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Chapter 4 “Atomic Structure”. Section 4.1 Defining the Atom Greek philosopher Democritus Greek philosopher Democritus (460 B.C. – 370 B.C.) suggested.

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Presentation on theme: "Chapter 4 “Atomic Structure”. Section 4.1 Defining the Atom Greek philosopher Democritus Greek philosopher Democritus (460 B.C. – 370 B.C.) suggested."— Presentation transcript:

1 Chapter 4 “Atomic Structure”

2 Section 4.1 Defining the Atom Greek philosopher Democritus Greek philosopher Democritus (460 B.C. – 370 B.C.) suggested (460 B.C. – 370 B.C.) suggested existence of atoms (Greek word existence of atoms (Greek word “atomos”) “atomos”) Believed atoms were indivisible and indestructible Believed atoms were indivisible and indestructible His ideas agreed with later scientific theory, but didn’t explain chemical behavior - was not based on scientific methods – only philosophy His ideas agreed with later scientific theory, but didn’t explain chemical behavior - was not based on scientific methods – only philosophy

3 Dalton’s Atomic Theory (experiment based!) 3)Atoms of different elements combine in simple whole-number ratios to form chemical compounds 4)In chemical reactions, atoms are combined, separated, or rearranged – but never changed into atoms of another element. 1)All elements composed of tiny indivisible particles called atoms 2)Atoms of same element identical. Atoms of any one element are different from all other elements. John Dalton (1766 – 1844)

4 Sizing up the Atom  Elements subdivided into smaller particles – called atoms, and they still have properties of that element  1.0 x 10 8 copper atoms in a single file, they would be approximately 1 cm long  individual atoms are observable with instruments such as scanning tunneling (electron) microscopes

5 Section 4.2 Structure of the Nuclear Atom One change to Dalton’s atomic theory - atoms are divisible into subatomic particles: One change to Dalton’s atomic theory - atoms are divisible into subatomic particles: Electrons, protons, and neutrons Electrons, protons, and neutrons

6 Discovery of Electron electron 1897 - J.J. Thomson used cathode ray tube to deduce presence of negatively charged particle…….the electron

7 Modern Cathode Ray Tubes  CRT’s pass electricity through gas contained - very low pressure. TelevisionComputer Monitor

8 Mass of the Electron 1916 – Robert Millikan determines mass of electron: 1/1840 the mass of hydrogen atom; has one unit of negative charge The oil drop apparatus Mass of the electron is 9.11 x 10 -28 g

9 Conclusions from the Study of the Electron: a)Cathode rays have identical properties regardless of element used to produce them. All elements must contain identically charged electrons. b)Atoms are neutral, so there must be positive particles in atom to balance negative charge of electrons c)Electrons have so little mass that atoms must contain other particles that account for most of mass

10 Conclusions from the Study of the Electron:  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 “neutron” – particle with no charge, but mass nearly equal to proton

11 Subatomic Particles ParticleCharge Mass (g) Location Electron (e - ) (e - ) 9.11 x 10 -28 9.11 x 10 -28 Electron cloud Proton (p + ) +1 1.67 x 10 -24 1.67 x 10 -24Nucleus Neutron (n o ) (n o )0 1.67 x 10 -24 1.67 x 10 -24Nucleus

12 Thomson’s Atomic Model Believed electrons were like plums embedded in + charged “pudding,” called “plum pudding” model. J. J. Thomson

13 Ernest Rutherford’s Gold Foil Experiment - 1911  Alpha particles - helium nuclei w/ + charge - The alpha particles were fired at thin sheet of gold foil  Particles that hit on the detecting screen (film) were recorded

14 Rutherford’s problem: 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

15 The Answers: Target #1 Target #2

16 Rutherford’s Findings a) Small nucleus b) Dense nucleus c) + charge nucleus  Most particles passed through  Few deflected  VERY FEW greatly deflected Conclusions: The Atom Song The Atom Song Atoms song - Mark Rosengarten “Like howitzer shells bouncing off of tissue paper!”

