Presentation on theme: "DEVELOPMENT OF THE ATOMIC MODEL From Democritus to Rutherford"— Presentation transcript:
1 DEVELOPMENT OF THE ATOMIC MODEL From Democritus to Rutherford
2 c.400 B.C. Ancient Greek Philosophy Everything in the universe is made of one or more of the basic “elements:”Earth, Fire, Water, Air
3 What makes up the elements? ARISTOTLEMatter is infinitely divisible; no matter how small a piece is, it can always be divided into smaller piecesDEMOCRITUSThere exists a “smallest piece” of matter, which cannot be divided any further.These pieces are called “ατομοσ,” or “atoms”
4 Whose Argument Prevails? Aristotle’s viewpoint enjoyed the support of most of the world because he was more well-known and because Democritus had no evidence to back up his claim since these “atoms” would be too small to see.
5 Antoine Lavoisier 1743-1794 Father of Modern Chemistry Chemical Revolution –Stated the Law of Conservation of MassOxygen for combustionDecomposed Water into two gases, hydrogen and oxygen and then reformed the exact same amount of water.
6 1803: John Dalton Dalton was an English schoolteacher Began teaching mathematics and chemistry at the age of 12Revived the idea of Democritus’ “smallest piece” of matter
7 Dalton’s Atomic Theory All matter is made of tiny particles called “atoms”Atoms are indivisible and indestructibleAtoms of the same element are identicalAtoms of different elements differ in some fundamental wayAtoms combine in simple whole number ratios to form compoundsAtoms are rearranged in chemical reactions but cannot be created or destroyed
8 Three Laws Explained by the Atomic Theory Law of Conservation of Mass (Antoine Lavoisier)Law of Definite Proportions - compounds always contained the same mass ratio of one element to another. (Joseph Proust)Law of Multiple Proportions – When elements combine in different ratios, each new ratio is a unique compound. (John Dalton)
9 So what?These results could only be explained by assuming that matter was made of atoms – tiny building blocks – and that these atoms only came in certain sizes.Dalton’s View of an atom
10 1897: J.J. Thomson English physicist Worked with Cathode-Ray Tubes (CRTs)Credited with the discovery of the electron
11 CRTs Mysterious particles emanated from the cathode end These particles were deflected by magnetic and electric fieldsThey were very small and negatively chargedThese particles were called “electrons” and were assumed to be a part of all matter
13 Thomson’s Atomic Model ElectronsPositively charged “goo”A.K.A. the “Plum-Pudding Model”
14 : Robert MillikanSet out to discover the charge of a single electronFamous experiment called the “oil-drop experiment”Using his results and the charge-to-mass ratio from Thomson, the mass of an electron was found
15 The Oil Drop Experiment To view an animation of this experiment click belowOIL DROP EXPERIMENT
16 1910: Ernest Rutherford Expert in radiation Famous “Gold-Foil Experiment”Discovered the presence of the nucleus by firing alpha particles at a sheet of gold foil
17 The Gold Foil Experiment To view an animation of this experiment click belowGOLD FOIL EXPERIMENT
20 Rutherford’s Atomic Model ElectronsEmpty SpaceNucleusPositively chargedMade of “protons”
21 Gold Foil Conclusions The atom has a nucleus The nucleus has a positive chargeThe nucleus is very small and very denseMost of the atom is empty spaceThe electron resides in the region outside the nucleus
22 1932: James ChadwickA fellow researcher with Rutherford, Chadwick discovered years later that the nucleus was not made of only one particle – the proton – but of two particles.This second particle was called the “neutron” because it had no electrical charge
23 Subatomic ParticlesElectron (e-) – mass 1/1840 amu, charge -1, found in space around the nucleus Proton (p+) – mass of 1 amu, charge +1, found in the nucleus Neutron (n) – mass of 1 amu, no charge, found in the nucleus
24 The AtomMade up of 3 fundamental subatomic particles: protons, neutrons, and electronsVery small and very dense nucleus (nucleus make up over 99% of atom’s mass)Nucleus contains protons and neutronsElectrons occupy the empty space outside the nucleus# of protons = # of electrons
25 What’s in the atom?Nucleons – particles in the nucleus (protons and neutrons)The combined total of the protons and neutrons is called the mass numberThe number of protons is called the atomic number.The atomic number identifies the element.Electrons found in the space outside the nucleusLots of empty space
26 IsotopesParticles with the same number of protons and electrons but different numbers of neutrons.Have different mass numbers.Have different masses.React the same chemically.
