Unit 3 The Atomic Theory Pre-Assessment – Draw a colorful model of what you think the structure of the atom looks like. This is your model from past science classes and what you know the structure of the atom to be.

Origin of the term “Atom” The word “atom” comes from the Greek word atomos. The word atom means Unable to be cut, something that cannot be divided further.

Ancient Theories of the Atom Democritus’ (450 BC) “all matter that makes up the world around us is actually composed of tiny invisible particles. (atomos)”

Antoine Lavoisier Elements as a simple substance cannot be broken down. (1777) From this he devised a theory of the formation of chemical compounds from elements. Law of Conservation of Matter – Matter is neither created nor destroyed in a chemical reaction.

Law of Conservation of Matter: *was beheaded at the height of the French Revolution.

18th Century Theories:Proust A given compound always contains the same elements in the same proportions by mass. Law of Constant Composition: (1799) Joseph Louis Proust found that a given compound always contains the same elements in the same proportions by mass.

Modern Theories of the Atom John Dalton (1805) used what was known about the atom to establish the Atomic Theory of Matter

Dalton’s Theory 4 postulates #1 – Each element is composed of extremely small particles called atoms.

Dalton’s Theory #2 – All atoms of a given element are identical , but they differ from those of any other element.

#3 – Atoms are neither created nor destroyed in any chemical reaction. Dalton’s Theory #3 – Atoms are neither created nor destroyed in any chemical reaction.

Dalton’s Theory #4 – A given compound always has the same relative numbers and kinds of atoms.

Unit 3 The Atom Theory Wednesday 10/21 Intro the Atom with video and outlining Chapter 3. Thursday 10/22 Guiding Readings on JJ Thomson and Ernest Rutherford for Performance Task Friday 10/23 Discussion on JJ Thomson and Ernest Rutherford Monday 10/26 Performance Task #1 on Gold Foil Experiment Wednesday 10/28 Subatomic Particles

What was the view of the model before the Cathode Ray Tube Experiment?

JJ Thomson’s Model The Plum Pudding Model Diffuse cloud of (+) charge with (-) charges throughout.

J.J. Thomson (IN 1896) He discovered the first subatomic particle – the electron

http://highered. mcgraw-hill http://highered.mcgraw-hill.com/sites/0072512644/student_view0/chapter2/animations_center.html# The stream of electrons was attracted to the + pole of the magnet and repelled by the (-) pole of the magnet. Using the charge to mass ratio he was able to calculate the mass to being 2000x lighter than a hydrogen atom (smallest atom known).

Summarize JJ’s Work Because the particles in the cathode ray tube were much lighter than the smallest known element he concluded that these are electrons that are (-) charged. He knew that positive charges also had to be present so he came up with the Plum Pudding Model.

Robert Millikan - 1909 He measured the charge of an electron. http://highered.mcgraw-hill.com/sites/0072512644/student_view0/chapter2/animations_center.html#

New Zealand Scientist Ernest Rutherford(1871-1937) Had performed experiments to find that atoms were made of positively charged particles moving around a very tiny positively charged nucleus.

Rutherford’s Gold Foil experiment He did an famous experiment whereby he shot a stream of alpha particles at a thin piece of gold (Au) foil expecting most of the particles to be deflected-instead most of them passed directly through the foil and onto the photographic film

If JJ Thomson’s model was correct Rutherford’s Model

Rutherford experiment animation http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf Rutherford’s Atomic Structure Model + =positively charged particles - = electrons

Comparison of Atomic Models Dalton’s Model Thomson’s Model Rutherford’s Model

YOU WILL NEED: NOTEBOOK PERIODIC TABLE

Protons (positive) are bigger than electrons Protons (positive) are bigger than electrons. The part of an atom that gives an element its identity. Neutrons (neutral) are slightly bigger in mass than a proton. Electrons (negative) charge, have a mass of close to zero-negligible

Atomic Mass Units (amu): roughly equal to the mass of a proton or neutron. **the mass of an atom is measured in amu’s 1 amu = 1.66x10-24g

Particle Location Charge(C) Mass (g) Mass (amu) Proton Inside nucleus 1.602 x 10-19 1.673x10-24 1.0073=1 Neutron 1.675x10-24 1.0087=1 Electron Outside nucleus -1.602 x 10-14 9.109x10-28 0.0006=0

Every element has a unique atomic number which is the number of protons in the atom. Elements atomic number is just above the chemical symbol on periodic table. Atoms are electrically neutral-means number of protons always equal number of electrons.

Electron Number An Element’s atomic number also indicates number of electron in its atoms.

Ions Ions are atoms that have lost or gained one or more electrons giving them a positive or negative charge!

If an ion gains an electron it has a negative charge. If an ion loses an electron it has a positive charge. For example: Na (Sodium), has originally 11 electrons but when an electron is lost it becomes a positive ion. Na+

A neutral magnesium atom (atomic number=12) has 12 protons/electrons. If it loses 2 electrons it becomes an ion with a charge of 2+. Number of protons 12 Number of electrons - 10 Charge of Ion 2+

Ion Protons Electrons 19. Cu²+ 20. As3- 29 33 27 36

Isotopes Isotopes are atoms that have the same number of protons but different numbers of neutrons Most elements in the first two rows of the periodic table have at least 2 isotopes with one being more common than the other In nature, elements are almost always found as a mixture of isotopes

Differences between isotopes? Isotopes react in the same way as others of the same element

Atomic mass: weighted average of the masses of the existing isotopes of an element. Atomic Number/ number of protons Atomic mass

What was Dalton’s 2nd Postulate? Was it correct? Pennies manufactured prior to 1982 were 95% copper, 5% zinc. Pennies manufactured after 1982 are 5% copper, 95% zinc. Pennies manufactured in 1982 are a mix of the older and newer types. So pennies seem to have 2 ‘isotopes.’ Hmmm

Isotopes of Hydrogen The most common isotope of hydrogen has no neutrons at all There's also a hydrogen isotope called deuterium, with one neutron, and another, tritium, with two neutrons.

To identify an isotope more specifically, chemists add a number after the elements name. ex. Carbon-11 Carbon-12 Carbon-14 This number is called the isotope’s mass number and is the sum of the isotope’s number of protons and neutrons. What is the atomic mass on your Periodic Table? Which of the 3 isotopes of Carbon is the most abundant?

Complete Chemical Symbols Mass number 141 Ba2+ 56 Charge Atomic number Number of protons = Atomic # Number of neutrons = Mass # - Atomic # Charge = Atomic # – Number of electrons

Complete the Table Chemical Symbol No. of Protons No. of Electrons No. of Neutrons Atom or Ion? 38Sr2+ 46 45 50 14 29 Atom 90

WARM UP What is the atomic mass unit of a proton? Neutron? Electron? What is the atomic mass of an element represent?

Complete the Table Chemical Symbol No. of Protons No. of Electrons No. of Neutrons Atom or Ion? 38Sr2+ 46 45 50 14 29 Atom 90