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Unformatted text preview: Chapter 4 Chapter “Atomic Structure” Pre-AP Chemistry Charles Page High School Stephen L. Cotton Stephen Section 4.1 Defining the Atom Section OBJECTIVES:
Describe Democritus’s ideas about atoms. about Section 4.1 Defining the Atom Section OBJECTIVES:
Explain Dalton’s atomic theory. theory. Section 4.1 Defining the Atom Section OBJECTIVES:
Identify what instrument is used to observe individual atoms. atoms. Section 4.1 Defining the Atom Section The Greek philosopher Democritus (460 The Democritus B.C. – 370 B.C.) was among the first to B.C. B.C.) suggest the existence of atoms (from the Greek word “atomos”) the He He believed that atoms were indivisible and indivisible indestructible indestructible His ideas did agree with later scientific His theory, but did not explain chemical behavior, and was not based on the scientific method – but just philosophy scientific Dalton’s Atomic Theory (experiment based!) Dalton’s
1) All elements are composed of tiny indivisible particles called atoms 2) Atoms of the same element are identical. Atoms of any one element are different from those of any other element. John Dalton (1766 – 1844) 1) Atoms of different elements combine in simple whole-number ratios to form chemical compounds 2) In chemical reactions, atoms are combined, separated, or rearranged – but never changed into atoms of another element. Sizing up the Atom Sizing Elements are able to be subdivided into smaller and smaller particles – these are the atoms, and they still have properties of that element If you could line up 100,000,000 copper atoms in a single file, they would be approximately 1 cm long Despite their small size, individual atoms are observable with instruments such as scanning tunneling (electron) microscopes Section 4.2 Section Structure of the Nuclear Atom OBJECTIVES:
Identify three types of subatomic particles. subatomic Section 4.2 Section Structure of the Nuclear Atom OBJECTIVES:
Describe the structure of atoms, according to the Rutherford atomic model. Rutherford Section 4.2 Section Structure of the Nuclear Atom One One change to Dalton’s atomic theory is that atoms are divisible atoms into subatomic particles: into Electrons, protons, and neutrons are examples of these fundamental particles particles There are many other types of There particles, but we will study these three particles, Discovery of the Electron Discovery
In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle: the electron Modern Cathode Ray Tubes athode ay Television Computer Monitor Cathode ray tubes pass electricity through a gas that is contained at a very low pressure. Mass of the Electron Mass
Mass of the electron is 9.11 x 10-28 g The oil drop apparatus 1916 – Robert Millikan determines the mass of the electron: 1/1840 the mass of a hydrogen atom; has one unit of negative charge Conclusions from the Study of the Electron: of
a) Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons. b) Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons c) Electrons have so little mass that atoms must contain other particles that account for most of the mass Conclusions from the Study of the Electron: of Eugen Goldstein in 1886 observed what is now called the “proton” particles with a positive charge, and a relative mass of 1 (or 1840 times that of an electron) 1932 – James Chadwick confirmed the existence of the “neutron” – a particle with no charge, but a mass nearly equal to a proton Subatomic Particles Subatomic
Particle Electron (e-) Proton (p+) Neutron (no) Charge Mass (g) 9.11 x 10-28 Location Electron cloud Nucleus Nucleus -1 +1 0 1.67 x 10-24 1.67 x 10-24 Thomson’s Atomic Model Thomson’s J. J. Thomson Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model. Ernest Rutherford’s Ernest Gold Foil Experiment - 1911 Alpha particles are helium nuclei The alpha particles were fired at a thin sheet of gold foil Particles that hit on the detecting screen (film) are recorded Rutherford’s problem: Rutherford’s
In the following pictures, there is a target hidden by a cloud. To figure out the shape of the target, we shot some beams into the cloud and recorded where the beams came out. Can you figure out the shape of the target? Target #1 Target #2 The Answers: The Target #1 Target #2 Rutherford’s Findings Rutherford’s Most of the particles passed right through A few particles were deflected VERY FEW were greatly deflected
“Like howitzer shells bouncing off of tissue paper!” Conclusions:
a) The nucleus is small b) The nucleus is dense c) The nucleus is positively charged The Rutherford Atomic Model The Based on his experimental evidence: The atom is mostly empty space All the positive charge, and almost all All the mass is concentrated in a small area in the center. He called this a “nucleus” in The nucleus is composed of protons The and neutrons (they make the nucleus!) make The electrons distributed around the The nucleus, and occupy most of the volume nucleus, His model was called a “nuclear model” Section 4.3 Section Distinguishing Among Atoms OBJECTIVES:
Explain what makes elements and isotopes elements isotopes different from each other. different Section 4.3 Section Distinguishing Among Atoms OBJECTIVES:
Calculate the number of neutrons in an atom. neutrons Section 4.3 Section Distinguishing Among Atoms OBJECTIVES:
Calculate the atomic mass of an element. an Section 4.3 Section Distinguishing Among Atoms OBJECTIVES:
Explain why chemists use the periodic table. the Atomic Number Atomic Atoms are composed of identical Atoms identical protons, neutrons, and electrons protons, How How then are atoms of one element different from another element? different Elements are different because they Elements contain different numbers of PROTONS PROTONS The “atomic number” of an element is of the number of protons in the nucleus number # protons in an atom = # electrons Atomic Number Atomic
Atomic number (Z) of an element is the number of protons in the nucleus of each atom of that element.
