History of the Atom Dalton’s Theory (1803)

John Dalton’s Theory All matter is made up of tiny, indestructible particles called atoms All atoms of a given element have identical physical and chemical properties Atoms are neither created nor destroyed (law of conservation of mass) Atoms of different elements form compounds in whole-number ratios Example: the formula for water is H2O not H2.1O1.3

Dalton’s Theory Some of these postulates now have exceptions Atoms can be broken apart in nuclear reactions Atoms of a given element can have different physical and chemical properties (isotopes)

J.J. Thomson’s Plum-Pudding Model (1897) His model portrays the atom as a big ball of positive charge that contains small particles of negative charge embedded in it

Thomson’s Model (Honors Only) Discovered the charge of an electron by observing cathode rays in a cathode ray tube

Thomson’s Model (Honors Only) From his observations, he concluded that cathode rays are streams of negatively charges particles with mass Another scientist, Millikan, was able to determine the mass of an electron based upon Thomson’s work (1909)

Rutherford’s Model (1909) Rutherford was able to make two key observations and conclusions based on his “gold-foil” experiment He disproved Thomson’s model

Rutherford’s Model The Experiment He bombarded a thin piece of gold foil with positively charged alpha particles (much smaller than the atom)

Rutherford’s Model Observations Almost all the alpha particles passed through foil without deflection Small percent slightly deflected Some were largely deflected A few even reflected back in the direction from where they had come Demo

Rutherford’s Model of the Atom Conclusions (1911) Atom is mostly empty space All the positive charge in an atom is concentrated in a small area (+ dense nucleus) this area is positive since positively charges particles were deflected from it (like repels like)

Bohr Planetary Model 1913 Model displayed electrons traveling in orbits around the nucleus Electrons are only found in orbits (principle energy levels) not in between

Bohr Model The PEL approximates how far the electron is from the nucleus PEL (n) Shell Max # of Electrons 2(n)2 1 K 2 L 8 3 M 18 4 N 32 5 O 50 6 P 72 7 Q 98

Bohr Model The electrons distance from the nucleus is related to their specific amount of energy (quanta) As you move away from the nucleus , the energy in each PEL increases

Bohr Model Ground State – when electrons are in their lowest energy level Quantum Leap When electrons jump between energy levels Electrons can only absorb a fixed amount of energy (quanta) to move to a higher energy level Electrons can only jump to levels that are not completely filled with electrons Heat, light, and electricity are all stimuli that can excite an electron

Bohr Model Excited State Electrons are in higher energy levels Acquired when an electron absorbs energy and becomes unstable Electrons quickly return to ground state, emitting the same amount of energy absorbed, usually in the form of light Every element gives off a unique pattern of colors (line spectrum) which can be used to identify the element

Bohr Model Spectrum of different elements

Bohr Model Line spectrums are observed with an instrument called a spectroscope

Electron Configurations An electron configuration tells you how many electrons there are in each energy level. Mg (2-8-2) has 2 electrons in PEL 1, 8 electrons in PEL 2, and 2 electrons in PEL 3 The amount of numbers in the electron configuration tells you how many energy levels are occupied with electrons Mg (2-8-2) has 3 energy levels containing electrons

Valence Electrons Are the electrons in the outermost energy level of an atom (last # in the electron configuration). Example: Al (2-8-3) has 3 valence electrons Valence electrons determine the chemical properties of an element.

The Kernel Includes the nucleus and all non-valence electrons What element is this? What is the electron configuration? How many valence electrons? Kernel?

Electron Configurations of Excited State Atoms Cl Ground State 2-8-7 (17 e-) Cl Excited State 2-7-8 (17 e-) 2-8-6-1 (17 e-) Only jump one electron Electrons can only jump into energy levels that aren’t filled You are not losing or gaining any electrons, the total amount of electrons is the same

Excited State Electron Configurations Determine a possible excited state electron configuration for: Mg Li S F Answers: Mg: 2-8-1-1 or 2-7-3 Li: 1-2 or 2-0-1 S: 2-7-7 or 2-8-5-1 F: 2-6-1 or 1-8

Electron Configurations for Ions Vs. Atoms Mg Atom 2-8-2 Mg Ion 2-8

Electron Configurations of Atoms vs. Ions Find the electron configurations of the following ions K+1 Al+3 P3- Answers: 2-8-8 2-8