Atomic Structure: Nucleus, Electrons, Isotopes

• The basic unit of matter is the atom
• It comprises a central nucleus surrounded by negatively charged electrons

Atomic Nucleus

• The nucleus contains protons and neutrons
• Protons are positively charged particles
• Neutrons are electrically neutral
• The number of protons determines the element's atomic number and its chemical properties

Electrons

• Electrons are negatively charged particles that orbit the nucleus in specific energy levels or shells
• The arrangement of electrons determines how an atom interacts with other atoms to form chemical bonds

Atomic Number and Mass Number

• Atomic number (Z) is the number of protons in the nucleus of an atom
• It uniquely identifies a chemical element
• Mass number (A) is the total number of protons and neutrons in an atom's nucleus
• The number of neutrons can vary, leading to isotopes of the same element

Isotopes

• Isotopes are variants of a chemical element which have the same number of protons and electrons, but different numbers of neutrons
• Isotopes have the same atomic number but different mass numbers
• Some isotopes are stable, while others are radioactive, decaying over time
• Radioactive isotopes have various applications in medicine, archaeology, and industry

Ions

• An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving the atom a net positive or negative electrical charge
• Cations are positively charged ions formed when an atom loses one or more electrons
• Anions are negatively charged ions formed when an atom gains one or more electrons

Electron Configuration

• The electron configuration describes the arrangement of electrons in the energy levels and sublevels within an atom
• Electrons fill the lowest energy levels first, following the Aufbau principle
• Hund's rule states that electrons individually occupy each orbital within a subshell before doubling up in any one orbital
• The Pauli exclusion principle states that no two electrons in an atom can have the same set of four quantum numbers

Valence Electrons

• Valence electrons are the electrons in the outermost energy level of an atom
• These electrons are responsible for the chemical properties of the atom
• Atoms tend to gain, lose, or share valence electrons to achieve a stable electron configuration, usually with eight valence electrons (octet rule)

Chemical Bonds

• Chemical bonds are the attractive forces that hold atoms together to form molecules and compounds
• Ionic bonds are formed through the transfer of electrons between atoms, creating ions that are attracted to each other due to opposite charges
• Covalent bonds are formed through the sharing of electrons between atoms
• Metallic bonds are formed through the delocalization of electrons among a lattice of metal atoms

Molecules and Compounds

• A molecule consists of two or more atoms held together by chemical bonds
• A compound is a substance that consists of two or more different elements chemically bonded together in a fixed ratio
• Molecules can be either elements (e.g., O2) or compounds (e.g., H2O)

States of Matter

• Matter exists in different states, including solid, liquid, gas, and plasma
• The state of matter depends on the temperature and pressure
• Solids have a fixed shape and volume
• Liquids have a fixed volume but take the shape of their container
• Gases have no fixed shape or volume and can be compressed
• Plasma is a state of matter in which a gas becomes ionized and carries an electrical charge

Periodic Table

• The periodic table is a tabular arrangement of the chemical elements, organized by atomic number, electron configuration, and recurring chemical properties
• Elements in the same group (vertical column) have similar chemical properties
• Elements in the same period (horizontal row) have the same number of electron shells
• Metals are typically located on the left side of the periodic table
• Nonmetals are typically located on the right side of the periodic table
• Metalloids (semimetals) have properties intermediate between metals and nonmetals

Atomic Spectra

• Atoms can absorb or emit energy in the form of electromagnetic radiation, such as light
• When an atom absorbs energy, its electrons jump to higher energy levels
• When an electron returns to a lower energy level, it emits energy in the form of a photon
• The energy of the photon corresponds to the difference in energy between the two levels
• Atomic spectra are unique to each element and can be used to identify the element in a sample