The Structure of the Atom And Electrons in Atoms Chapters 4 and 5 The Structure of the Atom And Electrons in Atoms

Early Theories of Matter Democritus (460-370 B.C.) Named atom (atomos) What holds atoms together?

Early Theories of Matter Aristotle (384-322 B.C.) Atoms move through empty space? There is no “nothingness”

Early Theories of Matter John Dalton (1766-1844) First Atomic Theory Schoolteacher Atoms are divisible into subatomic particles Atoms of the same element have slightly different masses

Defining an Atom The smallest particle of an element that retains the properties of the element. About 1 X 10-10 m in diameter. Can be seen with a scanning tunneling microscope. Nanotechnology – molecular manufacturing – atom-by-atom building of machines the size of molecules

Discovering the Electron William Crookes (1800’s) Vacuum tube – saw flash of light in a darkened room Flash due to light-producing coating – led to television and computer monitors Flash called a cathode rays – travel from cathode to anode & negatively charged (late 1800’s) Changing electron and gas inside tube doesn’t change ray – these negative particles are found in all matter.

Discovering the Electron J.J. Thomson (late 1890’s) Determined the charge-to-mass ratio Mass must be less than a hydrogen atom Plum Pudding Model of atom Plum pudding Model

Discovering the Electron Robert Millikan (1909) Determined charge of electron 1/1840 mass of a hydrogen atom

The Nuclear Atom Ernest Rutherford (1911)

The Nuclear Atom Atom contains: Creates nuclear model of atom Mostly empty space Tiny, dense nucleus which is positively charged Creates nuclear model of atom

Other Subatomic Particles Rutherford (1920) Concluded nucleus contains proton Proton as equal but opposite charge of electron James Chadwick (1932) Discovered neutron Neutron has no charge Chadwick was Rutherford’s coworker

Subatomic Particles

How Atoms Differ Moseley (shortly after Gold Foil) Atoms of each element contain a unique number of protons Atomic Number= #protons Identifies the atom For neutral atoms only: # protons = # electrons Atomic number is always a whole number

Isotopes Isotopes – atoms that contain the same number of protons but different number of neutrons. Most elements contain a mixture of isotopes. The relative abundance of each isotope is constant. Ex: potassium 93.25% have 20 neutrons, 6.7302% have 22 neutrons, 0.0117% have 21 neutrons

Isotopes Mass Number = #protons + #neutrons

Simple Practice Mg 25 12 Zn 30 35 Be 4 9 Hg 120 80 Atomic Number Mass # of Protons # of neutrons # of electrons Mg 25 12 Zn 30 35 Be 4 9 Hg 120 80 12 13 12 65 30 30 4 5 4 80 200 80

Mass of Atoms Atomic mass unit – 1/12 of a carbon-12 atom. Atomic Mass – weighted average mass of the isotopes of that element. 1 proton = 1.007 amu 1 neutron = 1.009 amu 1 electron = .0005 amu

Calculating Atomic Masses 6X has mass of 6.015 amu and abundance of 7.50%. 7X has mass of 7.016 amu and abundance of 92.5%. (6.015)(.0750) + (7.016)(.925) = 6.94 amu

More Challenging Problems! Cu-63 has a mass of 62.940 amu and an abundance of 69.17%. Find the mass and abundance of the other isotope. Boron has two isotopes with the masses of 10.013 amu and 11.009 amu. Find the abundance of each isotope.

Radioactivity Nuclear Reactions – changes an atom’s nucleus. Atom changes into a new element Due to unstable nuclei Radiation contains rays and particles emitted from a radioactive material. Radioactive decay is the spontaneous emission of radiation.

Types of Radiation

Types of Radiation Radiation Type Symbol Mass (amu) Charge Alpha  or 4 2+ Beta e- or 1/1840 1- Gamma 00

Nuclear Reactions Mass numbers and Atomic numbers on both sides of the reaction must be equal Practice Problem:

Chapter 5 Electrons in Atoms

Electromagnetic Radiation Electromagnetic Radiation is a form of energy that has wave-like behavior. 4 properties of waves: wavelength, amplitude, speed and frequency. Scientists need to explain behaviors of elements: emit light (color) when heated.

Properties of Waves Frequency()- number of waves that pass a given point per second. (hertz or 1/s or s-1) Speed (c)- is constant for all waves. 3 x 108 m/s

Calculating Properties of Waves What is the frequency of light with a wavelength of 5.80 x 10-7 m? A radio station broadcasts with a frequency of 104.3 MHz. What is the wavelength of the broadcast?

Particle Nature of Light Max Planck (1900) discovered that matter can gain or lose energy in small, specific amounts called quanta. Equantum= h Planck’s Constant (h)=6.626 x 10-34J·s Wave model of light can not explain why heated objects only emit certain frequencies of light.

Practice Problems What is the energy of a wave with a frequency of 6.25 x 1019Hz? What is the frequency of a wave that contains 8.64 x 10-18J of energy? A wave contains 4.62 x 10-15J of energy. Determine its wavelength.

Photoelectric Effect Photoelectric effect – electrons are emitted from a metal’s surface when light of a certain frequency shines on it. Frequency (color) of light, not brightness of light determines if electrons are emitted. Einstein (1905)- light has wave-like properties but is also a stream of tiny particles or bundles of energy called photons. Photon – a piece of EM with no mass and carries a quantum of energy.

Atomic Emission Spectrum When atoms absorb energy they become excited. Atomic Emission Spectrum- unique set of frequencies emitted by excited atoms.

Bohr Model of the Atom Bohr (1913) proposed why the emission spectrum of hydrogen is not continuous. Electrons can have only certain “energy states” Ground State - the lowest allowable energy state. Excited State – energy state of an electron when it gains energy

Bohr Model of the Atom Model did not work well for atoms other than hydrogen.

Electrons as Waves Louis de Broglie (1924) thought Bohr’s model had electrons having similar properties to waves. de Broglie equation: Predicts that all moving particles have wave properties.

Heisenberg Uncertainty Principle When viewing an electron, a photon of light hits it and changes the velocity and position of the electron. It is impossible to know precisely both the velocity and position of a particle at the same time.

Quantum Mechanical Model of the Atom Schrödinger (1926) derived an equation that treated hydrogen’s electron as a wave. Allows electron to have only certain energy but does not give path of electron. Atomic orbital – a 3-D region around the nucleus in which the electron can be found 90% of the time.