Atomic Theory & the Atom. it s elemental

Transcription

1 Atomic Theory & the Atom it s elemental

2 Our view of the atom has changed over time

3 the ATOM the smallest particle of an element that still retains the chemical properties of that element

4 Here is a model of an atom. - what does this model tell you about the atom? - What is something this model doesn t tell you?

5 Models of the Atom ATOMIC PUDDING

6 Key question: How are the smallest bits of matter described? Hank Green the nucleus

7 the ATOM our current model 3 principal particles Protons positively charged Neutrons no charge Electrons negatively charged

8 the ATOM Nucleons: protons and neutrons in the nucleus Neutrons are bound to protons by the strong nuclear force one of the strongest known forces! When neutrons are found outside the nucleus they breakdown quickly into electrons and protons.

9 the ATOM: electrons electrons: (e-) are continually in motion around the nucleus of an atom. Responsible for ALL the chemical properties of an atom. Hank - electrons The smallest of the 3 atomic particles and considered not to contribute mass to the atom.

10 atoms are neutral in charge Proton has + charge equal in magnitude to charge of electron Atoms are electrically neutral # of protons = # of neutrons

11 Subatomic particles Particle physics and nuclear physics study these particles and how they interact.

12 Elementary particles Protons and neutrons are composite particles made of even smaller particles.

13 protons and neutrons electrons

14 Dalton s atomic theory (still relevant today) 1. All matter is composed of atoms 2. Atoms of a given element are identical 3. Atoms cannot be divided, created or destroyed 4. Atoms combine in whole number ratios and in chemical reactions, are combined, separated or rearranged

15 Current electron cloud model of an atom. How many atoms do you think there are in this gold nugget?

16 Modern Atomic Theory Evidence through discovery and experimentation shapes our model of the atom.

17 Hank Green Crash course video The Atomic Model Through Time But let s take a step back and look at the evidence that changed our vision of the atom over time. The Cloud Model

18 An attempt to explain our world. The Egyptians (3000 BC) formulate the theory of the Ogdoad, or the "primordial forces", from which all was formed. These were the elements of chaos, numbered in eight, that existed before the creation of the sun.

19 The Greeks (450 BC) Empedocles asserts that all things are composed of four primal elements: earth, air, fire, and water, whereby two active and opposing forces, love and hate, or affinity and antipathy, act upon these elements, combining and separating them into infinitely varied forms.

20 440 BC Gives us the term atom or Atomos meaning uncuttable

21 Natural philosophers such as Aristotle and Democritus used deductive reasoning in an attempt to explain the behavior of the world around them. Leucippus and Democritus (440 BC) propose the idea of the atom, an indivisible particle that all matter is made of. This idea was largely rejected by natural philosophers in favor of the Aristotlean view

22 John Dalton Solid Sphere Model Dalton proposed a modern atomic model based on experimentation. All matter is made of atoms. Atoms of an element are identical. Each element has different atoms. Atoms of different elements combine in constant ratios to form compounds. Atoms are rearranged in reactions. His ideas account for the law of conservation of mass (atoms are neither created nor destroyed) and the law of constant composition (elements combine in fixed ratios).

23 JJ Thompson Plum Pudding Model 1897 Thompson a British scientist, zapped atoms with electricity. He observed that negatively charged particles were removed. He proposes that atoms are similar to a fluid-filled sac with negative particles floating inside. The fluid is positive and solid particles are negative. His ideas account for the discovery of the electron

24 1911 Ernest Rutherford His ideas account for the discovery of the positive nucleus of the atom

25 While protons give an atom mass, protons alone don t account for the mass of the atom. Since Rutherford s time, it was known that an atom s mass is a bit more than twice its number of protons (atomic number) and that essentially all the mass of the atom is concentrated in the nucleus. In the 1930 s it was presumed that the fundamental particles were protons and electrons, but that required that electrons were bound in the nucleus to partially cancel the charge of the protons. But it was also known that there just wasn't enough energy available to contain electrons in the nucleus.

26 Niels Bohr 1913 Solar system model Electrons orbit at different distances from the nucleus

27 more mass discovery of the neutron Until 1932, the atom was known to consist of a positively charged nucleus surrounded by enough negatively charged electrons to make the atom electrically neutral. Lord Ernest Rutherford (1917) had postulated the existence of a neutral particle This stimulated a search for the particle. However, its electrical neutrality complicated the search because almost all experimental techniques of this period measured charged particles

28 The neutron was finally discovered in 1932 when James Chadwick used scattering data to calculate the mass of this neutral particle. Chadwick proved that there was a neutral component with a mass approximately equal to that of the proton. He called it the neutron in a paper published in the February 17, 1932, issue of Nature. In 1935, Sir James Chadwick received the Nobel Prize in physics for this work.

29 The search was over. Chadwick had found a new elementary particle, the third basic component of the nucleus. It increased the mass of elements without adding electrical charge. This changed our view of the nucleus.

30 neutrons Neutrons are particles found in the nucleus. They have no charge, but do add mass to the atom. The mass of a neutron is considered equal to the mass of a proton.

31 Atom Clash of the Titans

32 Erwin Schrodinger (1924) Austrian physicist Developed the electron cloud model (current model) The exact path of electrons cannot be predicted. Instead they exist in a specific energy region or shell surrounding the nucleus. The region referred to as the electron cloud is an area where electrons are likely to be found at any given time.

