NOTES ON The Structure of the Atom

Transcription

1 NOTES ON The Structure of the Atom Chemistry is the study of matter and its properties. Those properties can be explained by examining the atoms that compose the matter. An atom is the smallest particle of an element that has the properties of the element. An element is a substance whose atoms have the same number of protons. History of the Atom Concept Many Greek Philosophers thought matter was formed of air, earth, fire and water. All matter had properties of hot, wet, dry or cold (see figure 4-1). These early nonscientific beliefs were not completely dispelled until the 1800s. Democritus Greek philosopher; coined the term atomos in reference to his idea that everything is made of small pieces; B.C. Summary of Democritus ideas: Matter is composed of empty space through which atoms move. Atoms are solid, homogeneous, indestructible, and indivisible Different kinds of atoms have different sizes and shapes. The differing properties of mater are due to the size, shape and movement of atoms. Apparent change in matter result from change in the groupings of atoms and not from changes in the atoms themselves. Democritus was criticized for not being able to answer the question What holds atoms together? Aristotle Greek philosopher; ( BC) Summary of Aristotle s ideas: Matter is continuous, like a cheese being cut in half over and over Supported the idea that all matter was composed of four elements, earth, air, fire, and water Did not believe that the nothingness of empty space could exist Discredited Democritus and his misconception was accepted until the 17 th century, because Aristotle was consider so great. Dalton School teacher from England ( ) revived the theory of the atom and began developing the modern atomic theory. Dalton s atomic theory All matter is composed of extremely small particles called atoms. All atoms of a given element are identical, having the same size, mass and chemical properties. Atoms of a specific element are different from those of any other element. Atoms cannot be created, divided into smaller particles, or destroyed. Different atoms combine in simple whole-number ratios to form compounds. In a chemical reaction, atoms are separated, combined or rearranged. +

2 So.What is an atom? See figure on p. 86 and figure 4-6. This is what an atom looks like. Crookes English Physicist:; developed cathode ray tube (CRT) (1800s); made Thompson s experiments possible and led to the development of the television Cathode ray tube or Crooke s tube, pass electricity through a tube that is void of air. Radiation is emitted from the cathode end and strikes a light-producing chemical at the other end. See Transparency #11 Scientists concluded that Cathode rays were actually stream of charged particles. The particles carried a negative charge. These negative particles are called Electrons. (The exact value of the negative charge was not known.) J.J. Thompson English physicist ( ) cathode ray tube/magnet experiments; In 1890s he determined: o The ratio of charge to mass of charged particles. ( electrons) o Concluded that the charged particle had a mass much smaller than hydrogen, meaning that Dalton was wrong!! Atoms could be divided. o Discovered electron and proton using charged gases and measuring the deflection of the particle ray o Developed plum-pudding model of the atom (1903) Millikin American Physicist; ( ) oil drop experiment o In 1909 he calculated the charge of the electron to be 1 o Calculated the mass of a single electron from Thompson s experiment (9.1*10-28 g ) o Which led to the question If electrons are part of all matter and they possess a negative charge, how is it that all matter is neutral? This led to the plum-pudding model. Rutherford New Zealander ( ) o gold foil experiment using alpha particle source( ) o Draw figures from Transparency #12 here

3 o working with Bohr, Marsden, & Geiger o led to discovery of the dense, small nucleus with positive charge, and mostly empty space o developed bees around a hive model (1911) o This explained the neutral nature of matter: the positive charge of the nucleus balancing the negative charge of the electrons. Did not explain the atom s mass. o 1920 refined the concept of the nucleus. Nucleus contains a proton carrying a charge of +1 o Draw a diagram of the atom. Niels Bohr Danish physicist who worked in Rutherford s (1913) o Proposed the quantum model o developed the planetary model of the atom o still useful today for many concepts; called Bohr model in his honor Draw model here. The Subatomic Particles There are 3 subatomic particles found in the atom, the proton (p + ), the electron (e - ) and the neutron (n ) Properties of subatomic particles PARTICLE SYMBOL LOCATION RELATIVE CHARGE Electron RELATIVE MASS Proton Neutron On the periodic table you can find this information: atomic number average atomic mass 16 S Sulfur You can also be given the information in: Isotopic Notation 32 S 16 Mass number notation S-32

4 Atomic number the number of protons in an element. If an atom is neutral, this also equals the number of electrons. This is written as the bottom number in isotopic notation Average atomic mass the average mass of all isotopes for that particular element. This is usually a decimal number NOT a whole number. Mass Number the mass of one particular atom or isotope. This has to be a whole number. If it s not given to you, you round the average atomic mass to a whole number and that becomes the mass number. This is written as the top number is isotopic notation or as the whole number after the chemical symbol in Mass number notation. Finding the number of protons, electrons and neutrons for any atom Number of protons = atomic number (ALWAYS!!!) Number of electrons = atomic number (IF the atom is neutral and if it s not add electrons for negative charges and subtract electrons for positive charges.) Number of neutrons = mass number atomic number (ALWAYS!!!) Examples: How many protons, neutrons and electrons are there in Silver? How many neutrons, electrons and protons are there in Chlorine? How many protons, electrons and neutrons are there in Mg 23? How many electrons, neutrons and protons are there in Br? Examples: Complete the table. Isotope Composition Data ELEMENT CHEMICAL ATOMIC MASS PROTONS ELECTRONS NEUTRONS SYMBOL NUMBER NUMBER Neon 22 Calcium 46 Oxygen 17 Iron 57 Zinc 64 Mercury 204 Ions atoms that have the same number of protons, but differing numbers of electrons. They are not neutral, they have a charge. If an atom has a positive charge, it has lost some electrons. If an atom has a negative charge, it has gained some electrons. Example: How many protons, neutrons and electrons are there in Na +1?

