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Chapter 4 - The Structure of the Atom Atomic Models PIONEERS OF THE ATOM Democritus Lavoisier Dalton Thompson Milliken Rutherford DEMOCRITUS - Was a Greek philosopher that began wondering what the universe was made of. - He proposed that all the matter in the universe is composed of tiny particles. - These particles are indivisible. - He called these particles atomos (means indivisible). ARISTOTLE - Aristotle was much more popular and he opposed the idea of atoms. - He believed that matter was continuous. So, chemistry died for about 1400 years, until the 1700’s. FOUNDATIONS OF ATOMIC THEORY - The predominate belief by the mid 1700’s was the modern definition of an element as a substance that cannot be broken down by ordinary chemical means. - It was also believed that elements combined to form compounds that have different physical and chemical properties than those of the elements that formed them. - However, there was controversy as to whether elements always combined in the same ratio when forming a particular compound. - In the 1790’s, the study of matter was revolutionized by a new emphasis on the quantitative analysis. - Scientists’ investigations were made more accurate by new improved balances. Antoinne Lavoisier took advantage… - He began to gather evidence about how matter behaved before and after a reaction. - He discovered that the total mass of the ingredients of a chemical reaction was the same as the total mass of the results. Two other fundamental ideas were investigated… Law of Definite Proportions - States that a chemical compound is always composed of the same elements in the same proportion by mass. 1 Law of Multiple Proportions - States if two different compounds are composed of the same two elements, then the ratio of the elements is always a ratio of small whole numbers. John Dalton’s thoughts… - In 1808, John Dalton proposed an explanation for the Law of Conservation of Mass, the Law of Definite Proportions, and formulated the Law of Multiple Proportions. - He reasoned that elements were composed of atoms and that only whole numbers of atoms can combine to form compounds. - His theory can be summarized in five statements: o All matter is composed of extremely small particles called atoms. o Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties. o Atoms cannot be subdivided, created, or destroyed. o Atoms of different elements combine in simple whole number ratios to form chemical compounds. o In chemical reactions, atoms are combined, separated, or rearranged. Dalton and his contemporaries - The earliest theorists thought the atom was hard and round, much like tiny marbles or ball bearings. Structure of the Atom - Although Dalton thought atoms were indivisible, investigators in the late 1800’s proved otherwise. - It soon became clear that atoms are actually composed of several basic types of smaller particles. - And it’s the numbers and arrangements of these subatomic particles that determines the identity of the atom. - The first discovery of a subatomic particle resulted from the investigations into the relationship between electricity and matters. - In the late 1800’s, many experiments were performed in which electric current was passed through various gases at low pressure. These were carried out in tubes called cathode-ray tubes. - Investigators noticed that when current was passed through a cathode ray tube, the surface of the tube directly opposite the cathode would glow. - The cathode ray traveled from the cathode to the anode when current was passed through the tube. - The cathode ray tube opened the door for another scientist, J.J. Thompson. J.J. Thompson - Thompson’s investigations supplied evidence that the cathode ray could be deflected by applying a positively charged electric field. - This helped Thompson discover that the cathode ray was negatively charged. 2 - Thompson was able to measure the ratio of the charge of cathode ray particles to their mass. He found that the ratio was the same regardless of the metal used as the cathode. Thompson concluded that all cathode rays are composed of identical negatively charged particles. Which were later called electrons. Thompson’s experiments revealed that the electron has a very large charge in relation to it’s mass. Robert Millikan - In 1909, Robert Millikan, performend and ingenious experiment to calculate the mass of an electron. - He discovered that the mass of the electron is about 1/2000th the mass of the simplest atom (hydrogen). - Hil oil drop experiment showed that event hough the atoms of different elements are very different, their electrons are identical. - An electron is an electron - The difference between two different atoms is their number of electrons not their type of electrons. - He also calculated that the electron’s mass is 9.109x10-31 kg. What we know so far… - Thompson’s and Millikan’s ideas: o Atoms are in face divisible o Electrons are present in atoms of all elements. o One of the atom’s fundamental particles is negatively charged. o Atoms are electrically neutral, so there must be a (+) charge to balance out the (-). o Because electrons are basically massless there must be something else that accounts for the atoms mass. 3 Other significant discoveries… - In 1886, when E. Goldstein observed a cathode ray tube and found rays travelling in the opposite direction of that of the cathode rays. o He called the rays canal rays and concluded that they were composed of positive particles. - It was soon discovered that the positive particles mass is about 2000 times that of the electron - In 1932, the English physicist James Chadwick confirmed the existence of yet another subatomic particle. o The neutron, which is a particle with no charge o Has a mass nearly equal to that of the proton - Therefore the subatomic particles are the electron, proton, and neutron. The atomic nucleus - When subatomic particles were discovered, scientists wondered how these particles were put together in an atom. - This was a difficult question to answer, given how tiny atoms are. - Most scientists thought it likely that the electrons were evenly distributed throughout an atom filled uniformly with positively charged material. Ernest Rutherford - In 1911, Rutherford performed on of the most brilliant investigations ever conceived. o Rutherford set out to test the Thompson model of the atom. o The test used relatively massive radioactive alpha particles. o Alpha particles (α) are helium atoms that have lost their two (2) electrons and have a double positive charge because of the two (2) remaining protons. - In the experiment, Rutherford directed a narrow beam of alpha particles at a very thin sheet of gold foil. o According to the prevailing theory, the alpha particles should have passed easily through the gold, with only a slight deflection due to the positive charge thought to be spread out in the gold atoms. 