33 Schrodinger What was the impact of Schrodinger's atomic cloud model? Erwin Schrodinger s atomic cloud model revolutionized the way scientists viewed the structure of the atom. Building on the work of Neils Bohr, Schrodinger demonstrated that it was impossible to determine the exact location of the electron at a particular point in time. Instead, Schrodinger s model showed that an electron could be found in some portion of an electron cloud at any specific point in time.

34 Schrodinger s work largely took the form of a probability equation. In essence, the equation demonstrated that while the electron was more likely to be found at a specific point at a given point in time, it was impossible to determine whether or not the electron actually was there.

35 Experimental results cannot provide any more definitive answers regarding the location of the electron at a particular point in space and time. Visible light s wavelengths are too large to view atomic structures, so light microscopes are of no use to atomic investigations. Normally, scientists examine very small objects with electron microscopes. Electron microscopes fire electrons, rather than photons of light, at the object to be seen. However, electrons used in electron microscopes cannot provide imagery of other electrons because they are the same size and they will cause the original electron s position to change.

36 More on Atoms ATOMIC NUMBER & ATOMIC MASS

37 Key Question How are the atoms of one element different from those of another element?

38 You will be able to: distinguish between atomic number, mass of an atom, and average atomic mass describe the structure of an atom and draw a simple atomic model of an atom extract information from the periodic table related to atomic structure and atomic mass

39 Atomic number Every atom has a distinct number of protons in its nucleus. remember: The number of protons determines an atom s identity boron carbon nitrogen 5 protons 6 protons 7 protons

40 Also remember: Elements are arranged in the Periodic table based on their atomic number (# of protons) the atomic number identifies the element

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42 How are atomic mass and atomic number related? Theoretically, atoms have the same number of neutrons as they do protons. Neutrons and protons are responsible for the mass of an atom. Atomic mass Atomic number So..if you know the atomic number, you can predict the approximate atomic mass.

43 Elements are often symbolized with their mass number and atomic number mass # e.g. Oxygen: 16 O 8 atomic # These values are given on the periodic table. For now, round the mass # to a whole number. These numbers tell you a lot about atoms. # of protons = # of electrons = atomic number # of neutrons = mass number atomic number Calculate # of e, n 0, p + for Ca, Ar, and Br.

44 Atomic Mass p + n 0 e Ca Ar Br P 35 N Bromine

45 Atomic mass The atomic mass (m a ) is the mass of an atom, expressed in atomic mass units (AMU). The atomic mass is the total mass of protons, neutrons and electrons in a single atom.

46 average atomic mass Why does the periodic table show atomic mass numbers that are not whole numbers?

47 Subatomic Heavyweights: Isotopes

48 A chemist investigating a sample of lithium found that some lithium atoms have a lower mass than other lithium atoms. The chemist drew models of the two different types of lithium atoms, as shown below. 1. What is different about the two atoms? 2. What is the atomic number of each atom? 3. What is the atomic mass of each atom?

49 Key Question How can atoms of the same element be different?

50 You will be able to: define isotope and write and interpret the symbol for a specific isotope determine the average atomic mass of an element based on the natural abundance of isotopes of that element predict the number of protons, neutrons, and electrons in the most abundant isotope of an atom, based on average atomic mass

51 isotopes Atoms of an element that have different numbers of neutrons are known as isotopes. While an atom is identified by its number of protons (atomic number), the same atom can have different numbers of neutrons

52 A helium atom has an atomic number of 2 How many protons does it have? How many electrons does it have? How many neutrons does it have?

53 Calculate average atomic mass First you need to know the % abundance of each isotope. The average atomic mass of an element is the weighted average of the masses of the isotopes in a sample of the element.

54 Calculate the Atomic Mass of Neon (.77)(20) + (.12)(22) + (.11)(21) = The sample contains 100 atoms of neon = 20.35

55 Isotope notation A Z X Often, at least one isotope is unstable. It breaks down, releasing radioactivity.

56 Properties of isotopes are important There are 90 naturally occurring elements with roughly 250 stable isotopes, and over 3200 unstable or radioactive isotopes. Different isotopes of the same element often have completely different properties -- making some of them very useful

57 Isotopes are used in a wide variety of applications medical imaging in the diagnosis of a wide range of ailments cancer treatment and other therapeutic applications; As a ''fingerprint'' used in forensic analysis; Smoke detectors; Batteries that power NASA satellites; to enable new sources of energy such as nuclear fusion;

58 Calculating % abundance The percentage of each isotope of an element that occurs in nature is called the natural percent abundance of the isotope.

59 Try Problem #1: Nitrogen is made up of two isotopes, N-14 and N-15. Given nitrogen's atomic weight of , what is the percent abundance of each isotope? (14) (x) + (15) (1 - x) = Notice that the abundance of N-14 is assigned 'x' and the N- 15 is 'one minus x. The two abundances always add up to one (or, if you prefer, 100%)

60 to solve: (14) (x) + (15) (1 - x) = Solving gives: 14x x = x = = and 1 - x = So. 99.3% N-14 And 0.7% N-15

61 Problem #2: Copper is made up of two isotopes, Cu-63 ( amu) and Cu-65 ( amu). Given copper's atomic weight of , what is the percent abundance of each isotope?

62 To solve: ( ) (x) + ( ) (1 - x) = Solve for x: x = OR 69.15% (the abundance for Cu-63) = or 30.85% (abundance for Cu-65)