5 How many protons, neutrons and electrons are there in O 2-? Isotopes atoms that have the same number of protons, but differing numbers of neutrons. (REMEMBER.the number of protons is ALWAYS the same for each element. ) Isotopes of the same elements have essentially the same chemical behavior, because it s the number of electrons that determines an atom s behavior. For example, in a banana, which is a rich source of potassium, 93.25%of the potassium atoms have 20 neutrons, % will have 22 neutrons, and a scant 0.011% will have 21 neutrons. Isotopes have different mass due to differing numbers of neutrons. o Mass number is a whole number that represents the sum of protons and neutrons in the nucleus. o Write the mass numbers for the isotopes of potassium. Sample: See transparency #4 Give the data in the table, calculate the atomic mass of unknown element X. Then, identify the unknown element, which is used medically to treat some mental disorders. ISOTOPE ABUNDANCE FOR ELEMENT X ISOTOPE MASS (amu) PERCENT ABUNDANCE 6 X % 7 X % Nuclear Concepts Nuclear Reactions involve a change in the atom s nucleus What do chemical reactions involve? Radioactivity substances spontaneously emit radiation Radiation the particles and energy that are spontaneous emitted in radioactivity Alpha particles helium nucleus (2 protons, 2 neutrons) ; positively charged; low energy, can be stopped by paper or cloth Beta particles high-speed electron emitted by the nucleus at the instant of decay; negatively charged; higher energy and more penetrating therefore need a few millimeters of aluminum to stop them Gamma rays high energy X-rays; travel at the speed of light; no mass or charge; can penetrate most materials; stopped by thick lead

6 Nuclear equations show the atomic number and mass number of the particles involved in a nuclear reaction. (Both the mass number and the atomic number are conserved in a nuclear reaction.) Alpha radiation: Beta radiation: Gamma radiation:

7 NOTES- Nuclear Chemistry REVIEW CONCEPTS: Draw a model of the atom List the subatomic particles with their relative masses and place them on the model What is an isotope? What is isotropic notation? How is a nuclear reaction different from a chemical reaction? NUCLEAR RADIATION Table 25-1 Characteristics of Chemical and Nuclear Radiation Chemical Reactions Nuclear Reactions Occur when bond are broken and formed Occur when nuclei emit particles and/or rays. Atoms remain unchanged, though they may be rearranged Involve only valence electrons Atoms are often converted into atoms of another element May involve protons, neutrons, and electrons Associated with small energy changes. Associated with large energy changes. Reaction rate is influenced by temperature, pressure, concentration and catalysts. Reaction rate is not normally affected by temperature, pressure or catalysts. Table 25-2 Properties of alpha, beta, and gamma radiation Property Alpha ( ) Beta ( ) Gamma ( ) Composition Description of radiation Charge Relative penetrating power

8 RADIOACTIVE DECAY Write the isotopic notation for Lead 206. How many protons, electrons and neutrons are there? How many protons, electrons and neutrons are there in C-14? Write the isotopic notation. Beta decay ( 0-1 ) occurs when a radioisotope has too many neutrons and is above the band of stability. Alpha decay ( 4 2 He) occurs with all nuclei with more than 83 protons, because there are too many protons and neutrons in the nucleus. Positron emission and electron capture occur for nuclei with low n/p ratios lying below the band of stability. o Positron emission ( 0 1 ) involves a positron which is a particle with the same mass as an electron, but with a positive charge. Positron emission converts protons into neutrons. o Electron capture ( 0-1 e) is when a nucleus pulls in an electron and combines with a proton to form a neutron. Gamma decay ( 0 0 ) energy is released and there is no change to the n/p ratio. Table 25-3 Summary of Radioactive Decay Processes Type of Radioactive Particle emitted Decay Alpha Decay Change in mass number Change in atomic number Beta Decay Positron emission Electron capture Gamma emission Write a balanced nuclear equation for the alpha decay of Thorium 230.

9 RADIOACTIVE DECAY RATES Half-life is the time required for one-half of a radioisotope s nuclei to decay into products. Iron-59 is used in medicine to diagnose blood circulation disorders. The half-life of iron-59 is 44.5 days. How much of a mg sample will remain after days? If the passing of five half-lives leaves 25.0 mg of a strontium-90 sample, how much was present in the beginning? FISSION AND FUSION Nuclear fission is the splitting of a nucleus into fragments. This produces a very large release of energy. Nuclear power plants use fission reactions to generate power. Figure 25-16, Figure 25-17, Figure A self-sustaining process in which one reaction initiates the next is called a chain reaction. The neutrons released from U-235, produce more neutrons which propagate the chain reaction. Critical mass is the mass need to sustain a chain reaction. Subcritical mass is when the mass is too small to sustain a chain reaction, because the neutrons escape before colliding with more U-235. Supercritical mass is when the mass is much greater that the critical mass and the chain reaction escalates rapidly. This can lead to a nuclear explosion. Nuclear Fusion, also known as thermonuclear reactions is the combining of atomic nuclei. This produces a very large amount of energy. Nuclear fusion is the process that power our sun. Fusion reactions required extremely large temperatures, 40,000,000K, to initiate and sustain the reaction. The reaction that powers our sun is: