1. Structure of the Atom Democritus

2. Structure of the Atom The first person to be credited with the modern theory of the atom was an English school teacher by the name of John Dalton. Modern scholarship has identified four basic ideas in Dalton's chemical atomic theory. 1) chemical elements are made of atoms (minute, discrete, indivisible, and indestructible particles, with nothing inside) 2) the atoms of an element are identical in their masses 3) atoms of different elements have different masses 4) atoms only combine in small, whole number ratios such as 1:1, 1:2, 2:3 and so on *Dalton’s Atomic Theory was published in 1808, but not accepted by all scientists.

3. Structure of the Atom John Dalton

4. Structure of the Atom Dalton’s model of the atom held true until the end of the 1800’s. A scientist named J.J. Thomson (a British physicist and Nobel laureate) disproved the theory that nothing was inside of the atom with a device called a Crooke’s Tube. With this he discovered the existence of the electron.Britishphysicist Nobel laureateCrooke’s Tube Thomson concluded that the strength of the magnetic field exerted on the particle beam could not bend the lightest of known elements as much as the beam was being bent. He concluded that it must be a negative sub-atomic particle coming from the atoms inside of the tube.

5. But, atoms are neutral! How can there only be a negative particle inside of the atom if they are suppose to be neutral? Thomson's discovery of the negatively charged electron raised theoretical problems for physicists, because atoms as a whole are neutral. Where was the neutralizing positive charge and what held it in place? Between 1903 and 1907 Thomson tried to solve the mystery by adapting an atomic model that had been first proposed by Lord Kelvin in1902.

6. Lord Kelvin…. to the rescue (William Thomson) Kelvin, in 1903, proposed that the atom have the newly discovered electrons embedded somehow in a sphere of uniform positive charge, this sphere being the full size of the atom.

7. Structure of the Atom JJ Thomson concluded that matter is not negatively or positively charged, so a positive sub-atomic particle must also exist to supply matter with a neutral charge. (This was later proven through experimentation by other scientists.) The negative sub-atomic particle became known as the electron and the positive sub-atomic particle as the proton. JJ Thomson adopted Lord Kelvin’s model, it became known as the: Plum Pudding Model. The atom was still thought of as a miniscule particle. Inside of the atom were the protons and electrons equally distributed. The analogy of Plum Pudding was the positive protons were the pudding and distributed throughout the pudding were the negative electrons (plums).

8. Structure of the Atom JJ Thomson

9. Structure of the Atom At the turn of the century, the Plum Pudding model was the accepted model. That would change in 1911 due to a man named Ernest Rutherford.

10. Structure of the Atom Rutherford’s Gold Foil Experiment

11. Structure of the Atom Rutherford’s Gold Foil Experiment

12. Structure of the Atom Rutherford’s Gold Foil Experiment (Summary) The positive alpha particles (  ) were expected to pass through the thin piece of gold foil. The great majority of the alpha particles passed through, but an estimated 4 of every 10,000 alpha particles had its path deflected. Rutherford likened this to a bullet being stopped by tissue paper. Rutherford then proposed a model of the atom that had protons and electrons with the charges separated. The protons were put in the center of the atom and only occupied an area of 10 -13 cm. This center became known as the nucleus. The electrons were placed outside of the nucleus and occupied an area of 10 -8 cm. Thus the nucleus of an atom occupies 1/100,000 of the area of the atom. The idea of the nucleus being part of the atomic model was accepted in 1919.

13. Structure of the Atom The modern model of the atom was set in 1932 when E. Rutherford and James Chadwick were able to show a third particle existed in the atom within the nucleus with the proton. The third particle was neutral and accounted for missing mass that was in the atom. It became known as the neutron. Relative to an electron, a proton is 1,836 times more massive then an electron. Relative to an electron, a neutron is 1,839 times more massive then an electron. The nucleus of the atom held the great majority of the mass of an atom, yet only occupies 1/100,000 the area in an atom.

14. Structure of the Atom The mass of an atom is dependent upon the number of protons and neutrons in the nucleus. The number of protons for an element must always be the same (e.g. oxygen always has 8 protons), but the number of neutrons can be different. Atoms of an element can have different numbers of neutrons in the nucleus, thus different atomic masses. The mass of a proton and a neutron are assigned an atomic mass of 1 amu (atomic mass unit). The sum of the protons and neutrons in the atom account for the mass of the atom. The mass of the electrons are discounted towards the mass of the atom because of how miniscule it is compared to the mass of the proton and neutron.

15. Structure of the Atom In fact, the mass that electrons contribute to the overall mass of an atom is like…. peeing in the ocean, it doesn’t make a difference!! Actual mass of an electron-9.11 x 10 -31 kg Actual mass of a proton -1.6726 x 10 -27 kg Actual mass of a neutron -1.6749 x 10 -27 kg

16. Structure of the Atom The symbol for an isotope is like the generic representation below.

17. Structure of the Atom Atomic Size

18. Structure of the Atom Isotopes In case you were wondering, on the left is the insignia for Albuquerque Isotopes minor league baseball team and their mascot, Orbit.

19. Structure of the Atom Finding the average mass of an element is a lot like calculating your grade for this class. Your grade is weighted, which mean different categories are worth different proportions to your overall grade. Weighting Scale Assessments - 60% Labs- 30% Homework- 10% Now, let us figure out a grade for a student… Ralph Wiggum’s Report Card Homework- 95% Labs- 78% Assessments- 62%

20. Structure of the Atom The average atomic mass of an element is found by adding together the product of the mass of the isotope an percent abundance. (XXX amu x AA.AA%) + (YYY amu x BB.BB%) = Average Atomic Mass Percent Abundance of Carbon