Unit 2 Notes PDF

What do we know about atoms? 1) What are atoms made of? (parts How do you of the atom) know?????? 2) How big (or small) are atoms? 3) Draw a picture of a simple atom in your notebook. We have come a long way in the understanding of our world. Going back to famous Greek philosoph...

What do we know about atoms? 1) What are atoms made of? (parts How do you of the atom) know?????? 2) How big (or small) are atoms? 3) Draw a picture of a simple atom in your notebook. We have come a long way in the understanding of our world. Going back to famous Greek philosophers (500-600BC), humans questioned how our universe works. The original notion was the earth was made of Earth, Wind, Fire and Water Around 400 BC there was one philosopher who came forward with a new notion of the universe. Democritus was the first to say the world is built by smaller particles. He called these particles “atoms”. His ideas were not received with open arms. 1. All elements are composed of tiny indivisible particles called atoms. 2. Atoms of the same elements are identical. 3. Atoms of different elements can physically mix or chemically combine together in whole number ratios. 4. Atoms cannot be subdivided, created or destroyed. 5. Chemical reactions occur when atoms are separated, joined or rearranged. Atoms of one element never change into other elements.  Section 1 The Atom: From Chapter 3 Philosophical Idea to Scientific Theory Foundations of Atomic Theory The transformation of a substance or substances into one or more new substances is known as a chemical reaction. Law of conservation of mass: mass is neither created nor destroyed during ordinary chemical reactions or physical changes  Section 1 The Atom: From Chapter 3 Philosophical Idea to Scientific Theory Foundations of Atomic Theory, continued Law of definite proportions: a chemical compound contains the same elements in exactly the same proportions by mass regardless of the size of the sample or source of the compound Law of multiple proportions: if two or more different compounds are composed of the same two elements, then the ratio of the masses of the second element combined with a certain mass of the first element is always a ratio of small whole numbers  How small is an atom? In one copper penny there are 6.24 x 1022 atoms To compare the earth’s population is only 6 x 109 people If you put 100 000 000 copper atoms side by side they would make a line only 1 cm long. Why can’t we see Atoms? Dalton Lied!!! Well not exactly……his theories were investigated further. Atoms have since been shown to be able to be divided into smaller particles. (subatomic particles) Atoms are made of three smaller, subatomic particles. Protons, Neutrons and Electrons Electrons are the negatively charged subatomic particles. JJ Thomson (English physicist) discovered electrons in 1897. He discovered these by using a cathode ray tube. Passing electric current through gases, then observing how the electric beam reacts to a magnet. Thomson tried experimented with different gases in the tube with the same results. Thus concluded electrons were apart of all atoms. Afterwards Millikan found the charge and mass of an electron about 1/1840 of a proton. Video In the 1900s scientist continued studying nuclear chemistry. They were using particle accelerators to bombard atoms. In 1932 James Chadwick was able to detect a neutral charged subatomic particle with the same mass of a proton. This particle was found to be the neutron Now we have Protons (+ charge), Neutrons (0 charge), and Electrons (- charge). Where do all these pieces fit? In 1911 Ernest Rutherford from Manchester, England came up with the Gold Foil Experiment. Ernie fired Alpha Particles (double positive charged) at a piece of gold foil. In 1911 Ernest Rutherford from New Zealand came up with the Gold Foil Experiment. Ernie fired Alpha Particles (double positive charged) at a piece of gold foil. Gold Foil Experiment Gold Foil Experiment Discovery of the Atomic Nucleus The results of their gold foil experiment led to the discovery of a very densely packed bundle of matter with a positive electric charge. Rutherford called this positive bundle of matter the nucleus.  Unit 2 Composition of the Atomic Nucleus Except for the nucleus of the simplest type of hydrogen atom, all atomic nuclei are made of protons and neutrons. A proton has a positive charge equal in magnitude to the negative charge of an electron. Atoms are electrically neutral because they contain equal numbers of protons and electrons. A neutron is electrically neutral.  Unit 2 The nuclei of atoms of different elements differ in their number of protons and therefore in the amount of positive charge they possess. Thus, the number of protons determines that atom’s identity.  Unit 2 Atomic Number Atoms of different elements have different numbers of protons. Atoms of the same element all have the same number of protons. The atomic number of an element is the number of protons of each atom of that element.  Visual Concepts Chapter 3 Atomic Number  Section 3 Counting Atoms Chapter 3 Isotopes Isotopes are atoms of the same element that have different masses. The isotopes of a particular element all have the same number of protons and electrons but different numbers of neutrons. Most of the elements consist of mixtures of isotopes.  Section 3 Counting Atoms Chapter 3 Mass Number The mass number is the total number of protons and neutrons that make up the nucleus of an isotope.  