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Dalton’s Atomic Theory (1808)

Dalton’s Atomic Theory (1808). All matter is made of tiny indivisible particles called atoms. Atoms of the same element are identical. The atoms of any one element are different from those of any other element.

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Dalton’s Atomic Theory (1808)

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  1. Dalton’s Atomic Theory (1808) • All matter is made of tiny indivisible particles called atoms. • Atoms of the same element are identical. The atoms of any one element are different from those of any other element. • Atoms of different elements can combine with one another in simple whole number ratios to form compounds. • Chemical reactions occur when atoms are separated, joined, or rearranged;however, atoms of one element are not changed into atoms of another by a chemical reaction. V.Montgomery & R.Smith

  2. Summary of the Atom • atoms are the smallest particles that can be uniquely associated with an element • each element has unique atoms • atoms are composed of e-, p and n • atoms are electrically neutral (# of e- = # of p) • for a single element, isotopes differ only in number of n (neutrons) • atoms have characteristic masses (atomic weights) • atoms combine with one another in definite, whole number proportions to make compounds

  3. ~ 10-10 m electron nucleus Mass > 10-26 kg Mass 9 x 10-31 kg

  4. ~ 1 – 7 x 10-15 m (1 – 7 fermi)

  5. atoms are dominantly empty space: electron orbits The Spacious Atom If an oxygen atom had a total radius of 100 km, the nucleus would be a ~1 m diameter sphere in the middle.

  6. In a simplistic model, electrons float around the nucleus in energy levels called shells. electron orbits Electrons in Orbit As the number of electrons increases, they start to fill shells farther out from the nucleus. In most cases, electrons are lost or gained only from the outermost shell.

  7. Atom Nucleus

  8. The Nuclear Model of the atom

  9. Mass of an electron is approximately 1/1840th of a proton or neutron. • Mass of a neutron is very close to the mass of a proton. • 1 atomic mass unit (amu or u)= 1.66054x10-24g • 1 g= 6.022 x1023amu • 1amu is defined as 1/12th the mass of an atom of carbon-12 .

  10. Subatomic Particles Particle Symbol Charge Relative Mass Electron e- 1- 0 Proton p+1+ 1 Neutron n 0 1

  11. Atomic Number(nuclear charge) 11 Na Atomic Number Symbol

  12. All atoms of an element have the same number of protons 11 Na 11 protons Sodium

  13. Number of Electrons • If an atom is neutral ; • The net charge is zero • Number of protons = Number of electrons • Atomic number = Number of electrons in a neutral atom

  14. Ions • Have a net electrical charge since the total number of electrons isn’t equal to the number of protons. • Can be anions (e->p+ ;formed as a result of gaining electrons; negatively charged), cations (p+>e-;formed as a result of losing electrons; positively charged). • e- + q(charge of the ion) = proton number

  15. In chemical rxns, atom never gain or lose protons. It’s the interaction of electrons.

  16. Mass Number(nucleon number) Counts the number of protons and neutrons in an atom A= p+ + n0

  17. Atomic Symbols • Show the mass number and atomic number • Give the symbol of the element (A)mass number 23Na 11 (Z)atomic number sodium-23

  18. Notation for Atoms only one isotope of carbon 12C 13C C only one isotope of carbon all isotopes of carbon

  19. Basic Definitions • “atomic number” = number of protons in the nucleus; • “mass number” = sum of protons + neutrons in the nucleus • “isotopic mass” = mass of a single isotope

  20. More Atomic Symbols 16 31 65 O P Zn 8 15 30 8 p+ 15 p+ 30 p+ 8 n 16 n 35 n 8 e- 15e- 30 e-

  21. Isotopes • Atoms with the same number of protons, but different numbers of neutrons. • Atoms of the same element (same atomic number) with different mass numbers Isotopes of chlorine 35Cl 37Cl 1717 chlorine - 35 chlorine - 37

  22. Since both isotopes have the same number of protons and electrons, they have identical chemical properties. • Since physical properties depend on the mass of particles as well, isotopes will often have slightly different physical properties such as density, mass, rate of diffusion etc.

  23. Natural abundances of isotopes Natural chlorine contains : 75 % 35Cl and 25 % 37Cl . 1717 • These percentages are known as the natural abundances of the isotopes and determined by “mass spectrometry.”

  24. Relative atomic mass • is weighted mean. Relative atomic mass has no unit since the atomic mass unit, amu is cancelled out in calculation w/ respect to C-12.

  25. Exercise 1 • The molar mass of iridium is 192.2 g / mol. What are the naturally occurring percentages of the two isotopes of Ir-191 and Ir-193?

  26. solution Iridium is a mixture of 40% 191Ir and 60 % 193Ir.

  27. Isotopes of Hydrogen element has the biggest abundance in nature.

  28. Radioactive isotopes • Are produced by exposing the natural element to a flux(flow) of neutrons in a nuclear reactor. The nucleus of an atom captures an additional neutron and form radioisotope.

