CHAPTER 4 Atomic Structure 4.1 Atoms Democritus first suggested the idea of atoms Indivisible & Indestructible 460 B.C. 370 B.C. Dalton s Atomic Theory 1. All elements are composed of submicroscopic indivisible particles called atoms 2. Atoms of the same element are identical. The atoms of any one element are different from those of any other element. Every sodium atom in the universe is the same Every magnesium atom in the universe is thesame 3. Atoms of different elements can physically mix together or can chemically combine with one another in simple whole number ratios to form compounds. 1
4. Chemical reactions occur when atoms are separated, joined, or rearranged. However, atoms of one element are never changed into atoms of another element as a result of a chemical reaction. Atom The smallest particle of an element that retains the properties of that element. Have a size ranging from: (5 x 10 11 m) (2 x 10 10 m) Protons Electrons Neutrons Parts of an atom Do not have unique properties! The protons, electrons, and neutrons in one atom are the same as in any other atom. The number of protons, electrons, and neutrons is what makes each atom unique. Scanning Tunneling Microscope Visualizes individual atoms and makes it possible to see them as a 3D image on a monitor. Atoms of Copper and Iron 2
4.2 The Structure of the Nuclear Atom Atoms can be broken down into more fundamental particles. In Chemistry we will learn about protons, electrons, and neutrons. Electrons Negatively charged subatomic particle 1/1840 the mass of 1 proton Millikan: discovered that an electron carries one unit of negative charge Electrons Discovered by Thomson using the Cathode ray tube experiment Electrons being repelled by magnet... Protons Positively charged subatomic particles Carries a one positive charge 3
Neutrons Subatomic particle with no charge Discovered by Chadwick Mass equals that of a proton The Atomic Nucleus The central core of an atom, composed of protons and neutrons Responsible for almost all the mass of an atom Protons Neutrons The nucleus has a positive charge, and it occupies a very small part of the volume of an atom. Nucleus & Energy Levels Proton Neutron Nucleus 99.99% of the mass of an atom Contains Protons and Neutrons Overall positive charge Very little volume of the atom Proton defines the atom Energy Level Contains only electrons 0% of the mass of an atom Overall negative charge Most of the volume of an atom Determines the properties of an atom 4
The negatively charged electrons in an atom occupy most of the volume of the atom. The Rutherford atomic model is known as the nuclear atom. The protons are located in the nucleus The electrons are around the nucleus The electrons occupy most of the volume of the atom Electrons: determine the properties of every atoms. Protons: define what each atoms is. 4.3 Atomic How are atoms of one element different from those of another element? Differences among elements result from differences in the numbers of protons in their atoms. Atomic The number of protons in the nucleus of the atom of that element. Example: The atomic number of Oxygen is 8 5
Remember that atoms are electrically neutral. Thus the number of protons in the nucleus of an atom must equal the number of electrons around its nucleus. The periodic table gives the atomic number of each element. Nucleons: protons & neutrons because they are in the nucleus. s, p, d, f: sub shells where electrons are found Mass Mass number is the total number of protons and neutrons in the nucleus of an atom. You can determine the composition of an atom of any element from its atomic number and its mass number. Example: Beryllium: atomic number 4 mass number 9 # p = 4 # e = 4 # n = 5 Aluminum Al Palladium Pd Atomic = Atomic = Mass = Mass = Proton = Proton = Electron = Electron = Neutron = Neutron = Mass of Electrons = Mass of Electrons = 6
