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4/26/2017 4.1 Studying Atoms Pearson Prentice Hall Physical Science: Concepts in Action Chapter 4 Atomic Structure Democritus (4th century B.C) said the universe was made of invisible units called atoms In 1808, an Englishman named John Dalton proposed a theory: 1. every element is made of tiny unique particles called atoms that cannot be subdivided 2. atoms of the same element have the same mass & atoms of different elements have different masses 3. compounds contain atoms of more than one element 4. in any particular compound, atoms of different elements always combine in the same way Objectives: 1. Explain Dalton’s atomic theory 2. Explain the contributions of Thomson and Rutherford Structure of the Atom Atomic Components • Atom-smallest piece of matter that still retains the property of the element. • Examples? • Hydrogen • Oxygen • Helium • Iron Click image to view movie • Gold Structure of the Atom Scientific Shorthand Thomson & Rutherford Q: What did Dalton notice that all compounds have in common? A: Compounds contain atoms of more than one element JJ Thomson (1856-1940) used a sealed tube of gas to show that particles are charged 1 4/26/2017 In Thomson’s experiment, the particles repelled the negative plate and were attracted to the positive plate Q: What was the charge on the particles in the beam of the gas? Thomson’s experiments provided the first evidence that atoms are made of even smaller particles Q: How do objects of the same charge behave when they come close to one another? Thomson developed the “plum pudding” model of the atom His model suggested that negative charges were evenly scattered throughout an atom filled with a positively charged mass of matter Ernest Rutherford (1871-1937) discovered fast moving particles in uranium that he called alpha particles Uranium is a radioactive substance He used a zinc sulfide screen, gold foil and alpha particles to produce a very famous experiment in which he showed that the positive charges are not evenly dispersed throughout the atom Rutherford concluded that atoms must have a concentrated central core Definition: a nucleus is a dense, positively charged mass located in the center of the atom According to Rutherford’s model, all of an atom’s positive charge is concentrated in its nucleus Today, much of an atom consists of empty space outside the positively charged nucleus 4.2 The Structure of an Atom Objectives: 1. Describe the three subatomic particles 2. Compare protons, electrons and neutrons in terms of physical properties 3. Explain how atoms of one element are different from atoms of other elements 4. Describe the difference between 2 isotopes of the same element Three Subatomic Particles Protons, neutrons & electrons are subatomic particles Definition: a proton is a positively charged subatomic particle that is found in the nucleus of an atom Definition: an electron is a negatively charged subatomic particle found in the space OUTSIDE THE NUCLEUS (ORBITALS) Definition: a neutron is a neutral subatomic particle found in the nucleus of an atom 2 4/26/2017 Structure of the Atom Atomic Components • Nucleus- positively charged center of the atom that contains protons and neutrons. Surrounded by electron cloud. Ernest Rutherford provided evidence for this. • Protonsparticles with an electrical charge of 1+. Structure of the Atom Quarks—Even Smaller Particles • Protons and neutrons are made up of smaller particles called quarks. • So far, scientists have confirmed the existence of six uniquely different quarks. Comparing Protons, Neutrons & Electrons Protons, neutrons and electrons can be distinguished by mass, charge & location in an atom The mass of a neutron is almost exactly equal to the mass of a proton Electrons are nearly 2,000 times less massive than either a proton or a neutron Structure of the Atom Atomic Components • Electrons- particles with an electrical charge of 1–. J.J. Thomson provided evidence that an atom contains negatively charged particles. • Neutrons -are neutral particles that do not have an electrical charge. Structure of the Atom Quarks—Even Smaller Particles • Scientists theorize that an arrangement of three quarks held together with the strong nuclear force produces a proton. • Another arrangement of three quarks produces a neutron Isotopes Definition: isotopes are atoms of the same element that have different numbers of neutrons and different mass numbers Isotopes: element has same atomic number, same # protons, different # neutrons, different mass # Q: Why do isotopes have different mass numbers for the same element? A: Because they have different numbers of neutrons 3 4/26/2017 Masses of Atoms Isotopes • Not all the atoms of an element have the same number of neutrons. • isotopes Atoms of the same element that have different numbers of neutrons Masses of Atoms Identifying Isotopes • average atomic mass- is the weightedaverage mass of the mixture of its isotopes. • For example, four out of five atoms of boron are boron-11, and one out of five is boron-10. • To find the weighted-average or the average atomic mass of boron, you would solve the following equation: Atoms of Different Elements Atoms of any given element ALWAYS have the same number of PROTONS Definition: atomic number of an element equals the number of protons Atoms of different elements ALWAYS have different numbers of PROTONS The number of PROTONS determines which element you have Masses of Atoms Identifying Isotopes • Models of two isotopes of boron are shown. Because the numbers of neutrons in the isotopes are different, the mass numbers are also different. • You use the name of the element followed by the mass number of the isotope