31 Electromagnetic Properties Wavelength – distance between consecutive waves.Frequency – the number of waves that pass a point in a given amount of time, usually 1 s.
32 Quantum Theory Proposed by Max Planck Two main ideas: Energy changes are not continuous but rather occur in small increments called “Quantums”.The energy of a quantum is directly proportional to the frequency of the radiation.
33 ….. Quantum theory continued. E = hνE = energy of a quantumh= Planck’s constant(6.63 x J.s)V = frequency
34 What is the energy of a quantum with a wavelength of 550 nm?
39 Questions?Why do the lines always occur at the same place and why only 4 lines?Ans: Only specific energy changes are possible in an atom. The lines are representations of those energy changes.
40 Bohr reasoned that if an electron could occupy an infinite number of possible orbits, its jumps from these orbits should give rise to an infinite number of different energy radiations……….THUSA continuous spectrum
41 Bohr’s Interpretation Electrons in atoms can only occupy certain fixed orbits or “energy levels”.These energy positions are quantized, meaning only certain values are possible within an atom.To move from one energy orbit to another one, an electron must absorb or emit a quantum of energy exactly equal to the energy difference between the two positions.
44 Since Bohr . . . The Modern Model The Quantum Mechanical ModelHas a nucleusElectrons are in a “cloud” of negative charge.An electron “orbit” is an area where the electron is most “likely” to be.
45 Quantum NumbersDescribe the properties of orbitals and electrons in those orbitals.Principal q.n. (n) – designates the main energy level or shell. (Bohr)Values: 1, 2, 3, 4, 5, ……n=1; means the electron is located in the first energy level, which has the lowest energy.
46 2. Angular momentum q.n. – (l) designates the shape of the atomic orbital. values: 0, 1, 2, 3, n-1 so if n=3, l can be 0, 1, or 2. if l = 0, (s) then it is spherical if l = 1, (p) then it is dumbbell if l = 2, (d) complex if l = 3, (f) complex
48 3. Magnetic q. n. (ml) – designates the orbital’s orientation in space 3. Magnetic q. n. (ml) – designates the orbital’s orientation in space. values – (from –l through 0 to +l) ex. If l = 1 (p) then Ml can = -1, 0, corresponds to px 0 corresponds to py +1 corresponds to pz
49 4. Spin q. n. (ms) – describes the spin of the electron on its axis; clockwise or counter clockwise. values - +1/2 or – 1/2
50 Electron Configurations Describes the arrangement of electrons in an atom.Each electron in an atom has a set of 4 q. n.
51 Rules that govern orbital filling Aufbau principle – electrons enter orbitals of lowest energy first.Hund’s rule – when electrons enter orbitals of equal energy, degenerate orbitals, each orbital receives one electron, with parallel spins before any receive two.Pauli exclusion principle – no two electrons in an atom can have the same set of 4 q.n. They must have opposite spins.
53 1869: Dmitri Mendeleev Russian chemist Arranged elements in tabular form so that elements with similar properties were in the same columnWhen listed in order by mass, elements generally repeat properties in groups of 8 (Law of Octaves)
54 The First Periodic Table Most tables at the time listed elements by massMendeleev also arranged elements by mass, but left several “holes” in his table and occasionally reversed the order of elements to fit the properties of others in that columnThe “holes” were later filled in with newly discovered elements that had the properties predicted by Mendeleev’s table.The reason for the reversal of elements was explained later by Henry Moseley, who noted that the elements were in order by atomic number (number of protons) rather than by mass
55 Introducing the Elements The Element SongPeriods- Horizontal Rows (7 periods)Groups/families – Verticle columns (18 groups)