Element Carbon Phosphorus Gold # of protons 6 15 79 Atomic # (Z) 6 15 79 Mass Number Mass
Mass number is the number of protons and neutrons in the nucleus of an isotope: Mass # = p+ + n0 Nuclide
Oxygen 18 75 Arsenic 31 Phosphorus p+
8 33 15 n0
10 42 16 e- Mass #
15 18 75 31 Complete Symbols Complete
Contain the symbol of the element, Contain the mass number and the atomic number. number. Mass Superscript → number Subscript → Atomic number X Symbols Symbols
s Find each of these: Find a) number of protons a) number b) number of neutrons neutrons c) number of electrons electrons d) Atomic number Atomic e) Mass Number Mass 80 35 Br Symbols Symbols
s If an element has an atomic If number of 34 and a mass number of 78, what is the: a) number of protons a) number b) number of neutrons number c) number of electrons number d) complete symbol complete Symbols Symbols s If an element has 91 protons and 140 neutrons what is the a) Atomic number a) Atomic b) Mass number Mass c) number of electrons number d) complete symbol complete Symbols Symbols s If an element has 78 electrons and 117 neutrons what is the a) Atomic number a) Atomic b) Mass number Mass c) number of protons number d) complete symbol complete Isotopes Isotopes Dalton Dalton was wrong about all elements of the same type being identical being Atoms of the same element can Atoms can have different numbers of neutrons. neutrons Thus, different mass numbers. These are called isotopes. These isotopes Isotopes Isotopes Frederick Soddy (1877-1956) proposed the idea of isotopes in 1912 1912
Isotopes are atoms of the same element having different masses, due to varying numbers of neutrons. Soddy won the Nobel Prize in Soddy Chemistry in 1921 for his work with isotopes and radioactive materials. isotopes Naming Isotopes Naming We We can also put the mass number after the name of the after element: element:
carbon-12 carbon-14 uranium-235 Isotopes are atoms of the same element having different masses, due to varying numbers of neutrons.
Isotope Protons Electrons Neutrons Nucleus Hydrogen–1 (protium) Hydrogen-2 (deuterium) Hydrogen-3 (tritium) 1 1 1 1 1 1 0 1 2 Isotopes Isotopes
Elements occur in nature as mixtures of isotopes. Isotopes are atoms of the same element that differ in the number of neutrons. Atomic Mass Atomic How heavy is an atom of oxygen? It depends, because there are different It kinds of oxygen atoms. kinds We are more concerned with the average We atomic mass. atomic This is based on the abundance This (percentage) of each variety of that element in nature. element We don’t use grams for this mass because We the numbers would be too small. the Measuring Atomic Mass Measuring Instead Instead of grams, the unit we use is the Atomic Mass Unit (amu) Atomic (amu) It is defined as one-twelfth the It mass of a carbon-12 atom. mass Carbon-12 chosen because of its isotope purity. Carbon-12 isotope Each Each isotope has its own atomic mass, thus we determine the average from percent abundance. average To calculate the average: To Multiply Multiply the atomic mass of each isotope by it’s abundance (expressed as a decimal), then add the results. results.
not told otherwise, the mass of the isotope is expressed in atomic mass units (amu) units If If Atomic mass is the average of all the naturally occurring isotopes of that element. Symbol Composition of % in nature Isotope
Carbon-12 Carbon-13 Carbon-14
12 Atomic Masses Atomic C C C 13 14 the nucleus 6 protons 6 neutrons 6 protons 7 neutrons 6 protons 8 neutrons 98.89% 1.11% <0.01% Carbon = 12.011 - Page 117 Question Knowns and Unknown Solution Answer The Periodic Table: The A Preview A “periodic table” is an arrangement of elements in which the elements are separated into groups based on a set of repeating properties The periodic table allows you to easily compare the properties of one element to another The Periodic Table: The A Preview Each horizontal row (there are 7 of them) is called a period Each vertical column is called a group, or family Elements in a group have similar chemical and physical properties Identified with a number and either an “A” or “B” More presented in Chapter 6 ...
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