Chapter Overview

The Structure of the Atom Chapter 4..\..\Movies\Atoms.MOV Early Theories of the Atom Subatomic Particles How Atoms Differ Unstable Nuclei & Radioactivity Ann Dau – CHEMISTRY 112

4.1 Early Theories of Matter Dalton’s Atomic Theory 1.All matter is composed of extremely small particles called atoms. 2.All atoms of a given element are identical. Atoms of different elements are different from one another. 3.Atoms cannot be created or divided into smaller particles or destroyed. 4.Different atoms combine in simple whole number ratios to form compounds. 5.In a chemical reaction, atoms are separated, combined, or rearranged. Dalton’s Atomic Model Atom - the smallest particle of an element that retains the properties of the element.

4.1 Early Theories of Matter E. Goldstein discovered the proton in J.J. Thomson discovered the electron in 1897 during cathode ray tube experiments in the late 1890’s.

4.1 Early Theories of Matter Robert A. Millikan determined the mass and charge of the electron in –one unit of negative charge –mass is 1/1840 of a hydrogen atom

4.1 Early Theories of Matter In 1911 Ernest Rutherford discovered the nucleus during his gold foil experiment.

4.1 Early Theories of Matter Ernest Rutherford’s gold foil experiment.

4.1 Early Theories of Matter Neils Bohr developed the planetary model of the atom –Electrons are in a particular path have a fixed energy –Energy level-region around a nucleus where the electron is likely to be moving

4.1 Early Theories of Matter Erwin Schrodinger developed the Quantum Mechanical Model –Describes the electronic structure of the atom as the probability of finding electrons within certain regions of space

4.1 Early Theories of Matter James Chadwick discovered the neutron in In 1913 Henry Mosley used X-rays to count the number of protons in the atomic nuclei of different atoms.

The Atomic Theory All elemental substances are made up of tiny indivisible particles called atoms. Atoms of different elements have different properties. In an ordinary chemical reaction, atoms of elements maintain their identities. Compounds are made when atoms of different elements combine...\..\Movies\atom model.MOV

Parts of the Atom Protons give atoms their identity!

Atomic Structure Nucleus Protons and neutrons Both about the same mass (Table 2.1 Text) About cm radius for gold atom Most of atomic mass found in nucleus Outer periphery electrons Less massive Electronic structure covered later..\..\Movies\atomic model.MOV

4.2 Subatomic Particles & the Nuclear Atom Located within the Nucleus –Proton (p + ) Positively charged particle (each carries a charge of +1) Relative mass = 1 amu Actual mass = X kg –Neutron (n 0 ) Neutrally charged particle Relative mass = 1 amu Actual mass = X kg Serves as the glue that holds the nucleus together as well as a buffer between the charges of protons and electrons

Subatomic Particles Located outside the nucleus in the electron cloud –Electron Negatively charged particle (each carries a charge of -1) Relative mass = 1/1840 amu Actual mass = 9.11 X kg The electron is the part of the atom that will function in bonding and reactions

4.3 How Atoms Differ Atomic Number –the number of protons in the nucleus of an atom –indicated at the top of the element’s block on the periodic table 8O Mg Oxygen has an atomic number of 8 There are 8 protons in an atom of Oxygen Magnesium has an atomic number of 12 There are 12 protons in an atom of Magnesium

Isotopes Atoms of the same element with the same number of protons, but different numbers of neutrons Since the atoms have different numbers of neutrons, they also have different mass numbers –Mass number = # of protons + # of neutrons

Abbreviating Isotopes Hyphen Notation –Simply write the Name of the atom, put a hyphen, and then write the mass number Carbon-12 vs. Carbon-14 –Carbon 12 has 6 protons and 6 neutrons –Carbon 14 has 6 protons and 8 neutrons Nuclear Designation –Element symbol is written in the center –Mass number goes in the upper left corner –Atomic number goes in the lower left corner C 12 6

Different Isotopes Identify the number of protons, neutrons, and electrons each of the following have. Boron-10 Boron Cl 66 Zn Br p + : ________ n o : ________ e - : ________ p + : ________ n o : ________ e - : ________ p + : ________ n o : ________ e - : ________

Sample Problem 2.2Determining the Number of Subatomic Particles in the Isotopes of an Element PROBLEM: Silicon(Si) is essential to the computer industry as a major component of semiconductor chips. It has three naturally occurring isotopes: 28 Si, 29 Si, and 30 Si. Determine the number of protons, neutrons, and electrons in each silicon isotope.

Calculating Atomic Mass Mass Number –the number of protons + neutrons in a given isotope Atomic Mass –The weighted average mass of all of the isotopes of that element [(Mass of isotope A)(percent abundance )] + [(Mass of isotope B)(percent abundance)]

Practice Calculating Atomic Mass Calculate the atomic mass of helium given the following information: There are two naturally occurring isotopes of helium: Isotope% AbundanceMass helium helium

Practice Calculating Atomic Mass There are two naturally occurring isotopes of helium: Isotope % Abundance Mass helium helium ( x ) + ( x ) = =

Practice Calculating Atomic Mass There are three naturally existing isotopes of silicon: silicon-28, silicon-29, and silicon-30. Their percents of natural abundance is listed respectfully: %, 4.70 %, and 3.09 %. Calculate the average atomic mass of silicon and express your answer in 4 significant digits.

Sample Problem 2.3 Calculating the Atomic Mass of an Element Silver (Ag: Z = 47) has 46 known isotopes, but only 2 occur naturally, 107 Ag and 109 Ag. Given the following data, calculate the atomic mass of Ag: IsotopeMass(amu)Abundance(%) 107 Ag 109 Ag

Atoms, Ions, and Isotopes Ions and isotopes are atoms…they are specific types of atoms. Ions: atoms that have a charge; either positive or negative What would allow an atom to be an ion? Isotopes: a relationship between two atoms in which they have the same number of protons and different numbers of neutrons. Are isotopes technically atoms of the same element?

Ions Atoms that have gained electrons to become ___________________ charged. Atoms that have lost electrons to become ___________________ charged. Key: Has the identity of the atoms changed? Why?

4.4 Unstable Nuclei & Radioactive Decay Nuclear Reactions –reactions that involve a change in the nucleus of an atom. Radioactivity –the spontaneous release of radiation. Radiation –rays and particles emitted by radioactive materials Radioactive atoms emit radiation because their nuclei are unstable. There are three main types of radiation –Alpha decay –Beta decay –Gamma decay

Alpha ( α ) radiation two protons and 2 neutrons Positive charge Symbols:  4  4 He 2 2 reduces the atomic number by 2 reduces the mass by 4

Beta ( β ) radiation Fast moving electron Negative charge Symbols:  0  increases the atomic number by 1 does not change the mass

Gamma ( γ ) radiation high energy radiation released with alpha and beta radiation symbol:  0  0 does not change the mass or atomic number

Half lives The time it takes for 1/2 of the mass of the isotope to be decayed. If I have a 60g sample and the half life is 2 years, how long will it take for there to be 7.5g left of the sample? 60g  30g  15g  7.5g 2 years 4 years 6 years So, it takes 6 years for the 60g sample to decay into 7.5g.