Atomic Theory: The Nuclear Model of the Atom

Transcription

1 Chapter 5 Atomic Theory: The Nuclear Model of the Atom Section 5.1 Dalton s Atomic Theory Goal 1 Precursors to John Dalton s atomic theory Law of Definite Composition The percentage by mass of the elements in a compound is always the same Identify the main features of Dalton s atomic theory. Law of Conservation of Mass In a chemical change, mass is converved Dalton s Atomic Theory (1808) Each element is made up of tiny, individual particles called atoms. Atoms are indivisible; they cannot be created or destroyed All atoms of each element are identical in every respect. Atoms of one element are different from atoms of any other element. Atoms of one element may combine with atoms of other elements, usually in the ratio of small, whole numbers, to form chemical compounds 1

2 Law of Multiple Proportions When two elements combine to form more than one compound, the different weights of one element that combine with the same weight of the other element are in a simple ratio of whole numbers. Section 5.2 Subatomic Particles Goal 2 An atom contains smaller parts known as subatomic particles. Electron 1 charge, amu Identify the three major subatomic particles by charge and approximate atomic mass, expressed in atomic mass units. Proton 1+ charge, amu Neutron no charge, amu Section 5.3 The Nuclear Atom 2

3 Goal 3 Describe and/or interpret the Rutherford scattering experiment and the nuclear model of the atom. The Nuclear Model of the Atom Every atom contains an extremely small, extremely dense nucleus. All of the positive charge and nearly all of the mass of an atom are concentrated in the nucleus. The nucleus is surrounded by a much larger volume of nearly empty space that makes up the rest of the atom. The space outside the nucleus is very thinly populated by electrons, the total charge of which exactly balances the positive charge of the nucleus. The diameter of an atom is approximately 100,000 times the diameter of the nucleus. If the nucleus was the size of a pea, there would be a 1 km distance from it to its nearest neighbor 1 km 3

4 Goal 4 Section 5.4 Isotopes Explain what isotopes of an element are and how they differ from each other. Goal 5 Goal 6 For an isotope of any element whose chemical symbol is known, given one of the following, state the other two: (a) nuclear symbol, (b) number of protons and neutrons, (c) atomic number and mass number. Identify the features of Dalton s atomic theory that are no longer considered valid, and explain why. Every atom of a particular element has the same number of protons. The number of protons in an atom of an element is the atomic number, Z Atoms are electrically neutral Protons have a 1+ charge Electrons have a 1 charge The number of electrons in an atom therefore must be equal to the number of protons All atoms of an element are not identical Some have more mass than others Since the number of protons and electrons in an atom of an element are fixed, there must be variation in the number of neutrons that causes this difference in mass Atoms of the same element that have different masses different numbers of neutrons are called isotopes 4

5 An isotope is identified by its mass number, A Mass number = A = Total number of protons + neutrons Mass number = # of protons + # of neutrons A = Z + # of neutrons Name of an Isotope Elemental name Mass number Example: A carbon atom with 6 protons and 6 neutrons Mass number: = 12 Name: carbon-12 What is the name of the carbon isotope with 7 neutrons? carbon-13 Symbol of an Isotope Known as a nuclear symbol # of protons + # of neutrons # of protons Sy mass number atomic number Sy A Z Sy Example: What are the nuclear symbols for carbon-12 and carbon-13? 12 6 C 13 6 C Section 5.5 Atomic Mass Goal 7 Goal 8 Define and use the atomic mass unit (amu). Given the relative abundances of the natural isotopes of an element and the atomic mass of each isotope, calculate the atomic mass of the element. 5

6 Masses of atoms are expressed in atomic mass units, amu 1 1 amu the mass of one carbon atom amu the mass of one carbon atom C atom: 6 p +, 6 n, 6 e Since the mass of a p + and a n are about the same, and since the mass of an e is about 0, 12 amu = mass of (6 + 6) (p + + n) 12 amu = mass of 12 nuclear subatomic particles 1 amu = mass of 1 nuclear subatomic particle 1 amu = 1.66 x g Mass Spectrum of Neon Atomic Mass (of an element): The average mass of all atoms of an element as they occur in nature. Chlorine has two natural isotopes: 75.78% is chlorine-35 at amu 24.22% is chlorine-37 at amu What is the atomic mass of chlorine? x amu = x amu = amu amu amu Goal 9 Section 5.6 The Periodic Table Distinguish between groups and periods in the periodic table and identify them by number. 6

7 Goal 10 When elements are arranged according to their atomic masses, certain properties repeat at periodic intervals (Mendeleev and Meyer, 1869) Given the atomic number of an element, use a periodic table to find the symbol and atomic mass of that element, and identify the period and group in which it is found. Periodic Table of the Elements A table that arranges the elements in order of atomic number with elements with similar properties in the same column Mendeleev had blank spaces in his periodic table. He averaged the properties of the surrounding elements to predict properties of as-yet undiscovered elements. Periods Horizontal rows in the periodic table Numbered 1 7, top to bottom (numbers usually not printed) Groups (or Chemical Families) Vertical columns in the periodic table U.S. numbering: A groups and B groups IUPAC numbering: 1-18, left to right 7

8 Main group elements Elements in the U.S. A-groups Transition elements Elements in the U.S. B-groups Metals Elements on the left of the stair-step line Nonmetals Elements on the right of the stair-step line Section 5.7 Elemental Symbols and the Periodic Table Goal 11 Given the name or the symbol of an element in Figure 5.9, write the other. aluminum argon barium beryllium boron Al Ar Ba Be B cobalt copper fluorine helium hydrogen Co Cu F He H Br Ca C Cl Cr bromine calcium carbon chlorine chromium I Fe Kr Pb Li iodine iron krypton lead lithium 8

9 magnesium manganese mercury neon nickel Mg Mn Hg Ne Ni silver sodium sulfur tin zinc Ag Na S Sn Zn N O P K Si nitrogen oxygen phosphorus potassium silicon 9