5Daltons Atomic Theory (~1800) p. 103) All elements are composed of tiny indivisible particles called atoms.Atoms of the same element are identical. The atoms of any one element are different from those of any other element.Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds.Chemical reactions occur when atoms are separated from each other, joined, or rearranged in different combinations. Atoms of one element are never changed into atoms of another element as a result of a chemical reaction.Atoms of element AAtoms of element BCompound made by atoms of elements A and BMixture of atoms of elements A and B
6Caveats of Dalton’s Atomic Theory All elements are composed of tiny indivisible particles called atoms.Atoms are not indivisible – they are made of subatomic particlesAtoms of the same element are identical. The atoms of any one element are different from those of any other element.Every atom has at least one isotope; one atom’s isotope is NOT identical to another isotope of the same atom.Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds.This is known as the Law of Definite Proportions – very important.Chemical reactions occur when atoms are separated from each other, joined, or rearranged in different combinations. Atoms of one element are never changed into atoms of another element as a result of a chemical reaction.Atoms of one element can change into an atom of another element as a result of a nuclear reaction.
74.2 – Structure of the Nuclear Atom Subatomic ParticlesElectron – discovered in 1897 by J.J. Thomson while experimenting with cathode ray tubes
8Thomson performed experiments that involved passing electric current through gases at low pressure. The result was a glowing beam, or cathode ray, that traveled from the cathode to the anode.Thomson found that a cathode ray is deflected by electrically charged metal plates.Thompson knew that opposite charges attract and like charges repel, so he hypothesized that a cathode ray is a stream of tiny negatively charged particles moving at high speed; now called electrons.To test his hypothesis, Thompson set up an experiment to measure the ratio of an electron’s charge to its mass.Also, the charge-to-mass ratio of electrons did not depend on the kind of gas in the cathode-ray tube or the type of metal used for the electrodes.
10Properties of Subatomic Particles Relative mass (mass of proton = 1) Proton – observed in cathode ray tubes also in 1886 by Eugen Goldstein.Neutron – confirmed in 1932 by James Chadwick.Properties of Subatomic ParticlesParticleSymbolRelative chargeRelative mass (mass of proton = 1)Actual mass (g)Electrone–1–1/18409.11 10–28Protonp+1+11.67 10–24Neutronn0
11Thomson’s Plum Pudding Model The Atomic NucleusHow are atoms structured?DaltonDemocritusThomson’s Plum Pudding Model
12In 1911, Ernest Rutherford and others performed the Gold Foil Experiment to test the plum pudding modelErnest Rutherford
14The results… Expected Actual It was expected that alpha particles would pass through the plum pudding model of the gold atom undisturbed.It was observed that a small portion of the alpha particles were deflected, indicating a small, concentrated positive charge (the nucleus!)ExpectedActual
15It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you. On consideration, I realized that this scattering backward must be the result of a single collision, and when I made calculations I saw that it was impossible to get anything of that order of magnitude unless you took a system in which the greater part of the mass of the atom was concentrated in a minute nucleus. It was then that I had the idea of an atom with a minute massive center, carrying a charge.—Ernest Rutherford
16Nucleus – tiny positively charge core of an atom Rutherford’s Nuclear Model of the AtomIs this the current model of the atom?NO…If an atom were the size of a football stadium, the nucleus would be about the size of a marble
184.3 – Distinguishing Among Atoms Atomic Number (Z) -The number of protons in an atom; identifies the element.In a neutrally charged atom, the number of protons (p+) equals the number of electrons (e-)
19The number of protons (p+) and neutrons (n0) in an atom. Mass Number (A) -The number of protons (p+) and neutrons (n0) in an atom.The mass number is NOT the atomic mass.ElementAtomic Number (Z)Protons (p+)Electrons (e-)Neutrons (n0)HOCa111???888???202020???The number of n0 depends on the mass number of the isotope
20Isotopes -Atoms of an element that have a different number of neutrons.ElementAtomic Number (Z)Protons (p+)Electrons (e-)Neutrons (n0)HOCa111888202020
22Chemical Symbols for Isotopes A is the superscriptZ is the subscript
23Determining the Composition of an Atom How many protons, electrons, and neutrons are in each atom?a. Be b. Ne c. Na9420102311
24Percent Abundance in Nature Naturally Occurring Isotopes of NeonPercent Abundance in Nature90.48% % %
25The masses of atoms are rarely expressed in grams. The C-12 isotope has been given a mass of exactly 12 atomic mass units (amu)The masses of all other elements are based on the mass of the C-12 isotope.Why is the mass of a carbon atom amu?
26Atomic Mass of Carbon = 12.011 amu Weighted average of all the naturally occurring isotopes of the element.amu98.93 %amu1.07 %amu%Atomic Mass of Carbon = amu
27Atomic masses are weighted averages. amu98.93 %amu1.07 %amu%Atomic Mass of Carbon = amuNo atom of carbon actually weighs amu. But a typical carbon atom averages amu.Atomic masses are weighted averages.
28There are 2 stable isotopes of silver Silver-107; amu; 51.84%Silver-109; amu; 48.16%Calculate the atomic weight of silver.Atomic Weight of Silver = amu
29Despite differences in the number of neutrons, isotopes of an element are chemically similar. Neutrons do not determine chemical reactivity; the electrons do.