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The History of Atomic Theory TEKS 3B, 3C and 5A Atomic Models This model of the atom is the “Bohr model”. In this model, the nucleus is orbited by electrons, which are in different energy levels. A model uses familiar ideas to explain unfamiliar facts observed in nature. A model can be changed as new information is collected. Atomic Models through Time The atomic model has changed throughout the centuries, starting in 400 BC, when it looked like a billiard ball → To the modern model theory that recognizes sub-particles of quarks, hadrons, gluons and more. Critical Understanding It is crucial to understand that atomic theory has changed over time as more has been learned. It is not stagnant and will continue to evolve as we learn! Let’s start WAY back in time with the ancient Greeks Who are these men? In this lesson, we’ll learn about the men whose quests for knowledge about the fundamental nature of the universe helped define our views. Men of Atomic Theories: 400 BC Democritus This is the Greek philosopher Democritus who began the search for a description of matter more than 2400 years ago. He asked: Could matter be divided into smaller and smaller pieces forever, or was there a limit to the number of times a piece of matter could be divided? Atomos His theory: Matter could not be divided into smaller and smaller pieces forever, eventually the smallest possible piece would be obtained. This piece would be indivisible. He named the smallest piece of matter “atomos,” meaning “not to be cut.” Atomos To Democritus, atoms were small, hard particles that were all made of the same material but were different shapes and sizes. Atoms were infinite in number, always moving and capable of joining together. This theory was ignored and forgotten for more than 2000 years! Why? The eminent philosophers of the time, Aristotle and Plato, had a more respected, (and ultimately wrong) theory. Aristotle and Plato favored the earth, fire, air and water approach to the nature of matter. Their ideas held sway because of their eminence as philosophers. The atomos idea was buried for approximately 2000 years. Dalton’s Model In the early 1800s, the English Chemist John Dalton performed a number of experiments that eventually led to the acceptance of the idea of atoms. Dalton’s Theory The main points of Dalton's atomic theory are: Everything is composed of atoms, which are the indivisible building blocks of matter and cannot be destroyed. All atoms of an element are identical. The atoms of a given element are different from those of any other element. Atoms of one element can combine with atoms of other elements to form compounds. Atoms cannot be created, divided into smaller particles, nor destroyed in the chemical process. A chemical reaction simply changes the way atoms are grouped together. Thomson’s Plum Pudding Model In 1897, the English scientist J.J. Thomson provided the first hint that an atom is made of even smaller particles. Thomson Model He proposed a model of the atom that is sometimes called the “Plum Pudding” model. Atoms were made from a positively charged substance with negatively charged electrons scattered about, like raisins in a pudding. Thomson Model Thomson studied the passage of an electric current through a gas. As the current passed through the gas, it gave off rays of negatively charged particles. Thomson Model This surprised Thomson, because the atoms of the gas were uncharged. Where had the negative charges come from? Where did they come from? Thomson concluded that the negative charges came from within the atom. A particle smaller than an atom had to exist. The atom had “parts”! Thomson called the negatively charged “corpuscles,” today known as electrons. Since the gas was known to be neutral, having no charge, he reasoned that there must be positively charged particles in the atom. But he could never find them. Ernest Rutherford - 1909 Also conducted experiments to study the atom He found that atoms have a nucleus – a small dense center That center was surrounded by electrons His work also concluded that within the nucleus were smaller particles, protons Rutherford’s Gold Foil Experiment Rutherford’s experiment involved firing a stream of tiny positively charged particles at a thin sheet of gold foil (2000 atoms thick) Most of the positively charged “bullets” passed right through the gold atoms in the sheet of gold foil without changing course at all. Some of the positively charged “bullets,” however, did bounce away from the gold sheet as if they had hit something solid. He knew that positive charges repel positive charges. This could only mean that the gold atoms in the sheet were mostly open space. Atoms were not a pudding filled with a positively charged material. Rutherford concluded that an atom had a small, dense, positively charged center that repelled his positively charged “bullets.” He called the center of the atom the “nucleus” The nucleus is tiny compared to the atom as a whole. Rutherford’s conclusion Rutherford reasoned that all of an atom’s positively charged particles were contained in the nucleus. The negatively charged particles were scattered outside the nucleus around the atom’s edge. Bohr Model In 1913, the Danish scientist Niels Bohr proposed an improvement. In his model, he placed each electron in a specific energy level. Bohr Model According to Bohr’s atomic model, electrons move in definite orbits around the nucleus, much like planets circle the sun. These orbits, or energy levels, are located at certain