Updating the Atomic Theory

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Updating the Atomic Theory

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Transcription:

Updating the Atomic Theory

On the basis of Dalton s Atomic Theory: An atom is the basic unit of an element that can enter into chemical combination.

Three major differences between modern atomic theory and Dalton s atomic theory 1. Atoms are NOT indivisible. They are made up of smaller particles: electrons, protons and neutrons. 2. Atoms CAN be changed from one element to another, but NOT by chemical reactions. 3. Atoms of the same element are NOT exactly alike. They can have different masses.

Interactions Between Electric Charges

+ positive and negative charges - objects with an equal amount of positive and negative charge are said to be electrically neutral

Forces between charges Electrostatic Force - objects with like charge repel + +

Forces between charges Electrostatic Force - objects with like charge repel + + - objects with opposite charge attract +

Forces between charges Electrostatic Force - objects with like charge repel + + - objects with opposite charge attract +

Forces between charges (cont ) - electrostatic force becomes greater the more excess charge - electrostatic force becomes smaller the greater the distance separating the charges

Searching for Atomic Structure

The Electron The charge-to-mass ratio of an electron was determined by physicist J.J. Thomson in a series of experiments done between 1908 and 1917. charge-to-mass ratio = 1.76 x 10 8 coulomb/g

The Electron Streams of negatively charged particles were found to emanate from cathode tubes, causing fluorescence. J. J. Thomson is credited with their discovery (1897). 2012 Pearson Education, Inc. Atoms, Molecules, and Ions

The Electron Thomson measured the charge/mass ratio of the electron to be 1.76 10 8 coulombs/gram (C/g). 2012 Pearson Education, Inc. Atoms, Molecules, and Ions

Cathode Ray Tube - cathode + anode High voltage When high voltage is applied to the electrodes a glow was noticed between them.

Cathode Ray Tube - cathode + anode High voltage When an object was placed in the path of the glow, it blocked part of the beam showing that the beam originated at the negative electrode.

Cathode Ray Tube - cathode Negative Electric Field + anode High voltage The fact that the rays were repelled by the negative electric field indicated that they had a negative charge with an e/m ratio of -1.76 x 10 8

J.J. Thomson s plum pudding model + - - + - + + - - +

The Atom, circa 1900 The prevailing theory was that of the plum pudding model, put forward by Thomson. It featured a positive sphere of matter with negative electrons imbedded in it. 2012 Pearson Education, Inc. Atoms, Molecules, and Ions

The Electron R.A. Millikan measured the charge of an electron to be 1.60 x 10 19 coulomb Since the charge-to-mass ratio is 1.76 x 10 8 coulomb/g, the mass of an electron must be 9.11 x 10 31 kg.

telescope X-ray source Oil droplets settle into a beam of X rays causing them to become charged with electrons.

Positively charged plate + telescope X-ray source Negatively charged plate - A positive plate attracts the negative oil droplets the negative plate repels them making the droplets stand still

Positively charged plate + telescope X-ray source Negatively charged plate Knowing the density of the oil, Millikan could calculate the mass, m, of each oil droplet (volume x density = mass ) the force of gravitational attraction was Force = m G the force of electrical force was Force = E e E is the applied voltage, e is the charge of the electron - m droplet G = E e e = -1.6 x 10-19 coulomb

Millikan Oil-Drop Experiment Once the charge/mass ratio of the electron was known, determination of either the charge or the mass of an electron would yield the other. 2012 Pearson Education, Inc. Atoms, Molecules, and Ions

Millikan Oil-Drop Experiment Robert Millikan (University of Chicago) determined the charge on the electron in 1909. 2012 Pearson Education, Inc. Atoms, Molecules, and Ions

Radioactivity Radioactivity is the spontaneous emission of radiation by an atom. It was first observed by Henri Becquerel. Marie and Pierre Curie also studied it. 2012 Pearson Education, Inc. Atoms, Molecules, and Ions

Radioactivity Three types of radiation were discovered by Ernest Rutherford: α particles β particles γ rays 2012 Pearson Education, Inc. Atoms, Molecules, and Ions

The Nucleus Ernest Rutherford (gold foil experiment) deduced the nuclear model of the atom, with an extremely small, dense and positively charged nucleus surrounded by empty space sparsely occupied by electrons.

Discovery of the Nucleus Ernest Rutherford shot α particles at a thin sheet of gold foil and observed the pattern of scatter of the particles. 2012 Pearson Education, Inc. Atoms, Molecules, and Ions

The Nuclear Atom Since some particles were deflected at large angles, Thomson s model could not be correct. 2012 Pearson Education, Inc. Atoms, Molecules, and Ions

The Nuclear Atom Rutherford postulated a very small, dense nucleus with the electrons around the outside of the atom. Most of the volume of the atom is empty space. 2012 Pearson Education, Inc. Atoms, Molecules, and Ions

The Proton The positively charged particles in the nucleus are called protons. They have the same charge as an electron but have almost 2,000 times more mass.

Rutherford s Atomic Model left one major problem It was known that the hydrogen atom contained one proton and that the helium atom contained two protons. So the mass ratio should have been 2:1. In reality the ratio is 4:1. There must be another particle...

The Neutron Was discovered by James Chadwick in 1932. It is neutral (uncharged) and has slightly greater mass than a proton. Mystery solved.

Shortcomings of Rutherford s Model

Classical physics states that a charged particle traveling in a curved path radiates energy. Electrons would continually give off energy, slow down and spiral in towards the nucleus.

The properties of atoms and molecules are not governed by the same physical laws as larger objects. Quantum Mechanics: the physics of the very small To be continued

The Modern View of Atomic Structure

A neutral atom (no electric charge) has the same number of protons and electrons. The area where the negatively charged electrons are found surrounds the nucleus The nucleus containing the positively charged protons and neutral neutrons is comparatively small but contains all most all of the mass in the atom

Atoms are composed of PROTONS + positively charged mass = 1.6726 x 10 27 kg NEUTRONS neutral mass = 1.6750 x 10 27 kg ELECTRONS negatively charged mass = 9.1096 x 10 31 kg

PROTONS + NEUTRONS neutral positively charged Nucleons General term for a particle found in the nucleus ELECTRONS negatively charged mass = 9.1096 x 10 31 kg

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