Atomic Structure and the Nucleus Chemistry Chapter 3 Atomic Structure and the Nucleus World of Chemistry Zumdahl Last revision Fall 2008

Structure of the atom We have all learned from an early age that the atom is composed of 3 tiny particles Particle Symbol Relative Electric Charge Actual Mass (g) Relative Mass Electron e- 1- 9.11 x 10-28 1/1840 = 0 Proton p+ 1+ 1.67 x 10-24 1 Neutron n0

And the atom has 2 main regions: the nucleus and the electron cloud. The periodic table can help us figure out the protons, neutrons and electrons for any element Nucleus Electron Cloud p+ no e- 19 K Potassium 39.098 Atomic number: This tells the number of protons for the element In a neutral atom e- = p+ = Symbol: The symbol always starts with a capital letter Name Average Atomic Mass: This is the average abundance of isotopes occurring in nature When this number is rounded to a whole number it becomes the Mass Number which is the total particles in the nucleus

Determining number of particles in an atom Protons = the atomic number of the element. The protons identify the atom Electrons = as long as the atom is neutral, electrons are equal to protons. Neutrons = the mass number tells the total particles in the nucleus so just subtract the atomic number from the mass number. 19 K Potassium 39.098 39 – 19p+ = 20 no

Isotopes: an element with various masses. This indicates the mass number of the element

Symbolic notation for atoms, ions and isotopes The charge implies electrons have been gained or lost and the atom is now an ion  + This is also known as the complete chemical symbol or it can be written as an isotopic name: Potassium-39  Mass number

Atomic Masses Elements occur in nature as mixtures of isotopes Carbon =98.89% 12C 1.11% 13C <0.01% 14C How can you represent all the isotopes in one mass value? 98.89 (12)= 1.11 (13)= .01 (14)= 1186.6amu + 14.4amu 1201.2amu .14amu 100 Remember, any given sample can have this abundance of isotopes. 1. Multiply the abundance of the element by the element’s mass 2. Add the masses and divide by 100 Carbon atomic mass = 12.01 amu

In the beginning… The big bang many particles were created and they would randomly collide and stick together. What if two protons came together? Neutrons are added as glue for the nucleus. If the balance of protons to neutrons (not always equal numbers) was not right the nucleus becomes unstable and radioactive and begins to spit off particles and/or energy to achieve a balance. These emissions are called radiation

Three Kinds of Nuclear Radiation Alpha decay: An alpha particle or helium nucleus is emitted from the nucleus. Beta decay: A beta particle or electron from the decay of a neutron is emitted from the nucleus. Gamma decay: A burst of gamma electromagnetic radiation is emitted from the nucleus.

Alpha Particles α Alpha particles contain 2p+ and 2n0 just like a helium nucleus. It’s a big particle compared to others and carries the lowest energy of the three. This kind of radiation can be blocked by paper or clothes.

Beta Particle β A Beta is an electron that is released from the nucleus. The electron comes from a neutron decaying. It breaks apart into a proton and an electron. The electron leaves and the proton stays behind. Beta radiation has more energy than alpha and needs at least 1 cm of aluminum or similar material to stop the radiation.

Gamma Radiation γ Gamma radiation is an electromagnetic wave with a very short wavelength and a very high frequency. It has very high energy. You would need 6 ft of concrete or 6 inches of lead to stop gamma rays.

Both interactions create large amounts of Energy!!!! Nuclear Reactions The nuclei can interact in two ways Fusion : Two nuclei come together and fuse to make a bigger nucleus. Fission : A large nucleus breaks apart and makes two or more smaller nuclei. Both interactions create large amounts of Energy!!!!

Half life How long does it take for a radioactive substance to spit off radiation and finally come to a stable nucleus (the right number of protons to neutrons)? The time it takes for half of a sample of radioactive material to become stable is called a Half-life. Half-life: The amount of time it take half the sample to become stable and safe.

Calculating half-life 1. Time gone by = # of half-lives gone by half-life 2. Cut sample in half for every half-life gone by. 3. If given an ending sample, multiply it by 2 for every half life gone by.

Calculations of half-lives How much carbon-14 would remain after 22,920 years if there was a 500 kg sample to begin with? Half life of carbon-14 is 5,730 years (this information would be provided or found in a book) Calculate how many half-lives have gone by: 22,920 years divided by 5,730 years = Cut the sample in half 4 times 500kg 4 250 kg 1 half-life 125 kg 2 half-life 3 half-life 62.5 kg 4 half-life 31.25 kg