Name Ch. 5 - Atomic Structure Pre-AP Modern Atomic Theory All atoms are made of three subatomic (smaller than the atom) particles: the protons, the electrons and the neutrons. (P.E.N. s) There are particles smaller than these subatomic particles such as quarks, gluons, mesons, muons, and others but we will limit our discussion to the three subatomic particles. Atoms have two separate parts. The nucleus and the electron cloud. 1
The nucleus (plural: nuclei) is made of protons and neutrons and is in the center of the atoms. The electron cloud has only electrons in it and surrounds the nucleus. Name Symbol Charge Location Mass in atomic mass unit (amu) Mass in p + n o e - nucleus nucleus outside nucleus amu amu amu grams 1.67x10-24 g 1.67x10-24 g 9.11x10-28 g All atoms are neutral therefore the number of protons equals electrons. 2
The atomic number (or Z) of an element is the number of protons in the atom. The number of protons determines which number of the atom. For example, all carbon atoms have protons. Z = p + The mass number (or A (sometimes M)) is the sum of the number of protons and neutrons. A = p + + n o A Z X Charge so 56 26Fe means A = mass number and Z = atomic number. 3
Most atoms have more than one isotope. Isotopes are two atoms with the same atomic number, but different mass numbers. In other words, they have the same number of protons, but a different number of neutrons. Isotopes are usually written as the name and the new mass number (ex. Name-A). so hydrogen-3 has a mass number of 3. Ex. Carbon has two isotopes: Carbon-12 and Carbon-14 Mass Number # of Protons # of Neutrons 4
Ions are atoms with a charge. Atoms become ions by gaining or losing electrons. Ex. A magnesium atom ( 24 12Mg) has p +, n o, e -. If it loses 2 e -, it will have p +, n o, e -. The two extra e - give the ion a charge. So, for an ion e - = p +, but e- = Z - charge Ex. Symbol Atomic #Mass # p + n o e - 14 N 14 N 3-14 N 5+ The number of protons determines the identity of atom, the number of neutrons determines which isotope it is. Atomic number = Z = p + 5
Mass number = A = p + + n o e - = Z charge A - Z = n o Ex. Assume all elements are atoms unless the information given indicates otherwise. Symbol Atomic # Mass # p + n o e - H 1 1 1 He 2 4 B 5 11 H + 1 C 6 6 6 N 3-7 8 8 16 18 9 10 17 18 18 30 65 28 6
Atomic Mass When massing atoms we use the atomic mass unit (amu (or u)). One atomic mass unit is 1/12 the mass of a carbon-12 nucleus. So, the carbon-12 nucleus has a mass of exactly 12 amu (12.000000...amu). Average Atomic Mass We discussed earlier that elements have different isotopes. Due to the fact that most elements have at least 2 or 3 isotopes, atomic masses cannot be calculated by just adding the mass of each p +, no, and e -. 7
Instead we find an average of the isotopes taking into account the relative abundance of each. 1. Change the percent to a decimal by dividing to a decimal 2. Multiply the decimal by the isotopes mass 3. Add all of the masses together 4. DO NOT DIVIDE!!! Ex. 1) What is the average atomic mass of hafnium if it is 5.000% 176.0 amu, 19.00% 177.0 amu, 27.00% 178.0 amu, 14.00% 179.0, and 35.00% 180.0 amu? 8
2) What is the average atomic mass of lead if it is 1.370% 204.0 amu, 26.26% 206.0 amu, 20.82% 207.0 amu, and 51.55% 208.0 amu? Chapter 28: Nuclear Chemistry Changes in the Nucleus; Types of 9
Radiation Nuclear reactions change the composition of the atom s nucleus. All three kinds of radiation are produced by nuclear reactions. We know that the nucleus is made of protons and neutrons. We also know that there should be no attraction between these kinds of particles. Nor should there be any attraction between two neutrons, but there should be repulsion between two protons. So, what keeps a nucleus from flying apart? 10
A force called the strong nuclear force which is only significant between subatomic particles that are very close. If a nucleus is not radioactive it is said to be stable. If a nucleus is radioactive it is said to be unstable. The more unstable the nucleus the more radiation it gives off and therefore the faster it changes to another element. All elements with atomic numbers higher than 92 are radioactive. Types of Radiation There are three types of radiation: alpha ( ), beta ( ), and gamma ( ). 11
Penetrating Name Compositio Symbol(s) Charge Mass Power Shielding n Alpha Beta Gamma Radioactive Decay When an atom emits one of these kinds of radiation it is undergoing radioactive decay. This process is called transmutation (or decay) because the original nucleus changing into a new nucleus releasing radiation in the process. Ex. Reaction 1: 238 92U 234 90Th + 4 2He Reaction 2: 234 90Th 234 91Pa + 0-1e 12
Notice that the total mass number and total atomic number are equal on both sides. Also notice that in both cases because the atomic number changed we ended up with a different element. Complete the following reactions. a. 240 96Cm + 4 2He d. 209 83Bi + 4 2He b. 243 95Am + 0-1e e. 40 19K 40 20Ca + c. 231 90Th + 0-1e f. 226 88Ra 222 86Rn + Nuclear Fission and Fusion Nuclear fission is the splitting of nuclei. Ex. 235 92U + 1 0n 141 56Ba + 92 36Kr + 3 1 0n 13
Fission is used in nuclear power plants to produce electricity Nuclear fusion is the combining of nuclei. Ex. 2 1H + 2 1H 4 2He Fusion is the source of energy for the sun and every other star in the universe, but it only occurs above 40000000 o C. It is hoped that we will one day we will be able to use fusion as the source for our electricity. 14