Chemistry 103 Lecture 5

Outline I. Review from last lecture II. Periodic Table –Atomic Mass III. Electronic Structure

Structure of the Atom An atom consists Of a nucleus that contains protons and neutrons Of electrons in a large empty space around the nucleus Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings

Atoms: Composed of 3 Subatomic Particles SymbolRelative Charge Mass Proton p or p x g Neutron n or n o x g Electron e x g

Atomic Symbols X QA Z Mass number = number of protons + number of neutrons Atomic number = number of protons Defines the element Charge = Number of protons - number of electrons

Atomic Symbols X QA Z Mass number = number of protons + number of neutrons Atomic number = number of protons Charge = number of protons – number of electrons

Atomic Symbols X QA Z Mass number = number of protons + number of neutrons Atomic number = number of protons Charge = number of protons – number of electrons

Atomic Symbol Notation vs. The Periodic Table Notation Ca Ca

An atom of zinc has a mass number of How many protons are in this zinc atom? A) 30B) 35C) How many neutrons are in the zinc atom? A) 30B) 35C) What is the mass number of a zinc atom that has 37 neutrons? A) 37B) 65C) 67 Learning Check

10 Write the nuclear symbols for atoms with the following subatomic particles: 47p +, 60 n, 47 e - ___________ 12p +, 12n, 10e - ___________ Learning Check

Determine the number of subatomic particles for each: Co-59 Pb Z = 82A = K + 19 F -

Exam type multiple choice question The radioactive isotope strontium-90, a component of fallout from atmospheric nuclear testing, is particularly hazardous because its 2+ ion may substitute for that of calcium in bone. 90 Sr 2+ has a. 38 protons, 52 neutrons, and 36 electrons b. 38 protons, 52 neutrons, and 38 electrons c. 38 protons, 52 neutrons, and 40 electrons d. 52 protons, 38 neutrons, and 50 electrons e. 52 protons, 38 neutrons, and 54 electrons

How Much Does an Atom Weigh? The mass of atoms is measured in units of amu –atomic mass unit Defined as 1/12 th the mass of 12 C Therefore, C-12 is exactly 12 amu on this scale Masses reported on the periodic table are weighted averages Takes into account the fact that element samples consist of mixtures of different isotopes

14 Atomic Mass The atomic mass of an element, Is listed below the symbol of each element on the periodic table Gives the mass of an “average” atom of each element compared to 12 C Is not the same as the mass number Na 22.99

Mass Number v. Atomic Weights/Mass 20 Ca Mass Number = the number of protons plus the number of neutrons ALWAYS A WHOLE NUMBER NOT FOUND ON THE PERIODIC TABLE! Atomic Weight/Mass = the mass of an average atom Found on the Periodic Table An average of the masses of all the isotopes of an element

16 Examples of Isotopes and Atomic Masses Most elements have two or more isotopes that contribute to the atomic mass of that element.

17 Calculating Atomic Mass The calculation for atomic mass requires the: Percent (%) abundance of each isotope Atomic mass of each isotope of that element Sum of the weighted averages mass isotope 1 (%) + mass isotope 2 (%) + …

Calculating an Average Exam Scores (10 students): 95, 85, 85, 82, 79, 79, 79, 79, 65, 51 How do you determine an average?

Calculating an Average Exam Scores (10 students): 95, 85, 85, 82, 79, 79, 79, 79, 65, =

Calculating an Average Exam Scores: 95, 85, 85, 82, 79, 79, 79, 79, 65, = Alternate way of writing: 95+(85 x 2)+82+(79 x 4) =

Calculating an Average Exam Scores: 95, 85, 85, 82, 79, 79, 79, 79, 65, 51 10% = 95, 1/10 x 100% = 10% 20% = 85, 2/10 x 100% = 20% etc…. Total percentages = 100% Weighting factor: (95 x 0.1)+(85 x 0.2)+(82 x 0.1)+(79 x 0.4)+ (65 x 0.1)+(51 x 0.1) = 77.9

22 Isotopes of Magnesium In naturally occurring magnesium, there are three isotopes. 24 Mg 25 Mg 26 Mg

23 Calculating Atomic Mass of Mg Isotopes Mass of IsotopeAbundance Weighted portion 24 Mg = amu x78.70/100=18.88 amu 25 Mg = amu x10.13/100= amu 26 Mg = amu x 11.17/100= amu Atomic mass (average mass) Mg = amu Mg 24.31

24 Calculating Atomic Mass of Mg Isotopes Mass of IsotopeAbundance Weighted portion 24 Mg = amu x.7870= amu 25 Mg = amu x.1013= amu 26 Mg = amu x.1117= amu Atomic mass (average mass) Mg = amu Mg 24.31

Group Work: Average Atomic Weights Naturally occurring carbon is composed of 98.93% 12 C and 1.07% 13 C. If the mass of one 12 C atom is 12 amu and the mass of one 13 C atom is amu, what is the average mass of naturally occurring carbon?

Atomic Mass Calculation Naturally occurring sulfur consists of 4 isotopes – calculate the atomic mass of S Sulfur Isotopes Atomic Mass % Abundance % % % %