2/3/2016 Today Problem Set 1 DUE online ◦ What makes a given Element at 11 pm next Monday unique? Keep up with the reading & Definition of Atomic Number (Z) ◦ Isotopes: viewing on the course How do they differ? How are website they the same? Please read the Definition of Mass Number (A) background to Experiment ◦ Atomic Mass: 2 on Emission Spectra & Atomic Mass Units & Average Mass Flame Tests Development of the Modern Atomic Theory In 1803, John Dalton proposed an atomic theory that is still the basis for many of our theories about the atom. 1. All matter is composed of atoms, which are tiny, indivisible particles. 2. A chemical reaction is a rearrangement of atoms to form different compounds. Atoms are neither created nor destroyed in a chemical reaction (the law of conservation of mass). 3. Atoms of one element cannot be converted into another element. Atoms of an element are identical in mass and other properties, and are different from every other element.** 4. A compound is a combination of atoms of two or more elements in specific ratios (the law of definite composition). **In some cases, elements can change identity, although this only happens in nuclear reactions. What about Neutrons? ISOTOPES are atoms of the same element (same # of protons) with a different numbers of neutrons. Isotopes of an element have nearly identical properties. ◦ The number of protons and electrons, which are the same in all isotopes of a neutral element, has much more to do with the chemical and physical properties of an element. 1 2/3/2016 Mass Number • The mass number (A) is the sum of the protons and the neutrons in the nucleus of an atom. mass number (A) = # protons (Z) + # neutrons • The name for an isotope is the element name followed by the mass number. Example: Carbon-12 • The symbol for an isotope is its element symbol along with its mass number (A) and atomic number (Z). Mass Number: Specifying the Isotope • Consider the isotope Phosphorus-31 symbol: 31 15P A = 31 (mass number = the sum of protons and neutrons) Z = 15 (atomic number = # of protons ) • In order to determine how many neutrons are in the nucleus of an atom simply subtract: • # neutrons = mass number (A) – atomic number (Z) = 31 – 15 = 16 neutrons Isotopes: All about the number of Neutrons ISOTOPES: Atoms of the same element (Z) but different mass number (A). Mass Number (A) = # of protons + # of neutrons 2 2/3/2016 Counting numbers of Protons & Electrons Which isotope of Carbon would have the same number of neutrons as oxygen-16? A. Carbon-12 B. Carbon-13 C. Carbon-14 D. Carbon-15 E. Carbon & Oxygen can’t have the same # of neutrons Isotopic Abundance Different elements have different numbers of isotopes, each with their own natural abundance Some elements are not stable & have NO NATURALLY occurring isotopes Mass Spectrometry Can be used to separate isotopes of an element. Schematic diagram of a mass spectrometer 3 2/3/2016 Mass spectrograph of Neon Three isotopes of Neon naturally occur in nature with varying abundance Atomic Mass: How heavy is one atom compared to another? A single water molecule has a mass of 2.99 x 10-23 grams. Working in grams to describe the mass of single atoms or molecules is not convenient. Instead, we typically express the mass of atom in terms of atomic mass units, amu, or simply u. Atomic Mass 1 • REFERENCE STANDARD: The atomic mass unit is defined as /12 of the mass of an atom of the carbon-12 isotope. 1 amu = 1/12 mass of Carbon-12 • Both the neutron and the proton have a mass of approximately 1 amu: 1 amu 1 proton 1 neutron • Electrons are very small: 1 electron 0.00055 amu (Roughly 1/1800 amu) • Because the mass number is the sum of protons and neutrons, the mass number is a whole number approximation of the atomic mass of an isotope. 4 2/3/2016 UNITS and Quantitative measurements Numbers often make no sense if we do not have some sort of reference or standard to compare them to. Nearly all numbers MUST be followed by a unit label. The unit indicates the standard against which the number is measured. Average Atomic Mass • Most elements are a mixture of two or more isotopes. • The percentage of an isotope in a naturally occurring sample of an element is called the isotopic abundance (or percentage abundance) of that that isotope. • The isotopic mass is the mass of a single atom of an isotope. • The average atomic mass is the weighted average of the masses of isotopes of an element. 