4 What happened… - The majority of the α particles passed straight through the gold atoms, without any deflection. - Even more surprisingly, a small fraction of the α particles bounced off the gold foil atoms at very large angles. - Some even bounced back to the source. - Based on the results, Rutherford suggested a new theory of the atom. - He proposed that the atom is mostly empty space. o This explained the lack of deflection of some of the particles. - He concluded that all the positive charge and almost all the mass is concentrated in a small core. o He called this region the nucleus. What do we know? - There are three fundamental particles that together make up the atom. 5 - - - - - o Composed of electrons, protons, and neutrons The protons and the neutrons make up the center of the atom. o Center of the atom is called the nucleus. The protons and neutrons occupy the mass of the atom. The electrons surround the nucleus and occupy most of the atom’s volume How, then, are atoms of hydrogen different from those of oxygen? o A subatomic particle from one type of atom looks like particles from another atom. It’s the numbers of protons in an atom that makes atoms different. o An oxygen atom has 8 protons in its nucleus. o A hydrogen atom has 1 proton in its nucleus The atomic number of an element is the number of protons in the nucleus of an atom of that element. o Since oxygen’s nucleus has 8 protons its atomic number is 8. The protons identify the type of atom, the only atom with 8 protons is oxygen. For each element listed, the number of protons equals the number of electrons. o Remember atoms are electrically neutral. In at atom, the number of electrons must equal the number of protons. o A hydrogen atom has 1 electron o An oxygen atom has 8 electrons The mass of an atom is concentrated in its nucleus and depends on the number of protons and neutrons. The total number of protons and neutrons in an atom is called the mass number. o A carbon atom, which has 6 protons and 6 neutrons, has a mass number of 12. If you know the atomic number and mass number of an atom of any element, you can determine the atom’s composition. The composition of any atom can be represented in shorthand notation. Isotopes - Every chlorine atom has 17 protons, without exception. o However, not every chlorine atom has 18 neutrons. o Atoms with the same number of protons but contain different numbers of neutrons are called isotopes. - Because isotopes of an element have different numbers of neutrons they have different mass numbers. - Isotopes are chemically alike because they have identical number of protons and eelectrons. o It’s the electrons and protons that are responsible for chemical behavior. 6 Ions - An element’s atoms are not always netural in charge. o When an atom loses or gains one or more of its electrons it becomes an ion. An ion that has more electrons than protons has a negative electrical charge. An ion that has fewer electrons than protons has a positive electrical charge. Atomic mass - The actual mass of a proton or a neutron is very small – 1.67 x 10-24 g - The protons + neutrons is the mass of the atom. - Therefore the mass of even the largest atom is incredibly small. - Scientists have determined the tiny masses of atoms by using an instrument called a mass spectrometer. - Finding the mass of an individual atom can provide useful information, but in general, these values are too small and impractical to work with. - Instead scientists compare the relative masses of atoms to a reference atom. o A standard has been set that defines the mass of all the other atoms. - The isotope chosen was the carbon-12 atom. - This isotope of carbon was assigned a mass of exactly 12 atomic mass units (amu). - 1 atomic mass unit (amu) is defined as 1/12th the mass of a carbon-12 atom. o Using that information we can say that a helium-4 atom has a mass 1/3 the mass of C-12. - A carbon-12 atom has 6 protons and 6 neutrons in its nucleus, and its mass is set as 12 amu. - The 12 protons and neutrons account for nearly all of the carbon’s mass. o Therefore, the mass of a single proton or a single neutron has a mass of 1 amu. - Since the mass of an atom is based on individual particles of matter, the atomic mass of an atom should be a whole number. o Yet on the periodic table the masses aren’t whole numbers. - The masses on the periodic table are weighted averages. - In nature, most elements exist as a mixture of isotopes. - Each isotope of an element has a fixed mass and a natural abundance. o Natural abundance is an indication of how often we would encounter a particular isotope. 7 - o One of the isotopes is almost always encountered more often so it gets more weight. Recall that the element Chlorine (Cl) has two (2) isotopes (Cl-35 and Cl-37). 75% of the chlorine atoms you’ll find in nature are Cl-35. o Which has 17 protons and 18 neutrons in its nucleus. 25% of the chlorine atoms you’ll find in nature are Cl-37. o Which has 17 protons and 20 neutrons in its nucleus. Since we are more likely to find a Cl-35 atom the average mass of chlorine is closer to 35 than to the true average of 36. Thereofore, their atomic mass of an atom is the weighted average mass of the atoms in a naturally occurring sample of the element. The mass number of an atom is the protons + neutrons of a particular atom. o If we need the mass number from the periodic table we take the closest whole number to the atomic mass found on the periodic table. 8