Atoms

• An atom is the basic unit of an element that can enter into a chemical combination
• Understanding atoms ultimately leads to understanding why certain elements behave or react in a certain manner
• The idea of an atom started as early as 5th century B.C. when Democritus expressed his belief that matter is made up of smaller, indivisible particles he then called 'atomos', meaning indivisible
• John Dalton formulated a precise definition of indivisible building blocks that we now call atoms in 1808
• Dalton's hypotheses


  • All matter consists of indivisible particles called atoms
  • All atoms of the same element are identical in terms of size, mass, and chemical properties. The atom of one element is different from the atom of another element
  • Atoms of different elements may combine in fixed proportions to form a compound
  • Chemical reactions involve reorganization of the atoms—changes in the way they are bound together. The atoms themselves are not changed in a chemical reaction
• Atoms are actually made up of even smaller particles we now know as subatomic particles, including protons, neutrons, and electrons
• Electron


  Negatively-charged subatomic particle discovered by Sir Joseph John Thomson
• Thomson proposed the plum pudding model of the atom
• Rutherford's gold foil experiment led to the discovery that atoms are mostly empty space with a positively-charged nucleus
• Chadwick's discovery of neutrons accounted for the unexplained mass ratio between hydrogen and helium
• Subatomic particles


  • Proton
  • Neutron
  • Electron
• Atomic number (Z)


  Number of protons in the atom's nucleus
• Mass number (A)


  Number of protons + number of neutrons
• Atomic mass


  Mostly attributed to the mass of protons and neutrons
• Bohr's planetary model


  Electrons revolve around a positive nucleus in a predetermined orbit
• Quantum mechanical model


  Electrons are most probably found in a three-dimensional space around the nucleus, known as the orbital
• Quantum numbers


  • Principal quantum number (n)
  • Azimuthal quantum number (ℓ)
  • Magnetic quantum number (m)
  • Spin quantum number (s)
• Principal quantum number (n)


  Refers to the main energy levels (or shells) of an orbital
• Azimuthal quantum number (ℓ)


  Pertains to the shape of the orbital
• Principal Quantum Number (n)


  • The first circle corresponds to n = 1, and the electrons occupying the n = 1 principal QN can be found anywhere within the space enclosed by the said energy level
  • The principal quantum number can take values from 1 to ∞, and the higher the value of n, the higher is the energy of the orbital, and the farther the electron from the nucleus
  • The higher the n value, the larger the atom is
• Azimuthal Quantum Number (ℓ)


  Pertains to the energy sublevels or subshells of the orbital, and has something to do with the shape of the orbital
• Magnetic Quantum Number (mℓ)

  Tells us about the orientation of orbitals in space, and takes the values between —ℓ to +ℓ, including zero
• Spin Quantum Number (ms)


  Pertains to the electron spin, which can only be either clockwise or counterclockwise, with possible values of +½ and —½
• We can use these quantum numbers to assign electronic configurations on each electron in a multi-electron system, but some rules/principles must be followed
• Electron Configuration


  The distribution of electrons among the various orbitals in an atom, molecule, or ion
• To write electron configuration correctly, you need to know: the number of electrons present in a certain species, the number of electrons each orbital can occupy, and the correct ordering of the orbitals
• Number of electrons each orbital can occupy

  • s orbitals - maximum 2 electrons
  • p orbitals - maximum 6 electrons
  • d orbitals - maximum 10 electrons
  • f orbitals - maximum 14 electrons
• Aufbau Principle


  As a proton is being added one by one to the nucleus to build up the element, electrons are likewise added to the orbital
• (n + ℓ) rule


  The added electrons must occupy an orbital with a lower (n + ℓ) value first, meaning the added electrons should occupy the orbital with lower energy first
• Hund's rule of multiplicity


  Every orbital in a subshell is singly occupied with one electron before any orbital is doubly occupied, and all electrons in singly occupied orbitals have the same spin
• Pauli's exclusion principle


  No two electrons can have the same set of four quantum numbers