Atomic Structure

Atomic Structure History - Crooks to Chadwick Mr. Shields Regents Chemistry U01 L02

History of the Atom In the period between the mid 1800’s to early 1900 ‘s our understanding of Atomic Structure Increased Dramatically

William Crooks (1832 -1919) • In the 1870’s Crooks experimented with Cathode • Ray Tubes (CRT’s)

William Crooks (1832 -1919) • Crooks discovered several things about Cathode • Rays: • Cathode Rays could spin a small windmill placed in the • path of the Ray moving down the tube • - This Suggested a Cathode Ray was a stream of • particles • 2. Cathode Rays could also bent in magnetic fields • - North pole deflected it one way • - South pole deflected it the other way Thus… Cathode rays must have a charge

Magnetic field bending a cathode ray beam source source Movie of cathode ray tube

In 1896 J.J. Thomson (1856 – 1940) began a series of experiments on Cathode Rays In 1897 he discovers the electron source

J.J. Thomson (1856 – 1940) Thomson Concluded that … Cathode rays are negatively charged particles almost 2000x lighter than the hydrogen atom He called these particles ELECTRONS AND THE CONSEQUENCE? Atoms are NOT indivisible marble like particles but in fact had aSUB-STRUCTURE!

electrons Equally positive medium J.J. Thomson (1856 – 1940) In 1899 Thomson proposed a new model of the atom: Atoms contain electrons And since Atoms were known to be electrically neutral… Electrons had to be imbedded in an equally positive medium to cancel the effect of the electron This came to be Known as the “PLUM PUDDING” model of the atom

J.J. Thomson (1856 – 1940) Here is what he wrote in 1899: "I regard the atom as containing a large number of smaller bodies which I will call corpuscles, these corpuscles are equal to each other.... In the normal atom, this assemblage of corpuscles forms a system which is electrically neutral. Though the individual corpuscles behave like negative ions, yet when they are assembled in a neutral atom the negative effect is balanced by something which causes the space through which the corpuscles are spread to act as if it had a charge of positive electricity equal in amount to the sum of the negative charges of the corpuscles.... The detached corpuscles behave like negative ions, each carrying a constant negative charge which we shall call for brevity the unit charge; while the part of the atom left behind behaves like a positive ion with the unit positive charge and a mass large compared with that of the negative ion."

ERNEST RUTHERFORD (1871 – 1937) Canadian - began studies of Radioactivity in the early 1900’s 1902 – Rutherford studied effect of an magnetic & electric fields on the invisible radioactive beam released from a sample of uranium ore He discovers three particles that Emanate from the atoms in the Radioactive source

+2 0 No charge -1 Deflection of a beam from uranium ore in an electric field Evidence that atoms are more complex than either the Dalton or Thompson models

RUTHERFORD BACK SCATTERING EXPERIMENT (1909-1911) Rutherfords’s Question: If an atom was a bunch of electrons immersed in a positively charged Medium (Plum pudding model) how were the electrons arranged?? Rutherford’s devised an experiment to look at how ALPHA particles (Helium Nuclei) interacted with a thin sheet of Gold foil (i.e. only a few stacked atoms thick)

Rutherford’s Experimental Results - 1910 source

Most particles 1:20,000 particles 1:8000 particles Experimental Nos. & why the alpha particles were deflected

If Thomson’s Plum Pudding Model was correct all alpha particles should have passed Straight through the foil with no deflection. (Why?) Rutherford’s exp’t: animation

RUTHERFORD BACK SCATTERING EXPERIMENT1911 - Puzzle solved Thompson’s PLUM PUDDING MODEL had to be WRONG! Rutherford concluded in 1911 that most of the mass of the atom is concentrated in a very small core at the atom’s center Rutherford name this CORE the NUCLEUS

RUTHERFORD BACK SCATTERING EXPERIMENT Indicated electrons occupies mostly empty space around the nucleus These electrons could be anywhere in this space. - e- do not reside in specific orbits. But… The problem was that Newtonian mechanics predicted the electron should eventually fall into the nucleus ! Obviously that would be a problem!

NIELS BOHR1885-1962 1913 – Bohr, working for Thompson, suggests Rutherford’s model is probably more accurate that Thomson’s Plum pudding model Thomson suggests he go work for Rutherford – So he did… To solve the Newtonian problem of the electron falling into The Nucleus Bohr suggests a hypothesis: “There can only be discrete orbits in which the electron can reside.”

The model Bohr proposed set up electron energy levels In which the e- resided. These were designated n=1, 2, 3 etc.

NIELS BOHR Electrons can not reside between these orbits, representing Specific energy levels . To move from one orbit to the next they had to absorb or Release very specific amounts of energy called “quantum“ This model worked exceptionally well for the Hydrogen atom But … As we’ll see later there Were problems with this atomic model

HENRY MOSELEY (1887 – 1915) Up to 1913 No one knew what made atoms of one element Different from atoms of another element! Mosley, a Student of Rutherford’s, made a discovery of fundamental importance in 1913 His studies of x-rays indicated their frequency shifted Linearly from one element to the next according to the equation: (Frequency)1/2 = N And N was an integer that shifted with each element

Moseley called N the “ATOMIC NUMBER”. He said N must represent the positive charge of the nucleus. What made one Element different from another was now understood! It was the Incremental Change in postive charge

Rutherford (in 1914) described Moseley's discovery thus: "Recently Moseley has supplied very valuable evidence that this rule [atomic numbers changing by one from element to element] also holds for a number of the lighter elements. By examination of the wave-length of the characteristic X rays emitted by twelve elements varying in atomic weight between calcium (40) and zinc (65.4), he has shown that the variation of wave-length can be simply explained by supposing that the charge on the nucleus increases from element to element by exactly one unit. This holds true for cobalt and nickel, although it has long been known that they occupy an anomalous relative position in the periodic classification of the elements according to atomic weights." By the way, Moseley was part of Rutherford's research group -- having arrived in Manchester just weeks before Rutherford published his great nucleus paper -- when he started his atomic number work. Rutherford was not all that excited by Moseley wanting to study X-rays, but the energy and enthusiasm of the younger man soon wore Rutherford down.

Rutherford - protons& neutrons 1920 – Rutherford bombarded Nitrogen atoms with alpha particles (helium nuclei) Bombardment Produced a positively charged hydrogen nuclei He called it the “Proton” Suggested protons were the basic positive charges in the Nucleus Hydrogen had one proton, helium two etc. Problem: Helium was considerably more than twice the mass of Hydrogen Rutherford therefore proposed the nucleus also contained an another uncharged particle with same mass as a proton (the Neutron)

James Chadwick (1891 – 1974) 1932 – Chadwick Performed Experiments that led to the Actually Discovery of the Neutron The neutron changes atomic mass but not the element - leads to the understanding of Isotopes Imagine… It was only about 75 years ago that Chadwick found and Studied this new nuclear particle – The Neutron

1/1836 1 1 1/1836 mass of the Proton 1/2000 1 1

Haven’t talked about This one yet! So what did we know? Different concepts of the atom over time

Atomic Structure