2.1 Atomic structure and the Periodic Table

• Atomic number: The number of protons found in the nucleus of an atom, which determines its chemical properties and defines its place in the periodic table.
• Density: A measure of mass per unit volume, typically expressed in grams per cubic centimeter (g/cm³) or kilograms per cubic meter (kg/m³), describing how tightly packed the particles of a substance are.
• Mass number: The total number of neutrons and protons present in the nucleus of an atom.
• Electron shells: Regions surrounding the nucleus of an atom where electrons are found, arranged in concentric layers or shells according to their energy levels.
• Electronic structure: The arrangement of electrons within an atom, including their distribution among different energy levels and within electron shells.
• Metals: Elements that conduct electricity well and have high melting points due to strong metallic bonds holding their atoms together.
• Non-metals: Elements that do not conduct electricity as well as metals and often exist as gases at room temperature.
• Electrostatic forces: Forces of attraction or repulsion between electrically charged particles, such as protons and electrons, resulting from their opposite or like charges.
• Energy levels: Distinct and quantized states of energy that electrons can occupy within an atom, determined by their distance from the nucleus.
• Highest energy level: The outermost electron shell of an atom, also known as the valence shell, which determines the atom's chemical properties and reactivity.
• Mass number: The total number of protons and neutrons present in the nucleus of an atom, used to identify different isotopes of an element.
• Outermost electron shell: The electron shell farthest from the nucleus of an atom, containing the valence electrons that are involved in chemical bonding and interactions.
• Periodic Table: A tabular arrangement of chemical elements based on their atomic numbers, electron configurations, and chemical properties, organized into periods (rows) and groups (columns) to illustrate trends and relationships among elements.
• Proton: A positively charged subatomic particle found in the nucleus of an atom, with a mass approximately equal to one-twentieth of the mass of a neutral hydrogen atom.
• Valence electrons: Electrons located in the highest energy level of an atom, responsible for its chemical behavior and ability to form bonds with other atoms.
• Atom: The smallest unit of matter that has all the characteristic properties of an element.
• Isotope:

  An isotope is a variant of an element that contains the same number of protons in its atomic nucleus but differs in the number of neutrons. This variance in neutron number results in isotopes having different atomic masses. Isotopes of an element exhibit similar chemical properties due to their identical number of protons but may have different physical properties, such as stability or radioactive decay rates, depending on their neutron count.
• Neutron: An uncharged subatomic particle found in the nucleus of an atom, with a mass approximately equal to one-twentieth of the mass of a neutral hydrogen atom.
• Electron configuration: The arrangement of electrons around the nucleus of an atom, determined by the principal quantum number and angular momentum quantum number of each electron orbital.
• Nucleus: The central part of an atom, consisting of protons and neutrons, which determines the identity of the element.
• Dmitri Mendeleev was a Russian chemist and inventor who devised the periodic table in the mid-nineteenth century
• The periodic table organizes all known elements in order of increasing atomic number, representing the number of protons in the atoms of each element
• Elements in the periodic table are arranged in periods, which are rows that repeat patterns of chemical properties
• Each vertical column in the periodic table is called a group, with elements in the same group having similar chemical properties
• Group 1 elements are alkali metals and have one electron in their outer shell
• Group 2 elements have two electrons in their outer shell
• Group 7 elements are halogens and have seven electrons in their outer shell
• Group 0 elements are noble gases and have full outer shells, making them very unreactive
• There is a zigzag line on the periodic table separating metals (left side) from nonmetals (right side)
• Transition metals are a special block of elements in the middle of the periodic table
• The position of mass and atomic numbers may vary in different periodic tables, but they all contain the same information
• Structure of an atom:

  • Nucleus contains protons (positively charged) and neutrons (neutral)
  • Electrons orbit around the nucleus in shells
  • Electrons are about 2000 times smaller than protons or neutrons and have a negative charge
• Information in a periodic table box:

  • Elemental symbol (e.g. Li for lithium)
  • Atomic number (number of protons)
  • Mass number (total number of protons and neutrons)
• Isotopes:

  • Atoms with the same number of protons but different numbers of neutrons
  • Example: Lithium-6 (3 neutrons) and Lithium-8 (5 neutrons) are isotopes of Lithium
• Radioactive decay:

  • Unstable isotopes decay into other elements by emitting radiation (alpha, beta, gamma, or neutrons)
• Electron energy levels:

  • Electrons are arranged in shells around the nucleus
  • Electrons can jump to higher energy levels if they gain enough energy (excited state)
  • Electrons emit energy as electromagnetic radiation when falling back to lower energy levels
• Ionization:

  • Outermost electron can absorb enough energy to leave the atom
  • Atom becomes a positive ion with more protons than electrons
  • Ionizing radiation can knock electrons off atoms
• Niels Bohr, in 1913, expanded upon Rutherford's atomic model by proposing that electrons orbit the nucleus in distinct electron shells or energy levels, for which he was awarded the Nobel Prize, revolutionizing our understanding of atomic structure.
• The arrangement of electrons in these shells around the nucleus is known as the electronic structure, with each shell capable of holding a specific number of electrons: the first shell accommodates up to two electrons, while the second and third shells can hold up to eight electrons each.