ATOMIC THEORY

DEMOCRITUS

• The Greek philosopher Democritus (460 BC- 370 BC) was one of the first to suggest the existance of atoms
• Democritus claimed that the atomd were indivisible and indestructible
• His ideas weren't based on scientific method and didn't explian chemical behavior of atoms

DALTON

• By using experimental methods, Dalton transformed Democritus' ideas of atoms into a scientific theory
• He studied the ways in which elements combine in ratios during after a chemical reaction
• He was an English school teacher and chemist

DALTON'S ATOMIC THEORY

• 1) all elements are composed of tiny indivisible particles called atoms
• 2) atoms of the same element are identical and are different than those of another element
• 3) atoms of different elements can mix or chemicaly combine in whole number ratios =form compounds
• 4) chemical reactions occur when atoms are separated, joined, or rearranged in a diff. combination

CHANGES IN HIS THEORY

• Atoms are now known to be divisible
• They can be broken down into smaller particles
• These are also called subatomic particles
• --electrons, neutrons, protons

JJ THOMPSON

• English physicist, JJ Thompson discovered electrons
• Electrons are negatively charged subatomic particles
• Thompson performed experiments involving passing electrifal currents through gases at low pressure
• His was known as the cathode ray tube experiment

WHAT HE DISCOVERED

• 1) the atom is breakable
• 2) the atom's structure has electrons suspended in a positively charged electric field
• -- it must have some sort of positive charge to balance it out

RUTHERFORD

• He tried to porve the plum pudding model
• This allowed him to see if there really is that much empty space in an atom
• His experiment was known as the gold-foil experiment:
• 1) used tiny alpha particles as a bullet
• 2) and gold foil as the target atom

WHAT HE DISCOVERED

• The atom contains a small dense region know as the nucleus
• The nucleus has essentially the entire mass of the atom
• It is also positively charged [(+) balances (-) electrons]
• He also thought that electrons were dispersed around the nucleus
• --but this was proved all wrong

Untitled Slide

• The atom is mostly empty space based on the conclusions of his experiments
• Rutherford proposed that the nucleus had a paticle that was equal to electrons
• These were known as protons (+)
• Still, there were unanswered questions about the mass of the nucleus
• Rutherford then proposed the existence of neutrons (neutral/ no charge)

BOHR

• Limitations in Rutherford's theory led to The Bohr model
• Niels Bohr was student of Rutherford in 1913
• He proposed that an electron is found in specific orbits around the nucleus
• These orbital paths have a fixed energy known as energy levels
• His theory only applied to the hydrogen atom

THE BOHR MODEL

• The energy of each orbit is characterized by a number
• --the larger the number, the more energy an electron in that orbit has
• --and is farther away from the nucleus
• When an electron falls form a higher energy level to a lower inner orbit,
• --It emits electromagnetic energy (also known as light)

BOHR MODEL: ENERGY TRANSITIONS

• When an atom gains energy, the electron leaps from a lower to higher level
• This is knwn as the EXCITED state of an atom
• --The atom has electrons at higher energy levels than it should
• When an electron leaps from a higher to lower energy orbit, energy is emitted
• --As a photon of light, or quantum of energy (returns to GROUND state)

SCHRODINGER

• Erwin Schrödinger's theory was known as Quantum Mechanics
• --the probability of finding an electron with a particular amount of energy at a specific location
• His equation predicts this theory

THE QUANTUM ATOMIC MODEL

• Determines:
• 1) the allowed energies an atom can have
• 2) how likely it is to find the electron in various locations around the nucleus

QUANTUM NUMBERS

• Each principal energy shell contains one or more subshells
• Subshells are identified with s, p, d, f
• Shells—> subshells—> orbitals
• Shape = probability map
• s-circular shape, p-dumbbell shape

ORBIT VS. ORBITALS

• An orbital is a region where there is a very high possibility of finding an electron
• --with a particular amount of energy (generally 90-95%)
• Orbit=pathway
• Orbital=probability
• Orbital shapes based on this probability and are solutions to Schrödinger's equation