9/30/2016Chapter N*1 1 Discovery of the Electron Cathode Rays In 1897 J.J. Thompson proposed the rather startling hypothesis that cathode rays were streams of negatively charged particles – electrons. -ve cathode +ve anode Red Hot – Energy Transfer

9/30/2016Chapter N*1 2 What’s in the Nucleus? Protons: Positively charged particle, 2000 times the size of the electron. Neutrons:Particles with no charge, Similar size to the proton

9/30/2016Chapter N*1 3 What Rutherford really did Rutherford shot alpha-particles (Helium nuclei) at a thin gold foil. He found that most went right through. However, some were deflected, and a percentage of those bounced right back at him! He said that “it was like firing a cannonball at tissue paper, and having it ricochet off!” Can you see how he concluded that the nucleus was a hard small sphere, and that most of the atom was empty space? (As opposed to a plum pudding?)

9/30/2016Chapter N*1 4 Discovery of the Nucleus Rutherford began studying the scattering of  -particles by thin foils of various metals. Because the foils consisted of so many layers of atoms, it seemed evident that there were no gaps between atoms for the a-particles to pass through. He discovered that occasionally an a-particle would be scattered into the backward hemisphere. No possible with “plum- pudding model” Radium Source (a-particles) Collimator Foil Detector

9/30/2016Chapter N*1 5

9/30/2016Chapter N*1 6

9/30/2016Chapter N*1 7 Vocabulary Electron: negatively charged fundamental particle Proton: positively charged fundamental particle Neutron: uncharged fundamental particle Nucleus: small, central unit in the atom that contains neutrons and protons Atom: smallest unit of an element

9/30/2016Chapter N*1 8 Key Concepts and Vocabulary: Atoms, Elements and Molecules Atom: smallest unit of an element Element: any of more than 100 fundamental substances that consist of atoms of only one kind Molecule: a collection of atoms, bound together. Molecules can be made from only one element, such as H 2 or O 2 Molecules can be made from different elements, such as H 2 O or CO 2

9/30/2016Chapter N*1 9 Parts of an Atom Each element in the Periodic Table has a different number of protons in its nucleus Protons have positive charge Change the number of protons  change elements This is called nuclear physics The element also has the same number of electrons Electrons have negative charge Change the number of electrons  ionize the element This is called chemistry Some elements also have neutrons Neutrons have no charge They are in the nuclei of atoms p e n

9/30/2016Chapter N*1 10 The Hydrogen Atom One electron orbiting a nucleus 1 proton = Z = atomic number 0 neutrons = N Total mass = A = Z+N =1 Singly ionized Hydrogen is missing one electron = 1 H + Add a neutron and you have Deuterium = 2 H = D Add a 2nd neutron and you have Tritium = 3 H = T p e 1H1H

9/30/2016Chapter N*1 11 The Helium Atom Two electrons orbiting a nucleus 2 protons = Z = atomic number 2 neutrons = N Total mass = A = Z+N =4 Singly ionized Helium is missing one electron = 4 He + Doubly ionized Helium is missing both electrons =  particle = 4 He ++ ppnn e e 4 He

9/30/2016Chapter N*1 12 If Helium loses one of its protons (and one of its electrons), it becomes a different element Isotopes and Elements If Helium loses one of its neutrons, it becomes an isotope pnn e 3 He p pn e e 3 H (Tritium)

9/30/2016Chapter N*1 13 History of the atom (continued) In the Bohr model, the electron can change orbits, accompanied by the absorption or emission of a photon of a specific color of light.

9/30/2016Chapter N*1 14 History of the atom (continued) Modern quantum theories lead to stable locations of electrons, which are not exact planetary orbits, but are characterized by specific quantum numbers. Each electron shell is characterized by a different principle quantum number, usually called n.

9/30/2016Chapter N*1 15 History of the atom (continued) In quantum theory, the electron shells are not fixed orbits, but clouds of probability. You can’t measure the exact location of the electron. Each electron orbital has a different shape, and no two electrons can be in the same orbital (unless they have opposite spins.)

9/30/2016Chapter N*1 16 History of the atom (continued): The spin of the electron is another quantum property. In the planetary model, it is similar to the spin of the Earth on its axis. There are two choices for the orientation of the electron’s spin axis: up or down. Some further questions: What is periodic about the periodic table? What types of chemical properties are used to classify the elements?

9/30/2016Chapter N*1 17 Periodic Table of the Elements Review: What are the numbers in each box?

9/30/2016Chapter N*1 18 Navigating the Periodic Table The rows are the “periods” Each period starts a new shell of electrons The periods are numbered starting with 1 at the top The columns are the “groups” Each group has similar chemical properties The groups are numbered starting with 1 at the left Elements with similar properties have the same number of electrons in the outermost shell

9/30/2016Chapter N*1 19 Electron shells and atomic structure The first shell will hold up to two electrons. The orbital is spherical, and called 1s. The first row of the periodic table consists of 2 elements with 1s electrons. The second (and third) rows each add eight electrons. The shells that are filled are made of a spherical orbital that holds 2 electrons, called 2s or 3s, and 3 non-spherical orbitals that hold a total 6 electrons, called 2p or 3p.

9/30/2016Chapter N*1 20 Key concepts: Periodic Table The number of elements in each of the horizontal rows (periods) in the periodic table, are due to the quantum rules that govern the electron orbitals. In vertical columns (groups) in the periodic table, similar chemical properties are due to the number of electrons in filled or unfilled shells. Completely filled shells make an atom very stable Elements with 1 or 2 electrons outside of filled shells or with 1 or 2 missing electrons are very chemically reactive. They always try to combine with other elements that can fill their shells.

9/30/2016Chapter N*1 21 Vocabulary Atomic number: the number of protons in the nucleus of an element Atomic mass: the total mass in one atom of an element Ionization: removal of electrons from an atom Quantum: a very small discrete unit of light or energy

9/30/2016Chapter N*1 22 Thinking deeper: The forces in the atom Electrons are bound to nucleus by the Coulomb (electromagnetic) force Protons in nucleus are held together by the strong nuclear force Neutrons can decay into protons by weak nuclear force, emitting an electron and an anti-neutrino. The weak force is also responsible for radioactivity. F = k q 1 q 2 r 2 n = p + e +

9/30/2016Chapter N*1 23 Thinking deeper: The forces in the atom Gravity and the electromagnetic forces both have infinite range but gravity is times weaker at a given distance The strong and weak forces are both short range forces (< m) The weak force is times weaker than the strong force within the nucleus There are four fundamental forces in physics.

9/30/2016Chapter N*1 24 Electrical Conductors: A deeper look The best conductors are Copper (Cu), Silver (Ag) and Gold (Au) Cu has Z=29, Ag has Z=47 and Au has Z=79 How are these electrons arranged? 29 = = So, why are these elements good conductors? 79 =

9/30/2016Chapter N*1 25 Key concepts: Electrical Conductors Conductors are usually (but not always) metals – they have electrons that move easily, as they are outside filled shells Insulators are materials that block the flow of electrons. They do not have easily removed electrons. Metals occur where the d-orbitals are being filled in the periodic table. When you flip the switch, individual electrons do not instantaneously move through the entire length of the wire. Rather, they quickly bump into the metal nuclei, as they slowly drift down the wire (at about 0.5 mm/sec)

9/30/2016Chapter N*1 26 Vocabulary Conductor: material that allows the flow of electricity Insulator: material that blocks the flow of electricity