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Chapter 2 Atoms and Elements 2.4 The Atom Dalton’s Atomic Theory Atoms ♣ Are tiny particles of matter. ♣ Of an element are similar and different from other elements. ♣ Of two or more different elements combine to form compounds. ♣ Are rearranged to form new combinations in a chemical reaction. Subatomic Particles Atoms contains subatomic particles. ♣ Protons have a positive (+) charge. ♣ Electrons have a negative (-) charge. ♣ Like charges repel and unlike charges attract. ♣ Neutrons are neutral. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings Rutherford’s Gold-Foil Experiment Structure of the Atom An atom consists ♣ Of a nucleus that contains protons and neutrons. ♣ Of electrons in a large, empty space around the nucleus. Atomic Mass Scale On the atomic mass scale for subatomic particles ♣ 1 atomic mass unit (amu) is defined as 1/12 of the mass of the carbon-12 atom. ♣ A proton has a mass of about 1 (1.007) amu. ♣ A neutron has a mass of about 1 (1.008) amu. ♣ An electron has a very small mass, 0.000549 amu. Particles in the Atom TABLE 2.6 Learning Check Identify each statement as describing a 1) proton 2) neutron 3) electron A. Found outside the nucleus. B. Has a positive charge. C. Is neutral. D. Found in the nucleus. Atomic Number The atomic number ♣ Is specific for each element. ♣ Is the same for all atoms of an element. ♣ Is equal to the number of protons in an atom. ♣ Appears above the symbol of an element. Atomic Number Symbol 11 Na Atomic Number and Protons Examples of atomic number and number of protons: ♣ Hydrogen has atomic number 1, every H atom has one proton. ♣ Carbon has atomic number 6, every C atom has six protons. ♣ Copper has atomic number 29, every Cu atom has 29 protons. ♣ Gold has atomic number 79, every Au atom has 79 protons. Learning Check State the number of protons in each. A. A nitrogen atom 1) 5 protons 2) 7 protons 3) 14 protons B. A sulfur atom 1) 32 protons C. A barium atom 1) 137 protons protons 2) 16 protons 3) 6 protons 2) 81 protons 3) 56 Number of Electrons in An Atom An atom ♣ Of an element is electrically neutral; the net charge of an atom is zero. ♣ Has an equal number of protons and electrons. number of protons = number of electrons ♣ Of aluminum has 13 protons and 13 electrons. The net charge is zero. 13 protons (13+) + 13 electrons (13 -) = 0 Mass Number The mass number ♣ Represents the number of particles in the nucleus. ♣TABLE Is equal to the 2.7 Number of protons + Number of neutrons Atomic Models Learning Check An atom of zinc has a mass number of 65. A. How many protons are in this zinc atom? 1) 30 2) 35 3) 65 B. How many neutrons are in the zinc atom? 1) 30 2) 35 3) 65 C. What is the mass number of a zinc atom that has 37 neutrons? 1) 37 2) 65 3) 67 Learning Check An atom has 14 protons and 20 neutrons. A. Its atomic number is 1) 14 2) 16 3) 34 B. Its mass number is 1) 14 2) 16 3) 34 2) Ca 3) Se C. The element is 1) Si Isotopes Isotopes ♣ Are atoms of the same element that have different mass numbers. ♣ Have the same number of protons, but different numbers of neutrons. Nuclear Symbols For an atom, the nuclear symbol gives the number of ♣ Protons (p+) ♣ Neutrons (n) ♣ Electrons (e-) 16 31 8 15 8 p+ 8n 8 e- 15 p+ 16 n 15 e- O P 65 30 Zn 30 p+ 35 n 30 e- Learning Check Naturally occurring carbon consists of three isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of the following. 12C 6 protons 13C 6 14C 6 ______ ______ ______ neutrons ______ ______ ______ electrons ______ ______ ______ Learning Check Write the nuclear symbols for atoms with the following subatomic particles: A. 8 p+, 8n, 8e- ___________ B. 17p+, 20n, 17e- ___________ C. 47p+, 60 n, 47 e- ___________ Solution 16O A. 8p+, 8n, 8eB. 17p+, 20 n, 17e- 8 37Cl 17 C. 47p+, 60n, 47e- 107Ag 47 Learning Check 1. Which of the following pairs are isotopes of the same element? 2. In which of the following pairs do both atoms have 8 neutrons? 15X A. 15X 8 7 B. 12X 6 C. 15X 7 16X 8 14X 6 Solution B. 12X 6 14X 6 Both nuclear symbols represent isotopes of carbon with six protons each, but one has 6 neutrons and the other has 8. C. 15X 7 16X 8 An atom of nitrogen (7) and an atom of oxygen (8) each have 8 neutrons. Isotopes of Magnesium In naturally occurring magnesium, Isotopes of Mg . there are three isotopes Atomic Mass 23.99 24.98 25.98 % abundance 78.70 10.13 11.17 24Mg 12 25Mg 12 26Mg 12 Isotopes of Sulfur A sample of naturally occurring sulfur contains several isotopes with the following abundances Isotope % abundance 32S 95.02 33S 0.75 34S 4.21 36S 0.02 32S, 33S, 34S, 36S 16 16 16 16 Atomic Mass The atomic mass of an element ♣ Is listed below the symbol of each element on the periodic table. ♣ Gives the mass of an “average” atom of each element compared to 12C. ♣ Is not the same as the mass number. Na 22.99 Some Elements and Their Atomic Mass Most elements have two or more isotopes that contribute to the atomic mass of that element. TABLE 2.9 32.07 amu Atomic Mass for Cl The atomic mass of chlorine is ♣ Based on all naturally occurring Cl isotopes. ♣ Not a whole number. ♣ The weighted average of 35Cl and 37Cl. Learning Check Using the periodic table, give the atomic mass of each element: A. calcium B. aluminum __________ __________ C. lead __________ D. barium __________ E. iron __________ Solution Using the periodic table, give the atomic mass of each element: A. calcium B. aluminum 40.08 amu 26.98 amu C. lead 207.2 amu D. barium 137.3 amu E. iron 55.85 amu Chapter 2 Atoms and Elements 2.7 Electron Energy Levels Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings Electromagnetic Radiation Electromagnetic radiation ♣ Is energy that travels as waves through space. ♣ Is described in terms of wavelength and frequency. ♣ Moves at the speed of light in a vacuum. speed of light = 3.0 x 108 m/s Electromagnetic Spectrum The electromagnetic spectrum ♣ Arranges forms of energy from lower to higher. ♣ Arranges energy from longer to shorter wavelengths. ♣ Shows visible light with wavelengths from 700400 nm. Electron Energy Levels Electrons are arranged in specific energy levels that ♣ Are labeled n = 1, n = 2, n = 3, and so on. ♣ Increase in energy as n increases. ♣ Have the electrons with the lowest energy in the first energy level (n=1)closest to the nucleus. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings Energy Level Changes ♣ An electron absorbs energy to “jump” to a higher energy level. ♣ When an electron falls to a lower energy level, energy is emitted. ♣ In the visible range, the emitted energy appears as a color. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings Energy Emitted Learning Check In each of the following energy level changes, indicate if energy is 1) absorbed 2) emitted 3) not changed A. An electron moves from the first energy level (n =1 ) to the third energy level (n = 3). B. An electron falls from the third energy level to the second energy level. C. An electron moves within the third energy level. Solution A. An electron in an atom moves from the first energy level (n =1 ) to the third energy level (n = 3). 1) absorbed B. An electron falls from the third energy level to the second energy level. 2) emitted C. An electron moves within the third energy level. 3) not changed Energy Diagram for Sublevels