Ch 5: Atomic Structure

warmup All matter is composed of very small particles called atoms. In middle school science you learned about the atom. 1. Draw a picture of a typical atom. Identify and label the location of the nucleus, electron cloud, electrons, neutrons, protons.

Objective  I can summarize Dalton's theories.  I can describe properties of protons, neutrons and electrons, who discovered them and where they are found in an atom.

Section 5.1 What is an atom? Describe it’s size  Definition: The atom is the smallest particle of an element that retains the characteristics of that element.  A typical atom is meters across -- that's one billionth of a meter Video: Just how small is an atom 5:20

What is an atom? A. An atom is the smallest particle that can exist B. An atom is the smallest particle that still has characteristics of an element C. An atom is a solid sphere, like a marble only really really small

Section 5.1 Dalton’s 4 Postulates (1808) 1. All elements are composed of tiny indivisible particles called atoms 2. Atoms of the same element are identical. Atoms of different elements are different from another element. 3. Atoms of different elements can chemically combine to form compounds 4. Any chemical reaction is simply a re-arrangement of atoms, but the atoms are not changed into different elements. John Dalton “Father of Modern Chemistry”

Section 5.2 Structure of the atom  Electrons  Discovered by J.J. Thomson in Sir Joseph John Thomson ( ) Nobel Prize for Physics (1906) Discovered the electron First to discover evidence for isotopes of stable elements.

Section 5.2 Structure of the atom  Thomson created and used a Cathode Ray Tube to study the existence of electrons.  He passed an electric current through a sealed vacuum. Negatively charged electrons are repelled by the negatively charge cathode, and are attracted to the positively charged anode causing a glowing cathode ray to appear Thomson - determined the presence of these electrons in all gases and concluded that electrons were part of all atoms

Section 5.2 The Atomic Nucleus  In Thomson’s “Plum Pudding Model”: Scientists assumed the negative electrons were evenly distributed throughout a positive atom.

Which of Dalton’s four theories did J.J. Thomson disprove in his famous CRT experiment? A. All elements are composed of tiny indivisible particles called atoms B. Atoms of the same element are identical. Atoms of different elements are different from another element. C. Atoms of different elements can chemically combine to form compounds D. Any chemical reaction is simply a re-arrangement of atoms, but the atoms are not changed into different elements.

Section 5.2 Structure of the atom  Robert Millikan - experimented to find the relative electrical charge of an electron to be –1  Millikan discovered electron mass to be 1/1840 of the mass of a hydrogen proton  Electrons in an atom determine its chemical properties Robert Millikan Nobel Prize for Physics 1923 first measured the electron charge and mass

Section 5.2 Structure of the atom  Protons  Since atoms are electrically neutral, there must be a particle that neutralizes the negative charge of an electron  Protons were discovered by Eugen Goldstein in 1886 who observed additional rays in a cathode ray tube that traveled in the opposite direction of the cathode ray.  Protons have a electrical charge of +1 and a mass of 1 amu.  The number of protons in an atom determines the type of element Eugen Goldstein discoverer of the proton

Section 5.2 Structure of the atom  Neutrons  James Chadwick discovered the existence of neutrons in 1932 using beryllium, alpha rays and known atomic masses.  Neutrons hold the protons together and thus contribute to the stability of the atomic nucleus.  Neutrons have a mass of 1 and no electric charge. James Chadwick Nobel Prize for Physics 1935

It has a very small mass, and a negative charge. What is it and where is it located in the atom? A. A Neutron is located outside the nucleus B. A Proton located in the nucleus C. An Electron is located in the nucleus D. An Electron is located in the electron cloud outside the nucleus.

Section 5.2 Structure of the atom  In 1909 Ernest Rutherford disproves the plum pudding model by bombarding gold foil with alpha particles(+). Ernest Rutherford Nobel Prize for Chemistry 1908

Section 5.2 Structure of the atom  According to the Thomson’s popular “Plum Pudding” atomic theory of the time, the alpha particles should pass through the foil with only a slight deflection  Instead, Rutherford observed that almost all particles went straight thru, but a few of the particles were deflected at odd angles or sometimes directly backwards!!  This result was completely unexpected. Gold Atom

Section 5.2 Rutherford’s planetary model  Rutherford uses this information to propose a new atomic theory.  The atom is mostly made up of empty space.  The positive charge of the atom, and most of the mass, is concentrated in a small area  This small, dense area is known as the nucleus, and is surrounded by orbiting electrons.

Section 5.2 Rutherford’s Experiment   Animations  Cathode Ray Tube  Alpha Particle Scattering …Rutherford’s experiment  = Alpha Particle 2 protons + 2 neutrons

In his famous gold foil experiment, Rutherford shot alpha particles into gold. What result did he prove? A. Atoms have a large diffuse nucleus, like a pudding. B. Atoms are made up of mostly empty space C. Atoms have a small dense positively charged nucleus D. The nucleus is actually much smaller and more dense than anyone knew

Atom Facts ElectronProtonNeutron Charge+10 Mass 1/1840 (almost negligible) 1 amu Location in Atomelectron cloudnucleus DiscovererJ.J. ThomsonGoldstein James Chadwick Function determines chemical properties determines an elements identity holds nucleus together

Atom Facts ElectronProtonNeutron Charge+10 Mass 1/1840 (almost negligible) 1 amu Location in Atomelectron cloudnucleus DiscovererJ.J. ThomsonGoldstein James Chadwick Function determines chemical properties determines an elements identity holds nucleus together