrogen Bonds

Hydrogen bonds are weak attractions between a hydrogen atom (already covalently bonded to a highly electronegative atom like oxygen or nitrogen) and another electronegative atom. These bonds are important in holding water molecules together and stabilizing large biological molecules like proteins.

Properties of Water

Polarity of Water

Water (H2O) is a polar molecule due to the unequal sharing of electrons between oxygen and hydrogen. Oxygen is more electronegative, so it pulls electrons closer, giving it a partial negative charge and hydrogen a partial positive charge.

Hydrogen Bonding in Water

• Hydrogen bonds form between the slightly positive hydrogen of one water molecule and the slightly negative 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 molecules stick together).

• Adhesion: Attraction between molecules of different substances (e.g., water and glass, causing a meniscus).

• 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, helping organisms regulate temperature (e.g., sweating).

• Versatile Solvent: Water dissolves many substances due to its polarity, 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).

Example: Adding HCl to water increases H+ concentration, making the solution acidic. Adding NaOH increases OH- concentration, making the solution basic.

Chemical Equations

Structure of Chemical Equations

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

• Reactants: Substances that undergo the reaction (left side of the equation).

• Products: Substances formed by the reaction (right side of the equation).

• Coefficients: Numbers placed before compounds to indicate the number of molecules or atoms involved.

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

Example Equation:

• 6CO2: 6 molecules of carbon dioxide, each with 1 carbon and 2 oxygen atoms.

• 6H2O: 6 molecules of water, each with 2 hydrogen and 1 oxygen atom.

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