Atoms & Elements

Concept of Matter and Chemical Elements

Matter is defined as anything that takes up space and has mass. All matter is composed of chemical elements, which are pure substances made of only one type of atom. Atoms are the smallest unit of an element and, therefore, the smallest unit of matter. Both living and non-living matter are made up of atoms.

• Matter: Anything with mass and volume (e.g., organisms, rocks, oceans).

• Chemical Element: Pure substance consisting of only one type of atom.

• Atom: Smallest unit of an element, fundamental building block of matter.

• Example: Atoms are the smallest units of matter.

Atomic Structure

Atoms are composed of three subatomic particles: protons, neutrons, and electrons. Each particle has a characteristic charge, mass, and location within the atom.

• Proton: Positively charged, mass of 1 atomic mass unit (AMU), located in the nucleus.

• Neutron: No charge, mass of 1 AMU, located in the nucleus.

• Electron: Negatively charged, negligible mass, located in electron shells orbiting the nucleus.

Elements of Life and the Periodic Table

Only a small subset of elements is found in living organisms. The periodic table arranges all known elements based on their chemical properties. About 97% of the mass of most life is composed of Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus, and Sulfur (CHNOPS).

• Major Elements: Required for life in large amounts (CHNOPS).

• Trace Elements: Required in small amounts.

Atomic Properties

Atomic Number, Mass Number, and Atomic Mass

Each atom of an element has unique properties:

• Atomic Number (Z): Number of protons in the nucleus; defines the element.

• Mass Number (A): Total number of protons and neutrons in the nucleus.

• Atomic Mass: Weighted average mass of all isotopes of an element.

Formula:

Electron Orbitals & Energy Shells

Electron Shells and Valence Electrons

Electron orbitals are three-dimensional regions around a nucleus where electrons are likely to be found. These are visualized as energy shells. Shells closer to the nucleus are lower in energy, while distant shells are higher in energy. Valence electrons are found in the outermost energy shell and are crucial for chemical bonding.

• 1st shell: Holds up to 2 electrons.

• 2nd shell: Holds up to 8 electrons.

• Valence shell: Outermost shell, determines chemical reactivity.

Octet Rule

Stability of Atoms

The octet rule states that atoms are more stable (less reactive) when their valence shells are fully occupied, typically with eight electrons. Atoms are most reactive when their outer valence shells are not full.

• 1st shell: Full with 2 electrons.

• 2nd shell: Full with 8 electrons.

• Octet Rule: Atoms seek to fill their valence shell to achieve stability.

Isotopes

Definition and Properties

Isotopes are atoms of the same element that differ in the number of neutrons. They have the same atomic number (number of protons) but different mass numbers (protons + neutrons). Atomic mass is the average mass of all isotopes.

• Example: Carbon-12, Carbon-13, and Carbon-14 are isotopes of carbon.

• Isotopes: Same element, different neutron count.

Radioactive Isotopes

Radioactive isotopes are unstable and break down, emitting energy in the form of rays or particles. The half-life is the time it takes for half of the radioactive atoms in a sample to decay. Radioactive isotopes are used in medicine and radiometric dating.

• Half-life: Time for half the atoms to decay.

• Applications: Medical imaging, cancer treatment, fossil dating.

Chemical Bonding

Introduction to Chemical Bonds

Chemical bonds are attractive forces between atoms, holding them together to form molecules and compounds. Molecules contain two or more chemically bound atoms, while compounds are molecules composed of two or more different elements. Chemical formulas reveal the composition and ratio of atoms in a molecule.

• Intramolecular Bonds: Interactions within a single molecule.

• Intermolecular Bonds: Interactions between atoms of different molecules.

Covalent Bonds

Covalent bonds are interactions between two atoms resulting from the sharing of electrons. There are two types of covalent bonds: nonpolar and polar, which arise due to differences in electronegativity.

• Electronegativity: Measure of an atom's attraction for electrons (scale 0-4).

• Nonpolar Covalent Bond: Equal sharing of electrons between atoms with similar electronegativities.

• Polar Covalent Bond: Unequal sharing of electrons between atoms with different electronegativities, resulting in partial charges (δ).

Noncovalent Bonds

Types and Properties

Noncovalent bonds are interactions between atoms resulting from full or partial charges, without sharing electrons. Types include ionic bonds, hydrogen bonds, van der Waals forces, and electrostatic bonds. These bonds are generally weaker than covalent bonds but are important in biological systems.

• Ionic Bonds: Electrical attractions between oppositely charged ions.

• Hydrogen Bonds: Interaction between a highly electronegative atom (F, O, N) and a hydrogen atom.

• Van der Waals Forces: Weak attractions due to temporary dipoles.

Ionic Bonding

Ions: Anions vs. Cations

Ions are atoms or molecules with a net electrical charge, resulting from the gain or loss of electrons. Anions are negatively charged ions (gain of electron), while cations are positively charged ions (loss of electron).

• Anion: Negatively charged ion.

• Cation: Positively charged ion.

Ionic Bonds

Ionic bonds are electrical attractions between oppositely charged ions. The transfer of electrons fills the valence shells of both atoms and creates charges.

• Example: Formation of ionic bond in sodium chloride (NaCl).

Hydrogen Bonding

Properties and Importance

Hydrogen bonds are interactions between a highly electronegative atom (such as F, O, or N) and a hydrogen atom. Individually, hydrogen bonds are weak, but collectively, they can be strong and are important in biology, including the properties of water and the structure of macromolecules.

• Hydrogen Bond: Weak interaction, but significant in large numbers.

• Importance: Water properties, DNA structure, protein folding.