Atomic Structure Chapter 4

4.1 Defining the Atom Democritus (460 BC – 370 BC) Had an atomic philosophy Matter was made of small invisible particles called atoms No scientific evidence, but it was logical

Dalton’s Atomic Theory John Dalton ( ) was an English chemist and school teacher Dalton’s theory was the first scientific theory of the atom

John Dalton looks like another famous John

Dalton’s Atomic Theory

1.Matter is composed of tiny indivisible atoms 2.All atoms of a given element are exactly the same 3.Different elements are made of different atoms 4.Atoms of different elements can combine in simple whole number ratios to form compounds 5.Chemical reactions involve separating, combining, or rearranging atoms.

How big are atoms? Pretty small Just 4 grams of helium (He) contains 6.02 x atoms! Most atoms have a radius in the range of 5 x m to 2 x m Atomic sizes are in the pico- and nanometer range

Can we see atoms? Amazingly, yes! STM Scanning tunneling microscope has made it possible to see atoms. These are gold atoms They are only 2.3 Å apart!

Here is a picture of a mixture of tin, lead, and silicon atoms! Custance's team demonstrated their "fingerprinting" technique by using an atomic force microscope (AFM) to distinguish atoms of tin (blue) and lead (green) deposited on a silicon substrate (red). (Credit: Oscar Custance) physicsworld.com/cws/article/news/ physicsworld.com/cws/article/news/ The colors are added to help distinguish the atoms

4.2 Structure of the Nuclear Atom When do I know I have learned this section? When… I can describe 3 types of subatomic particles I can describe the structure of the nuclear atom I can explain Thomson’s discovery –experiment and model of the atom I can explain Millikan’s discovery I can explain Rutherford’s discovery –experiment and model of the atom

Subatomic Particles A cathode ray is deflected by electrically charged plates. 4.2

Subatomic Particles In Dalton’s theory, atoms had no charge Thomson studied cathode rays (the cathode ray experiment) Resulted in discovery of the electron Electrons are negatively charged particles Discovered the charge to mass ratio of electron

JJ Thomson’s Plum Pudding Model Atom was a sphere Atoms were electrically neutral (-) charged electrons embedded in a (+) charged matrix Like a chocolate chip cookie

Millikan’s Oil Drop Experiment Discovered the actual quantity of charge for the electron Using Thompson’s charge/mass ratio, Millikan was also able to determine the mass of the electron Mass of electron is 1/1840 the mass of a hydrogen atom.

Protons If H atoms are neutral, and they give off negatively charged electrons, what must be left? Positive charge Proton discovered by Eugen Goldstein noticed a ray traveling in opposite direction of the electrons in the cathode ray tube Toward the negative end (cathode)

Neutrons Discovered by James Chadwick

Quarks Quarks are subnuclear particles Protons and neutrons are composed of quarks

The Atomic Nucleus Ernest Rutherford discovered the nucleus Gold foil experiment

Simulation of Rutherford’s Gold Foil Experiment

Interpretation of Rutherford’s Findings

Rutherford’s Conclusions All positive charge and nearly all the mass of the atom are concentrated in a very small nucleus Atom contains mostly empty space Rutherford’s model of the atom was like a mini solar system

Summary of Atomic Models DemocritusInvisible particles DaltonTiny homogeneous sphere 5 (4) statements ThomsonPlum pudding RutherfordNuclear atom

Thomson’s Model of the Atom Plum Pudding Negatively charged electrons embedded in a positive matrix

Rutherford’s Model Nuclear Atom Most of the volume of the atom is empty space All positive charge & most mass in a small nucleus Nucleus contains protons and neutrons Electrons occupy space outside the nucleus

Distinguishing Among Atoms Atomic Number = number of protons in nucleus Each element has unique atomic number Mass Number = number of protons and neutrons in nucleus # neutrons = mass # - atomic #

Practice

Symbols of Elements Elements are symbolized by one or two letters. First letter is always capitalized. Second letter, if there is one, is always lower case.

Atomic Number All atoms of the same element have the same number of protons: The atomic number (Z)

Mass Number The mass of an atom in atomic mass units (amu) is the total number of protons and neutrons in the atom.

Isotopes Isotopes are atoms of the same element with different masses. Isotopes have different numbers of neutrons C 12 6 C 13 6 C 14 6 C

Different Ways of Indicating Isotopes Chemical symbols 16 8 O 18 8 O Symbol and mass number O-16O-18 Name and mass number oxygen-16oxygen-18

Isotopes of Neon Atomic # Mass #

Atomic Mass Measured in amu’s (atomic mass units) because their mass is so small Definition: one atom of carbon-12 equals exactly 12 amu’s 1 amu = exactly 1/12 the mass of a C-12 atom 1 amu = x g

Different Isotopes Have Different Abundance & Atomic Masses IsotopeAbundance Atomic mass (amu) Chlorine % Chlorine % What is the atomic mass of chlorine on the periodic table?

Average Atomic Mass IsotopeAtomic Mass Abundance R ________ R ________ 124 Determining abundance of isotopes when average atomic mass is known Multiply the mass of each isotope by its relative abundance (decimal equivalent) and add the products

Periodic Table - Preview Allows you to easily compare and predict properties of elements Groups = Columns –Numbered 1-18 or –1A-8A (representative elements) –1B-8B (transition metals) Periods = Rows –Numbered 1-7