5 2/3/2016 Calculating Average Atomic Mass from Isotope Data Consider the element Magnesium, composed of three isotopes of the following percentage abundances: Isotope % Abundance Isotope Mass (amu) Mg-24 79.0 % 23.985 Mg-25 10.0 % 24.986 Mg-26 11.0 % 25.982 Understanding Percent Abundances Consider the only two naturally occurring isotopes of Boron: Isotope Isotope Mass (amu) % Abundance Boron-10 10.0129 ? Boron-11 11.0093 ? Which isotope is more abundant? Relative Masses: A convenient tool for relating one quantity of particles to another quantity of particles through measurements of mass Nut: 1 g 100 x 12 = 1200 Bolt: 2 g 1.2 x 103 objects 1200 g = 100 dozen = 2400 g 6 2/3/2016 Molar Mass: A connection between Macroscopic and Particulate nature of matter based on the relative masses of each atom Carbon-12: 12 amu Magnesium-24: ~24 amu Definition of 1 mole = 1 mol = Molar Mass: Indicates how many grams are in one mole of the substance 12 g/mol 24 g/mol Mole / Dozen Analogy • Like the mole, a dozen of something is a convenient way to talk about the number of items we tend to buy in those quantities: 1 dozen donuts = 12 donuts 3 dozen eggs = 36 eggs • The mole and the dozen make it easier to talk about large quantities of some specific item. In the case of a dozen, we are often specifying a quantity of eggs. • In the case of a mole, we are specifying a quantity of atoms, molecules or some other particle on the atomic scale. Molar Mass & Avogadro’s Constant: A connection between Macroscopic and Particulate nature of matter Relative Masses: Carbon: 12 amu Magnesium: ~24 amu 6.022 x 1023 particles 12 g = 1 mol = 24 g Molar Mass: Indicates how many grams are in one mole of the substance 12 g/mol 24 g/mol 7 2/3/2016 The Mole • A mole of anything is 6.02214 x 1023 of that particular thing. • In Chemistry, we work with very small particles, so we must work with a very large quantity of them. • The mole is a convenient number to count a large quantity of particles. • We can talk about a mole of anything, but we usually use it to talk about atoms, molecules, ions, and formula units – Matter at the microscopic, atomic, particle level. 6.02214 x 1023 is also called Avogadro’s number. Proceeding clockwise from the top samples containing one mole each: copper, aluminum, iron, sulfur, iodine, and (in the center) mercury. Subtle Differences in Isotopes: Neutrons affect the MASS & DENSITY of atoms Normal Water: Heavy Water: with hydrogen-1 with hydrogen-2 8 2/3/2016 Isotopic Fractionation: Subtle differences in physical & chemical properties can cause certain isotopes to concentrate in biological organisms or geological formations Ice Core Analysis: Using O-16/O-18 ratios to track temperature changes on Earth Isotope Signatures: A Characteristic Isotope Ratio that can be used as a Fingerprint TO A SMALL BUT MEASUREABLE DEGREE: Isotope signatures can be influenced by growth conditions, metabolism, moisture and nutrient availability, as well as other environmental factors like local climate, minerals & water supply. • Geology, Oceanography & Ecology: • O-16/O-18 Ratios: Temperature of precipitation (rain, snow, ice) • C-13/C-14 Ratios: Animal migration tied to food source • Anthropology & Archeology: • C-13/C-14 Ratios: Diet and migration of ancient peoples • N14/N15 Ratio: Vegetarian or Vegan? • Lead Ratios: Global trade patterns based on metal artifacts, lead-based pigments and glasses. • Forensics: • Oxygen & Strontium Ratios: • In teeth: where was a person born & raised? • In bone: where has a person lived the last 10 years? • Geographic origin of specific products: illegal drugs, explosives, animal products 9 2/3/2016 C-13 Isotope Signatures: Range of Natural Variation: 0.963-1.115% C-13 & N-15 Isotope Signatures: Analysis of Marijuana for Geographic Origins & Growing Methods C-13 & N-15 Isotope Signatures: Regional Grouping of Cocaine Samples 10.