Atoms & Elements

What is Matter?

Matter is anything that takes up space and has mass, including all living and nonliving things. All matter is composed of chemical elements, which are pure substances made of only one type of atom. Atoms are the smallest units of elements and, therefore, the smallest units of matter.

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

• Chemical Element: A pure substance consisting of only one kind of atom.

• Atom: The smallest unit of an element that retains its chemical properties.

Atomic Structure

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

• Proton: Positively charged particle found in the nucleus; mass = 1 atomic mass unit (AMU).

• Neutron: Neutral particle (no charge) found in the nucleus; mass = 1 AMU.

• Electron: Negatively charged particle found in electron shells/orbitals around the nucleus; mass ≈ 0 AMU.

Elements of Life

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

• Trace Elements: Elements required in small amounts for life.

Atomic Properties

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: The average mass of all isotopes of an element, weighted by their natural abundance.

For example, a carbon atom has 6 protons (atomic number 6) and typically 6 neutrons (mass number 12).

Electron Structure and the Periodic Table

Electron Orbitals & Energy Shells

Electrons occupy regions called orbitals, which are grouped into energy shells around the nucleus. Shells closer to the nucleus are lower in energy, while those farther away are higher in energy. The outermost shell is called the valence shell, and electrons in this shell are called valence electrons.

• The first shell holds up to 2 electrons; the second shell holds up to 8 electrons.

• Valence electrons determine an atom's chemical reactivity.

The Octet Rule

The octet rule states that atoms are most stable when their valence shell is fully occupied, typically with 8 electrons (except for hydrogen and helium, which are stable with 2). Atoms will gain, lose, or share electrons to achieve a full valence shell, making them less reactive.

• Atoms with incomplete valence shells are more reactive.

• Example: Neon (Ne) is unreactive because its valence shell is full.

Isotopes and Atomic Mass

Isotopes

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. This results in different mass numbers but identical chemical properties.

• Example: Carbon-12, Carbon-13, and Carbon-14 are isotopes of carbon, each with 6 protons but 6, 7, and 8 neutrons, respectively.

• Atomic Mass: The weighted average of all naturally occurring isotopes.

Radioactive Isotopes

Radioactive isotopes are unstable and decay over time, emitting energy in the form of radiation. The half-life is the time required for half of the radioactive atoms in a sample to decay. Radioactive isotopes are used in medicine (e.g., cancer treatment, imaging) and in dating fossils (radiometric dating).

• Example: Carbon-14 has a half-life of 5,730 years and is used in radiocarbon dating.

Chemical Bonding

Introduction to Chemical Bonding

Chemical bonds are attractive forces that hold atoms together in molecules and compounds. A molecule consists of two or more atoms chemically bonded together, while a compound contains two or more different elements.

• Chemical Formula: Indicates the types and numbers of atoms in a molecule (e.g., H2O, C6H12O6).

Intramolecular vs. Intermolecular Bonds

Bonds can be classified as intramolecular (within a molecule) or intermolecular (between molecules):

• Intramolecular Bonds: Hold atoms together within a molecule (e.g., covalent bonds in H2O).

• Intermolecular Bonds: Occur between molecules (e.g., hydrogen bonds between water molecules).

Covalent Bonds

Covalent bonds involve the sharing of electron pairs between atoms. There are two main types:

• Nonpolar Covalent Bonds: Electrons are shared equally due to similar electronegativities (e.g., H2, O2).

• Polar Covalent Bonds: Electrons are shared unequally due to differences in electronegativity, resulting in partial charges (e.g., H2O).

Electronegativity is a measure of an atom's ability to attract electrons in a bond (scale: 0–4).

Noncovalent Bonds

Noncovalent bonds do not involve sharing electrons. Types include:

• Ionic Bonds: Attraction between oppositely charged ions (cations and anions).

• Hydrogen Bonds: Weak attraction between a hydrogen atom covalently bonded to a highly electronegative atom (F, O, or N) and another electronegative atom.

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

Ions and Ionic Bonding

Ions: Anions vs. Cations

Ions are atoms or molecules with a net electrical charge due to the loss or gain of electrons.

• Anion: Negatively charged ion (gains electrons).

• Cation: Positively charged ion (loses electrons).

Ionic Bonds

Ionic bonds are formed by the electrical attraction between cations and anions. The transfer of electrons allows both atoms to achieve full valence shells, resulting in stable ions.

Hydrogen Bonding

Hydrogen Bonds

A hydrogen bond is an interaction between a hydrogen atom covalently bonded to a highly electronegative atom (F, O, or N) and another electronegative atom. Individually, hydrogen bonds are weak, but collectively, they can be strong and are crucial in biological systems (e.g., water properties, DNA structure).

• Example: Water molecules interact via hydrogen bonds, giving water its unique properties.