atoms and periodic table

Radioactivity

• Marie Curie named the spontaneous emission of particles and radiation from elements as radioactivity.
• Radioactive elements emit three types of rays: Alpha (α), Beta (β) and Gamma (γ)
• α rays are positively charged particles deflected by positively charged plates.
• β rays are electrons deflected by negatively charged plates.
• γ rays are high-energy rays that have no charge and are not affected by external fields.

Average Atomic Mass

• The average atomic mass of an element is the sum of the masses of its isotopes, each multiplied by its abundance.
• For example, Carbon-12 has a mass of 12 u and Carbon-13 has a mass of 13.0033548378 u. In a sample, 98.93% of Carbon atoms are Carbon-12 and the remaining are Carbon-13.

Mole (mol)

• Mole is the SI unit for the amount of substance containing as many elementary entities as there are atoms in exactly 12 g (or 0.012 kg) of carbon-12.
• The number of atoms in 12 g of carbon-12 is called Avogadro’s number (N A ) and its currently accepted value is 6.022 x 10^23.

Molar Mass

• Molar Mass (m) is the mass in grams or kilograms of 1 mole of units of a substance, such as atoms or molecules.
• The molar mass of carbon-12 is 12.00 g and 1 mole of carbon-12 contains 6.022 x 10^23 atoms.

Quantum Theory

• Max Planck discovered that atoms and molecules emit energy in discrete quantities called quanta.
• Classical Physics dictates that energy is continuous, while Quantum Theory limits energy to specific values.
• The difference between permitted energy levels of a system is called a quantum of energy and determines the energy increase when transitioning between those levels.

Nature of Light

• Heinrich Hertz discovered the photoelectric effect: electrons are ejected when light strikes the surface of certain metals.
• Electrons are only emitted when the incident light frequency exceeds a threshold value.
• The number of emitted electrons is proportional to the intensity of the light, while their kinetic energy depends on the light frequency.
• Albert Einstein proposed that light has particle-like qualities and that these particles have characteristic energy given by Ephoton = hv, where v is the frequency of light and h is Planck's constant.
• Planck’s constant is denoted by h and its value is 6.62607 x 10^-34 J s.

Properties of Waves

• A wave is a vibrating disturbance through which energy is transmitted.
• Wavelength (λ) is the distance between identical points on successive waves.

Electronic Configuration

• Electrons occupy orbitals in a manner that minimizes the energy of the atom.
• Aufbau principle states that the total energy of an atom depends on orbital energies and electronic repulsion, with orbitals generally filling in the order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p.
• Pauli Exclusion Principle states that no two electrons in an atom can have the same set of four quantum numbers, meaning only two electrons with opposite spins can occupy the same orbital.
• Hund's rule states that for degenerate orbitals, electrons will initially occupy orbitals singly with parallel spins.

Electron Configuration and the Periodic Table

• Alkali metals have one valence electron in an s orbital.
• Halogens have seven valence electrons in the configuration ns^2np^5.
• Noble gases have eight valence electrons in the configuration ns^2np^6, except for Helium which has only two electrons.

Blocks of Elements

• Elements can be classified into four blocks based on the subshells being filled:
  • s block: The highest principal quantum number (n) s orbital fills, including groups 1 and 2, plus Helium.
  • p block: The highest quantum number (n) p orbitals fill, including groups 13, 14, 15, 16, 17, and 18 (except He).
  • d block: The (n-1) d orbitals fill, encompassing groups 3 through 12.
  • f block: The (n-2) f orbitals fill, including the lanthanides and actinides.

Modern Periodic Table

• Dmitri Mendeleev arranged elements in order of increasing atomic mass, which closely resembles the modern periodic table.
• Elements were later rearranged in order of increasing atomic number.

Element Classification

• Elements are broadly classified into:
  • Metals: Found to the left of the periodic table, with the exception of hydrogen. They tend to lose electrons, are solid at room temperature (except mercury), shiny with good conductivity of heat and electricity, and are ductile and malleable.
  • Nonmetals: Found to the right of the periodic table, they tend to gain electrons, are brittle with poor conductivity, and are not malleable or ductile.
  • Metalloids: Located along the stair-stepped line in the periodic table, they have intermediate properties of metals and nonmetals, and are valuable semiconductors.

Periodic Properties

• Periods: The seven horizontal rows of the periodic table. Elements within a period have increasing atomic numbers from left to right.
• Groups/Families: Vertical columns in the periodic table, labeled using Roman numerals and letters (older) or numbers 1 through 18 (newer).

Effective Nuclear Charge

• The effective nuclear charge (Zeff) is the net positive charge experienced by an electron in an atom.
• Zeff = Z - σ, where Z is the actual nuclear charge and σ is the shielding constant, representing the repulsion from other electrons.

Atomic Properties

• Atomic Radius: Defined as half the distance between the nuclei of two adjacent atoms in a molecule or metal. Atomic radius increases down a group.
• Ionic Radius: The radius of a cation or anion.
  • Cations are smaller than their corresponding atoms
  • Anions are larger than their corresponding atoms
  • For isoelectronic ions, the ionic radius decreases with increasing positive charge and increases with increasing negative charge.
• Ionization Energy (Ei): The energy required to remove an electron from a gaseous atom. Ionization energy generally increases across a period and decreases down a group.
• Electron Affinity (Eea): The enthalpy change when an atom in the gas phase gains an electron.

Test your knowledge on radioactivity, the properties of alpha, beta, and gamma rays, as well as the concept of average atomic mass and moles. This quiz will cover essential concepts related to these fundamental topics in chemistry.