distances from the nucleus. Modern Theory - Wave or the Quantum Model The Wave Model Today’s atomic model is based on the principles of wave mechanics. According to the theory of wave mechanics, electrons do not move about an atom in a definite path, like the planets around the sun. The Wave Model In fact, it is impossible to determine the exact location of an electron. The probable location of an electron is based on how much energy the electron has. According to the modern atomic model, at atom has a small positively charged nucleus surrounded by a large region in which there are enough electrons to make an atom neutral. Electron Cloud: A space in which electrons are likely to be found. Electrons whirl about the nucleus billions of times in one second They are not moving around in random patterns. Location of electrons depends upon how much energy the electron has. Energy and the Electron Cloud: Depending on their energy they are locked into a certain area in the cloud. Electrons with the lowest energy are found in the energy level closest to the nucleus Electrons with the highest energy are found in the outermost energy levels, farther from the nucleus. The electron cloud model is used to accurately represent the structure of the atom Using models for understanding: The Bohr Model shows all of the particles in the atom In the center are circles. Each circle represents a single neutron or proton. Protons should have a plus or P written on them. Neutrons should be blank or have an N In a circle around the nucleus are the electrons. Electrons should have a minus sign or an e Electrons have special rules… Bohr models atoms’ electrons organized into energy levels. Each level can hold a specific number of electrons The first energy level can hold up to 2 electrons 2nd energy level – (up to) 8 electrons Electrons are not usually added to energy levels until the lower levels are filled The electrons in the outer most shell of any element are called valence electrons A few examples of the Bohr Model: Lithium has 3 electrons, 2 in the first level and 1 in the 2nd. It has 1 valence electron Fluorine has 9 electrons, 2 in the first level and 7 in the 2nd. It has 7 valence electrons Aluminum has 13 electrons, 2 in the first level and 8 in the 2nd and 3 in the 3rd It has 3 valence electrons Key points: In the Bohr model of the atom, the nucleus contains the majority of the mass of the atom in its protons and neutrons. In most cases, electrons fill the lower energy levels first, followed by the next higher energy orbital until it is full, and so on until all electrons have been placed. Atoms tend to be most stable with a full outer shell (one which, after the first, contains 8 electrons), leading to what is commonly called the “octet rule” Octet Rule The octet rule refers to the tendency of atoms to prefer to have eight electrons in the valence shell. atoms will lose, gain, or share electrons in order to have a full valence shell of 8 electrons. (Hydrogen is excluded because it can hold a maximum of 2 electrons in its valence shell. ) Atoms follow the octet rule because they always seek the most stable electron. Low atomic weight elements (the first twenty elements) are most likely to adhere to the octet rule. The Rule of 8 All atoms want to have a full valence shell - 8 electrons This makes them happy atoms that will not react with other atoms. Atoms with less than 8 electrons in their valence shell will react chemically (bond) with other atoms in one of 2 ways: Gain electrons to equal 8 Lose their valence electrons (to equal 8) In general, the fewer the electrons that need to be gained or lost, the easier and faster the atom will react More on Energy Levels Electrons are arranged in Energy Levels or Shells around the nucleus of an atom. • first shell - a maximum of 2 electrons • second shell - a maximum of 8 electrons • third shell - a maximum of 18 electrons Exception to the rule: In the Bohr Model of the atom, electrons fill different electron orbitals (also known as electron shells or energy levels) from the inside-out. The first orbital can only hold 2 electrons. The second orbital can hold 8 electrons, the third can hold up to 18, and the fourth up to 32, but the outermost orbital can never have more than 8 electrons. Only these outermost electrons, known as valence electrons, can interact with other atoms in chemical reactions. Valence Electrons The electrons farthest away from the nucleus in the “outermost energy level” are called valence electrons They affect the way an atom bonds Different atoms have a different number of valence electrons Atoms can have anywhere between 1 and 8 valence electrons Lewis (Dot) Structure This is another way to represent valence electrons (only shows these) The Lewis structure is used to represent the covalent bonding of a molecule or ion When an element is alone, the dots represent the valence electrons Crash Course video: https://www.youtube.com/watch?v=a8LF7JEb0IA or http://viewpure.com/a8LF7JEb0IA Atomic Models Limitations of the Bohr Model: Electrons do not orbit the nucleus of an atom like planets orbit the Sun Scale does not represent the actual size of an atom So, Why do we still learn the Bohr Model? Using this model makes it easier to understand the general structure of atoms. Activity: Homework: First page: Identify elements on the PT Use the Bohr model to differentiate among the three basic particles in the atom (proton, neutron, and electron) and their charges, relative masses, and locations. Hint: N = Atomic mass- # of P (remember atomic mass is P + N)