  29. Usages of radioisotopes • The rate of radioactive decay is used to date objects (C-14). • Naturally occurring C has a fixed proportion of C-14 due to exchange w/ C in the atmosphere. When the plant is dead, the exchange stops & the proportion of C-14 starts to decrease in the plant due to radioactive decay. This decayed amount is used to date the plant. After about 5,700 yrs, the proportion of C-14 falls to about half its initial value.

  30. Usages of radioisotopes 2. as tracers. • Radioactive isotope reacts chemically & biologically. For example the activity of thyroid gland can be measured w/ monitoring the increase in radioactivity of the gland after taking a drink including iodine radioisotopes (I-125 and I-131) since the thyroid gland absorbs the radioactive iodine when it works.

  31. Usages of radioisotopes 3. Source of gamma rays and therefore, source intense radioactivity. • Cobalt-60 is an example of such a radioactivity source. It’s used in radiation treatment for cancer and industrially as well

  32. Mass spectrometry • A mass spectrometer is an instrument which separates particles according to their masses , records the relative proportions of these, and determine natural abundances of the isotopes of an element. Therefore, it also allows us to calculate the atomic mass of an element. • The most accurate way for determining atomic and molecular weights is provided by mass spectrometer.

  33. Mass spectrometry • mass spectrometer, invented by the English physicist Francis William Aston (1877-1945) when he was working in Cambridge with J. J. Thomson. It was in his use of this instrument that the existence of isotopes of elements was discovered. • Aston eventually discovered many of the naturally occurring isotopes of non-radioactive elements. • He was awarded the Nobel Prize for Chemistry in 1922.

  34. Mass spectrometry accelerating C A B D F E

  35. A: a gaseous sample is very slowly introduced to the mass spectrometer. B: atoms/molecules are bombarded by a stream of high energy electrons to produce positive ions, mostly w/ a 1+ charge. These electrons collide w/ electrons in the particle knocking them out and leaving a positive ion. C: positively charged ions are accelerated high enough to make the particles pass through the slits and magnetic field by high electrical voltage on the negatively charged grid. With the slits, the ions were made a beam of ions.

  36. D: Fast moving ions enter a magnetic field produced by an electromagnet. Ions are deflected by a magnetic field into a curved path. The deflection of the ions depends on “charge to mass ratio(q/m). ”The more massive the ion, the less the deflection. The ions w/ equal mass and charge will deflect the same. E: By changing the strength of the magnetic field or the accelerating voltage on the negatively charged grid, ions of varying masses can be made to enter the detector at the end of the instrument.

  37. On the detector, ions are collected on a metal plate and the current flows through the metal plate to neutralise the ions and this current is recorded. • In this way, the relative abundances of ions of different masses in the sample can be determined and put into a graph called “mass spectrum.”

  38. F: The mass spectrometer must be at a high vacuum for its correct operation and its correct operation depends on particles being able to pass through it w/o colliding with any other particles.

  39. A: vapourised sample introduced • B: ionization by electron bombardment • C: Positive ions accelerated by electrical field • D: ions deflected by a magnetic field • E: detector records ions of a particular mass • F: vacuum prevents molecules colliding

  40. Mass spectrometer is used to identify the molecular structure of a compound or analyze mixtures of substances. • When a molecule loses an electron, it falls apart, forming fragments. Mass spectrometer measures the mass of these fragments, producing a chemical “fingerprint” of the molecule and providing clues about how the atoms were connected together in the molecule.

  41. Atomic weight measurements How was the atomic weight measured? • By mass spectrometry • This also measures % natural abundance for a given isotope • The graph is called as “mass spectrum.”

  42. P.53check out the graph.

  43. Atomic weight calculation There are three naturally occuring isotopes of neon (Ne): 20Ne isotopic mass = 19.99244018 amu 21Ne isotopic mass = 20.9938467 amu 22Ne isotopic mass = 21.9913855 amu the atomic weight is reported in text as: 20.1797 amu

  44. Learning Check 1 Naturally occurring carbon consists of three isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of these carbon atoms. 12C 13C 14C 6 6 6 #P _______ _______ _______ #N _______ _______ _______ #E _______ _______ _______

  45. Solution 12C 13C 14C 6 6 6 #P __6____ 6___ ___6___ #N __6___ __7___ ___8___ #E __6____ 6___ ___6___

  46. Learning Check 2 An atom of zinc has a mass number of 65. A. Number of protons in the zinc atom 1) 30 2) 35 3) 65 B. Number of neutrons in the zinc atom 1) 30 2) 35 3) 65 C. What is the mass number of a zinc isotope with 37 neutrons? 1) 37 2) 65 3) 67

  47. Solution An atom of zinc has a mass number of 65. A. Number of protons in the zinc atom 1) 30 B. Number of neutrons in the zinc atom 2) 35 C. What is the mass number of a zinc isotope with 37 neutrons? 3) 67

  48. Learning Check 3 Write the atomic symbols for atoms with the following: A. 8 p+, 8 n, 8 e- ___________ B. 17p+, 20n, 17e- ___________ C. 47p+, 60 n, 47 e- ___________

  49. Solution 16O A. 8 p+, 8 n, 8 e- 8 B. 17p+, 20n, 17e- 37Cl 17 C. 47p+, 60 n, 47 e- 107Ag 47

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