Al Shorthand Notation The atomic number is written as a subscript. The mass number is written as a superscript. 9 27 Example: Be Al 4 13 What can change in an atom? Protons: can never change Electrons: if the number changes, then an ion forms. Neutrons: if the number changes, then an isotope forms. If the proton number changes. Then the entire atom changes. If oxygen gains a proton, it becomes fluorine. If oxygen loses a proton, it becomes nitrogen. If the neutron number changes. Then a different version of the same atom forms. Oxygen 16 has 8 neutrons. Oxygen 17 has 9 neutrons. Oxygen 18 has 10 neutrons Isotopes of Elements The nuclei of the atoms of a given element must all contain the same number of protons, but the number of neutrons may vary. Isotopes vs. Allotropes Isotopes: atoms of the same element with different numbers of neutrons Allotropes: different forms of an element, usually when the same atoms are joined as compound. The way atoms are arranged Carbon exhibits both Isotopes: Carbon 12, Carbon 13, Carbon 14 Allotropes: graphite, diamond, and fullerenes 7
Isotopes Atoms that have the same number of protons but different numbers of neutrons Because isotopes of an element have different numbers of neutrons, they also have different mass numbers. Isotopes are chemically alike because they have identical numbers of protons and electrons. Hydrogen Hydrogen has three known isotopes: Hydrogen 1 (one proton, no neutrons) Protium Hydrogen 2 (one proton, one neutron) Deuterium Hydrogen 3 (one proton, two neutrons) Tritium Atomic Mass The masses of atoms are too small to work with so we use Carbon as the comparative atom. An Atomic Mass Unit (amu) is defined as one twelfth the mass of a carbon 12 atom. Proton=1 amu, Neutron=1 amu, electrons=0 amu Atomic Mass The atomic mass of an element is a weighted average mass of the atoms in a naturally occurring sample of the element. Atomic Mass vs. Mass There is a difference!! Average Atomic Mass is the mass on the periodic Table Mass is the actual mass of a given atom. 8
The weighted average mass reflects both the mass and the relative abundance of the isotopes as they occur in nature. Chlorine 35 Isotopes Chlorine 36 (54%) Relative Abundance (46%) Calculating the Atomic Mass of an Element You must know: The number of stable isotopes of that element The mass of each isotope The natural percent abundance of each isotope Average Atomic Mass of Carbon Carbon 12: 12 x 0.917 = 11.004 Carbon 13: 13 x 0.069 = 0.897 Carbon 14: 14 x 0.014 = 0.196 Add together products together 12.097 (Avg. atomic mass) Determining Average Atomic Mass of a "New" element Isotope Percentage Show Work Fontainium 41 87.3 % Fontainium 43 9.8 % Fontainium 44 2.9 % Average Atomic Mass Write the following in shorthand notation... Oxygen 15 Oxygen 16 Oxygen 17 Isotope or Common Isotope or Common Isotope or Common 9
States of Atoms Energy Levels of an Atom Ground State: electrons are at their lowest possible energy level Excited State: electrons have absorbed energy and jumped to a higher energy level Nucleus If an atom gains electrons, then. Then a negatively charged version of the same atom forms. If oxygen gains 2 electrons, it has 10 electrons and 8 protons. It now has a 2 charge If an atom loses electrons, then. Then a positively charged version of the same atom forms. If calcium lost 2 electrons, it has 18 electrons and 20 protons. It now has a 2+ charge. 10
Neutral Version Gain or Loss New electron number Examples of Ions Sulfur Magnesium Lithium Chlorine P=16 P=12 P=3 P=17 E=16 E=12 E=3 E=17 Gain 2 electrons Lose 2 electrons Lose 1 electron Gain 1 electron E=18 E=10 E=2 E=18 Charge S 2 Mg Li + Cl Cation vs. Anion Cation: Positive Ion More protons than electrons I am Positive I love my Cat. Anion: Negative Ion More electrons than protons Atomic Protons Electrons Neutrons Mass Atomic Protons Electrons Neutrons Mass Al 3+ Hydrogen 1 Hydrogen 3 H + Oxygen 16 Oxygen 14 O 2 Al 3+ 13 13 10 14 27 Hydrogen 1 1 1 1 0 1 Hydrogen 3 1 1 1 2 3 H + 1 1 0 0 1 Oxygen 16 8 8 8 8 16 Oxygen 14 8 8 8 6 14 O 2 8 8 10 8 16 Element Atomic # Neutral = Periodic Table Version Protons Electron Neutron Mass Fluorine 9 9 9 10 19 Chromium 24 24 24 32 56 Calcium 20 20 18 20 40 Iodine 53 53 54 74 127 Polonium 84 84 84 131 215 Ion, Isotope, Neutral Element Atomic # Protons Electron Neutron Fluorine 9 9 9 10 19 Mass Ion, Isotope, Neutral Neutral & Common Chromium 24 24 24 32 56 Isotope Calcium 20 20 18 20 40 Cation Iodine 53 53 54 74 127 Anion Polonium 84 84 84 131 215 Isotope 11
The Periodic Table: A Preview The Periodic Table allows you to compare properties of one element to another element Period Group 12