to identify each isotope: boron-10 and boron-11. The mass of an electron is insignificant Thomson’s experiment with the gas tube detected electrons Rutherford discovered the positive central core The mass of each proton or neutron equals one atomic mass unit (amu) Q: What is the charge of a proton? (neutron? electron?) Q: Where is the proton located in an atom? (neutron? electron?) Masses of Atoms Atomic Mass 4 4/26/2017 Definition: mass number of an atom is the sum of the protons plus the neutrons Neutrons + Protons = Mass # Q: Why aren’t electrons considered in the mass number of an atom? # neutrons = mass # - atomic # Q: What subatomic particle could be substituted for atomic number? Q: What 2 values represent mass#? Masses of Atoms Atomic Mass • The mass of each is approximately 1,836 times greater than the mass of the electron. Masses of Atoms Protons Identify the Element • The number of protons tells you what type of atom you have Masses of Atoms Atomic Mass • The mass of a proton is about the same as that of a neutron Masses of Atoms Atomic Mass • atomic mass unit (amu)- unit of measurement used for atomic particles • proton and neutron = 1 amu. Masses of Atoms Mass Number • mass number- sum of the number of protons and the number of neutrons in the atom. For example, every carbon atom has six protons. Also, all atoms with six protons are carbon atoms. • atomic number- The number of protons in an atom 5 4/26/2017 Masses of Atoms Mass Number • If you know the mass number and the atomic number of an atom, you can calculate the number of neutrons. number of neutrons = mass number – atomic number Losing and Gaining Energy Since the time of Democritus & Dalton the model of the atom has been revised In 1913, Niels Bohr theorized that atoms move in set paths around the nucleus like planets orbiting the sun The electron path is an electron’s energy level Energy level: any of the possible energies an electron may have in an atom How Electrons Behave in Atoms Definition: an electron cloud is a visual model of the most likely locations for electrons in an atom Scientists use the electron cloud model to describe the possible locations of electrons around the nucleus Instead of set paths like orbits, electrons are found in orbitals which have unique shapes 4.3 Modern Atomic Theory Objectives: 1. Explain what can happen when atoms gain or lose energy 2. Explain the model that scientists use to describe how electrons behave in atoms 3. Describe the most stable configuration of electrons in an atom Electrons in atoms must gain energy to move to a higher energy level or lose energy to move to a lower energy level The amount of energy gained (absorbed) to go higher or energy lost (released) can be measured Each element has its own unique set of energy levels The element itself determines the amount of energy gained or lost Bohr’s model does not explain all chemical observations Structure of the Atom The Electron Cloud Model • By 1926, scientists had developed the electron cloud model of the atom that is in use today. • electron cloudarea around the nucleus of an atom where electrons are found. 6 4/26/2017 Structure of the Atom The Electron Cloud Model • The electron cloud is 100,000 times larger than the diameter of the nucleus. • In contrast, each electron in the cloud is much smaller than a single proton. • Because an electron's mass is small and the electron is moving so quickly around the nucleus, it is impossible to describe its exact location in an atom. An electron cloud is a good approximation of how electrons behave in their orbitals Definition: an orbital is a region of space around the nucleus of an atom where there is a high probability of finding an electron The level with the least energy has only one orbital Higher energy levels have more than one orbital Section Check Electron Configurations Definition: an electron configuration is the arrangement of electrons in the orbitals of an atom The most stable configuration is the one in which the electrons are in orbitals with the lowest possible energies Definition: the ground state is when all electrons in an atom have the lowest possible energies Definition: excited state is when an electron is in an orbital with higher energy & less stability Question 1 Which is the smallest piece of matter that still retains the property of the element? A. atom B. quark C. neutron D. proton Section Check Section Check Answer Question 2 The answer is A. An atom is the smallest piece of matter that still retains the property of the element. What particles are found in the nucleus of an atom? A. protons and electrons B. protons and neutrons C. neutrons and electrons D. quarks and electrons 7 4/26/2017 Section Check Section Check Answer Question 3 The answer is B. Electrons are located in an electron cloud surrounding the nucleus of the atom. What is the name of the small particles that make up protons and neutrons? Answer Protons and neutrons are made of smaller particles called quarks. Section Check Section Check Question 1 Question 2 How is the atomic number of an element determined? The element helium has a mass number of 4 and atomic number of 2. How many neutrons are in the nucleus of a helium atom? Answer The atomic number of an element is equal to the number of protons in an atom of that element. Section Check Section Check Answer Question 3 Recall that the atomic number is equal to the number of protons in the nucleus. Since the mass number is 4 and the atomic number is 2, there must be 2 neutrons in the nucleus of a helium atom. How much of the mass of an atom is contained in an electron? Answer The electron’s mass is so small that it is considered negligible when finding the mass of an atom. 8