Unit 3: Atomic Structure - docfish.com

1 unit 3: Atomic Structure Chemistry unit 03: Structure of the Atom p. 1 OVERVIEW I. Atomic THEORY HISTORICAL DEVELOPMENT .. 2 A. THE EARLY HISTORY .. 2 B. EARLY PARTICLE RESEARCH .. 3 II. NUCLEAR CHEMISTRY COUNTING ATOMS .. 6 A. RADIOACTIVITY .. 6 B. COUNTING ATOMS .. 8 III. THE MOLE .. 10 Chemistry unit 03: Structure of the Atom p. 2 Atomic THEORY HISTORICAL DEVELOPMENT A. THE EARLY HISTORY Although science tries to understand the workings of the universe in a detached and objective fashion, it is important to keep in mind that science is a still human activity devised by humans, performed by humans, and interpreted by humans.

2 As such, what we know about the physical world comes from a history of human investigation. For much of western civilization Aristotelian ideals ( , the four-elements and that understanding the natural world involved inductive reasoning; , study guide 01: Introduction to Chemistry) held strong and alchemy predominated the study of the natural world. However, several important concepts began to emerge about nature, which, if not exactly contrary to the major paradigm, were inconsistent with it. For example, the idea of a small, indivisible piece of matter composing all substances began to emerge in the 1600 s in the writings of several prominent scientists, such as Isaac Newton and Robert Boyle, as the only way to explain certain phenomena.

3 Democritus had first proposed the idea of the indivisible atomos and described it with the analogy grains of sand making up the desert. Three important concepts lead John Dalton in 1808 to propose the Atomic theory. 1. The French chemist Joseph Louis Proust proposed the Law of constant composition ( , law of definite proportions) in 1794: the composition of a pure compound is always the same. For example, water is always composed of two parts hydrogen and one part oxygen, wherever it is. Even today, some people believe that compounds found in nature are somehow different than compounds made in the lab.

4 Another example is that vitamin C has the same composition whether it is synthesized in a chemistry lab or isolated from an orange or present in a lime. 2. Antoine Lavoisier, the French scientist and nobleman1, was the first to unambiguously state the Law of the conservation of mass: the total mass of matter stays the same in any experiment2. 3. John Dalton, an English schoolteacher, gave us the Law of multiple proportions (sometimes called Dalton s Law). Stated one way, if one element can combine with another element to form more than one compound, then the ratio of the masses of the elements is in small, whole numbers.

5 This law may be better understood by explaining with the following example (and using our current understanding of Atomic theory): a. Hydrogen can combine with oxygen to form either water (H2O) or hydrogen peroxide (H2O2). b. In H2O, g of oxygen combines with g of hydrogen. In H2O2, g of oxygen combines with g of hydrogen. c. Thus, the ratio of oxygen in H2O to that in H2O2 is / , or 2:1. 1 Lavoisier was a controversial figure ( , tax collector) and was executed by guillotine during the French revolution.

6 2 Following the introduction of the Atomic age, this has been amended to state that the total mass and energy of matter stays constant. Matter can be changed into energy according to Einstein s famous equation E = mc2. Chemistry unit 03: Structure of the Atom p. 3 In 1808, John Dalton proposed the Atomic theory that is summarized by the following statements: 1. All matter is composed of extremely small particles called atoms. 2. Atoms of a given element are identical in size, mass and other properties; atoms of different elements differ in size, mass and other properties.

7 (2) 3. Atoms cannot be subdivided, created or destroyed. (3) 4. Atoms of different elements combine in simple whole-whole number ratios to form chemical compounds, and 5. In chemical reactions, atoms are combined, separated or rearranged. Notes: (2) We still believe this if you substitute the works average mass for mass. This is because there are different isotopes for the same element. , carbon-12 and carbon-14 are the same size, have the same number of protons (which determine chemical behavior), but differ in Atomic mass: C-12 has 6 protons and 6 neutrons for an Atomic mass of 12 amu whereas C-14 has 6 protons but 8 neutrons for an Atomic mass of 14 amu.

8 (3) Atoms can be altered during nuclear reactions such as Atomic bombs and reactions in the Sun. Dalton described the atom as a solid, marble-like object. Relative Atomic sizes for the first two periods are shown. Figure 1. According to the Atomic theory, the mass of carbon dioxide equals the mass of the carbon plus the mass of the oxygen. B. EARLY PARTICLE RESEARCH If all matter is composed of very small atoms, what makes up the atoms? Are they solid, indivisible structures as was initially believed, or are they themselves, composed of even smaller parts. Because atoms are so small and we cannot look directly inside to see how they work, indirect methods were used.

9 1. CATHODE RAY TUBES AND THE ELECTRON Cathode ray tubes (CRTs) are glass tubes containing a gas at a very low pressure (Figure 2A). Metal electrodes are embedded in the glass and when a high voltage ( , 10,000 volts) of electricity are passed to the electrodes the gas glows. This glowing is called fluorescence. The picture in the tube-type of TV is a cathode ray tube. The tube filled with a gas in a partial vacuum was invented by the German glassblower Heinrich Geissler in 1857. In the 1880 s, the English scientist William Crookes noted that the glowing beam originzated from the negatively-charged electrode called the cathode.

10 Thus, the beam emitted from the cathode became known as the cathode ray and the tube, the cathode ray tube. Outside of popular entertainment, CRTs were used extensively for scientific investigation. In 1895, the Greman physicist Wilhelm Conrad Roentgen discovered that invisible, highly penetrating x-rays were also produced by the CRT. He made the first x-rays of his wife s hand. In 1897, Joseph John ( ) Thomson devised an experiment to test the nature of these cathode rays. He noted several important characteristics of the beam: (a) when the beam hit an internal paddle wheel (Figure 2B), it caused the Chemistry unit 03: Structure of the Atom p.