17 The Rutherford Atomic Model His experimental evidence: His experimental evidence: atom mostly empty space atom mostly empty space All positive charge, almost all mass in small center. “Nucleus” All positive charge, almost all mass in small center. “Nucleus” protons and neutrons make nucleus! protons and neutrons make nucleus! electrons distributed around nucleus…occupy most volume electrons distributed around nucleus…occupy most volume His model called “nuclear model” His model called “nuclear model” Rutherford’s Atom 3:08

18 Section 4.3 Atomic Number All atoms composed of identical protons, neutrons, and electrons All atoms composed of identical protons, neutrons, and electrons How then are atoms of one element different from another element? How then are atoms of one element different from another element? Elements different b/c they contain different # of PROTONS Elements different b/c they contain different # of PROTONS “atomic number” of element is number of protons in nucleus “atomic number” of element is number of protons in nucleus # protons in atom = # electrons # protons in atom = # electrons

19 Atomic Number Atomic number (Z) of element is # of protons in nucleus of each atom of that element. Element # of protons Atomic # (Z) Carbon66 Phosphorus1515 Gold7979

20 Mass Number Mass number is # of protons and neutrons in nucleus of an isotope: Mass # = p + + n 0 Nuclide p+p+p+p+ n0n0n0n0 e-e-e-e- Mass # Oxygen - 10 -3342 - 31 - 3115 8 8 18 Arsenic 7533 75 Phosphorus 15 31 16

21 Complete Symbols Contain symbol of element, mass number & atomic number. Contain symbol of element, mass number & atomic number. X Mass number Atomic number Subscript → Superscript →

22 Symbols n Find each of these: a) number of protons b) number of neutrons c) number of electrons d) Atomic number e) Mass Number Br 80 35

23 Symbols n If an element has an atomic number of 34 and a mass number of 78, what is the: a) number of protons b) number of neutrons c) number of electrons d) complete symbol

24 Symbols n If an element has 91 protons and 140 neutrons what is the a) Atomic number b) Mass number c) number of electrons d) complete symbol

25 Symbols n If an element has 78 electrons and 117 neutrons what is the a) Atomic number b) Mass number c) number of protons d) complete symbol

26 Isotopes Dalton was wrong about elements of same type being identical… Dalton was wrong about elements of same type being identical… Atoms of same element can have different numbers of neutrons. Atoms of same element can have different numbers of neutrons. different mass numbers different mass numbers isotopes isotopes

27 Isotopes Frederick Soddy (1877-1956) proposed idea of isotopes in 1912 Frederick Soddy (1877-1956) proposed idea of isotopes in 1912 Isotopes - atoms of same element with different masses, b/c varying #s of neutrons Won 1921 Nobel Prize in Chemistry Won 1921 Nobel Prize in Chemistry has a small crater named for him on the far side of the Moon. has a small crater named for him on the far side of the Moon.

28 Naming Isotopes We can also put mass number after name of the element: We can also put mass number after name of the element: carbon-12 carbon-12 carbon-14 carbon-14 uranium-235 uranium-235

29 IsotopeProtonsElectronsNeutronsNucleus Hydrogen–1 (protium) (protium)110 Hydrogen-2(deuterium)111 Hydrogen-3(tritium)112

30 Isotopes Elements occur in nature as mixtures of isotopes.

31 Atomic Mass  How heavy is an oxygen atom?  Depends, b/c different kinds of oxygen atoms exist.  We’re more concerned with average atomic mass.  Based on abundance (%) of each variety of that element in nature.  Don’t use grams - numbers tooooo small.

32 Measuring Atomic Mass Atomic Mass Unit (amu) Atomic Mass Unit (amu) one-twelfth mass of a carbon-12 atom. one-twelfth mass of a carbon-12 atom. Carbon-12 chosen b/c of its isotope purity. Carbon-12 chosen b/c of its isotope purity. Each isotope has own atomic mass Each isotope has own atomic mass we determine average from % abundance. we determine average from % abundance.

33 To calculate the average: Multiply atomic mass of each isotope by abundance (decimal), then add results. Multiply atomic mass of each isotope by abundance (decimal), then add results. If not told otherwise, mass of isotope expressed in atomic mass units (amu) If not told otherwise, mass of isotope expressed in atomic mass units (amu)

34 Atomic Masses IsotopeSymbol Composition of the nucleus % in nature Carbon- 12 12 C 6 protons 6 neutrons 98.89% Carbon- 13 13 C 6 protons 7 neutrons 1.11% Carbon- 14 14 C 6 protons 8 neutrons <0.01% Atomic mass is average of all naturally occurring isotopes of that element. 12.01 What is the average atomic mass of Carbon? What is the average atomic mass of Carbon? Atomic mass (amu) 12 13.00 14.00

35 - Page 117 Question Solution Answer Knowns and Unknown

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