CHEM 139: Zumdahl Chapter 4 page 1 of 11 CHAPTER 4: CHEMICAL FOUNDATIONS: ELEMENTS, ATOMS AND IONS Active Learning Questions: 2, 4, 7, 12, 14, 17, 22-23; End-of-Chapter Problems: 1-54, 85 (d,e,g), 98 (skip c), 99 (a,b,d), 100 (a,c,e,f), 101 (a-c,e), 102 (g,h), 103 (a,c,d,g) 4.1 THE ELEMENTS atom: smallest identifiable unit of an element – All matter is made up of atoms. → The properties of specific atoms determine the properties of matter with those atoms. Greek philosophers were the first to propose explanations for what was observed in nature. – Surprisingly, some of these Greek ideas led to similar modern ideas. Democritus (462-370 B.C.): proposed that all matter was made up of tiny, indivisible particles called atomos (meaning “indivisible”) or atoms. Empedocles (490-430 B.C.): suggested all matter was composed of four basic elements: air, water, fire, and earth. Aristotle (384-321 B.C.): accepted Empedocles idea and added a fifth element, heavenly ether, which is perfect, eternal, and incorruptible. This idea of only five basic elements was accepted for over 2000 years, until Dalton’s modern theory of atoms in the 1800s. In 1661 Robert Boyle (1627-1691), an English scientist, published The Skeptical Chymist. – He criticized the four-element explanation of matter, given that extensive experiments had indicated the existence of many more elements. – He advocated for the rigorous approach to scientific experimentation, that in order for a theory to be true, it must hold up to extensive testing and experimentation. These ideas have led to modern science as we know it. 4.3 DALTON’S ATOMIC THEORY John Dalton’s Modern Atomic Theory 1. Elements consist of tiny, indivisible* particles called atoms. 2. All atoms of an element are identical* and have the same mass and properties. 3. Atoms of one element will differ in mass and properties from atoms of another element. 4. Atoms combine in small whole number ratios to form compounds. – e.g., a H2O molecule has one O atom and 2 H atoms 5. Atoms can combine to form different compounds. – e.g., carbon and oxygen combine to form CO2 or CO *Later proven wrongCHEM 139: Zumdahl Chapter 4 page 2 of 11 4.5 THE STRUCTURE OF THE ATOM Michael Faraday, William Crookes, and many other scientists carried out experiments → discovery of electrons (e–), tiny negatively charged subatomic particles J.J. Thomson was given credit for discovering electron although evidence had accumulated for 20 years before his research group’s determination of the electron’s charge and mass. Eugen Goldstein (late 1880s) – carried out experiments on canal rays and found they consisted of positively charged subatomic particles → discovery of protons (p+) PLUM-PUDDING MODEL OF THE ATOM – Another scientist, William Thomson (no relation to J.J. Thomson) who was also known as Lord Kelvin, proposed that the atom was a uniform sphere of positively charged matter in which electrons were embedded. – J.J. Thomson adopted this idea and used the analogy with plum pudding to describe it. → Electrons are like raisins in a pudding of protons. THE NUCLEAR ATOM: PROTONS AND THE NUCLEUS – Ernest Rutherford was a scientist who did many pioneering experiments in radioactivity. – He had members of his research group test Thomson’s Plum-Pudding Model using radioactive alpha (α) particles. – The α particles are positively charged helium atoms. Rutherford's Alpha-Scattering Experiment – Alpha (α) particles were shot at a thin gold foil only a few atoms thick. – A circular detector was set up around the foil to see what happens to the α particles. – If Plum-pudding Model was correct, the α particles (which are much bigger than electrons) should go through the foil like bullets through tissue paper.CHEM 139: Zumdahl Chapter 4 page 3 of 11 Experimental results: – Most of the α particles went straight through, but some were deflected, and a few even bounced back! Rutherford’s interpretation of the results: – Most alpha (α) particles pass through foil. → An atom is mostly empty space with electrons moving around that space. – A few α particles are deflected or even bounce back. → Atom must also contain a very small dense region, and particles hitting this region are deflected or bounce back towards source. → small dense region = atomic nucleus (contains atom’s protons) → Why this is called the Nuclear Model of the Atom. Rutherford also estimated the size of the atom and its nucleus: Ex. 1: An atom is 100,000 times (105 or 5 orders of magnitude) bigger than its nucleus. If a nucleus = size of a small marble (~1 cm in diameter), indicate the length in meters then identify a common object that corresponds to that size for the following: a. 10 times bigger = ______ dm =______ m = _________________________ b. 100 times bigger = ____________ m = __________________________ c. 1000 times bigger = ____________ m = __________________________ d. 10,000 times bigger = ____________ m = __________________________ e. 100,000 times bigger = _______ m = ______ km = __________________________ nucleus (d~10-15 m)atom (diameter ~10-10 m)CHEM 139: Zumdahl Chapter 4 page 4 of 11 4.6 INTRODUCTION TO THE MODERN CONCEPT OF ATOMIC STRUCTURE Decades later, James Chadwick won the Nobel Prize winner for his discovery (1935) → neutron (n) = neutral subatomic particle Atoms are made up of subatomic particles: electron (e–): negatively charged subatomic particle (charge = –1) proton (p+): positively charged subatomic particle (charge = +1) neutron (n) = neutral subatomic particle (charge=0) Particle Symbol Location ChargeRelative Mass (amu*) electron e– outside nucleus –1 1/1836 ≈ 0 proton p+ inside nucleus +1 1 neutron n inside nucleus 0 1 *amu=atomic mass units Thus, most of the mass of an atom comes from the protons and neutrons in the nucleus. What is electrical charge? There are 4 fundamental forces: gravity, electromagnetic force, strong force, weak force force field (or field): a region in space where a force is effective Let’s focus on the electromagnetic force, which consists of electricity and magnetism. electrostatic force: the force resulting from a charge on an object – Two objects with the same charge (both negative or both positive)