Visual Concepts Chapter 3 Mass Number  Section 3 Counting Atoms Chapter 3 Designating Isotopes Hyphen notation: The mass number is written with a hyphen after the name of the element. uranium-235 Nuclear symbol: The superscript indicates the mass number and the subscript indicates the atomic number.  Section 3 Counting Atoms Chapter 3 Designating Isotopes, The number of neutrons is found by subtracting the atomic number from the mass number. mass number − atomic number = number of neutrons 235 (protons + neutrons) − 92 protons = 143 neutrons Nuclide is a general term for a specific isotope of an element.  Unit 2 Designating Isotopes, continued Sample Problem A How many protons, electrons, and neutrons are there in the following: 37 atom of chlorine-37? Cl 17 Atom of Chlorine – 34? 235 atom of Uranium – 235 ? U Atom of Uranium – 230? 92  Ions Ions are when elements gain (anions, negative charge) or lose (cations, positive charge) electrons. The element stays the same, so the number of the protons also stay the same! 1) Complete the table below Atom Name Atomic Atomic # Protons # # Mass Nuclear (hyphen Symbol Number electron Neutrons Number Symbol notation) Fe+2 30 Silver -108 46 K-1 40 -1 19 K 18 18 40  Nuclear Chemistry Chapter 21  Nuclear Chemistry Nuclear Chemistry Deals with the study of the nucleus. This deals with atoms protons, neutrons and their existence.  Types of Radiation Radiation Alpha particles Beta particles Gamma particles Radiation occurs when a nucleus is unstable and must alter is components to obtain stability. When we are looking at any type of radiation or radioactive decay we must remember that matter is conserved!  Why Radiation? What happens when positive and negative charges are near each other? What happens when the same charges are near each other? What holds the electrons to the atom? What is contained in the nucleus?  Coulombs Law q = charge r = radius (or distance) k =constant No calculations just qualitative  Nuclear Stability Isotopes with low fig 28.6 atomic numbers – Stable ratio is 1 neutron to 1 proton Isotopes with high atomic numbers – Stable ratio is 1.5 neutrons to 1 proton This creates the band of stability  Unit 2 Forces in the Nucleus When two protons are extremely close to each other, there is a strong attraction between them. The short-range proton-neutron, proton-proton, and neutron-neutron forces that hold the nuclear particles together are referred to as nuclear forces. The nuclear force attraction stabilizes the “+” “+” repulsion in the atom  Nuclear Stability Remember Nuclear fig 28.6 Forces. Protons have repulsive forces. Close distance between protons and neutrons keep the nucleus together.  Nuclear Stability and Decay Unstable isotopes will undergo decay to achieve a more stable ratio of neutrons to protons The type of decay depends on the ratio of neutrons to protons – Too many neutrons. Turn neutrons to protons. (beta decay). Or the nucleus loses mass. (alpha decay) – Too many protons, the atom captures electrons and turn protons to neutrons, this emits a positron  Alpha particles Helium nuclei – Contains 2 protons and 2 neutrons – Net charge of +2 – Has a mass of 4 amu High mass limits penetrability – Looks like: 42He or α  Alpha particles in a reaction Alpha radiation is emitted from U-238 238 92 U →23490Th + 42He Is matter conserved? Yes! Now you try! Alpha radiation is emitted from Rn-222 222 218 4 86 Rn → 84 Po+ 2 He Is matter conserved? Yes  Beta particles Fast moving electrons formed by the decomposition of a neutron in to a proton and the fast moving electron They have a negligible mass – Consequently they are more penetrating than alpha particles They have a charge of -1  Beta particles in a reaction The general reaction 1 1 0 0 n→ 1 H + -1 e What does that mean? A neutron is converted into a proton So, the mass number remains the same but the atomic number increases by one  Beta particles in a reaction C-14 is a beta emitter, show the decay process 146C →147N + 0-1e Is matter conserved? Yes! Now you try 4019K → 4019K → 4020Ca + 0-1e  Gamma Rays High energy electromagnetic radiation given off by a radioisotope Often emitted with alpha and beta particles Gamma rays have no mass and no charge, so they do not alter the atomic number or the mass number Gamma particles have the largest penetration ability  Gamma particles in a reaction 230 226 4 90 Th→ 88 Ra + 2 He + γ – When the alpha particle is released a huge amount of energy is also released (the gamma particle)!  Is the following Alpha emission or beta emission? 234 0 234 Th --→ B + Pa 90 -1 91 238 4 234 92 U --→ 2 He + Th 90  Warm Up Astatine – 210 goes through alpha decay, beta decay and alpha decay in that order to become stable. Write the reactions that represent this process.  Radioactive All nuclei with atomic numbers greater than 83 are radioactive These nuclei have both too many neutrons and too many protons to be stable – So most undergo decay Most emit alpha particles,  Fission When some radioactive nuclei are bombarded with neutrons they undergo splitting of a nucleus into smaller fragments called fission Neutrons are released from fission reaction, creating a chain reaction  Nuclear Fission & Energy Nuclear fission can release an enormous amount of energy – Ex. Fission of 1kg of U-235 release the same amount of energy as 20,000 tons of dynamite Energy is calculated by E=mc2 E = Energy m = difference of mass c = speed of light In fusion and fission a very small amount of mass of is converted to energy and visa versa.  Nuclear Fusion Fusion occurs when nuclei combine to produce a nucleus of greater mass Only take place at temps greater than 40,000,000˚C Ex. 4 1 1 H + 2 0 -1 e → 4 2 He + energy  Half Life Half life - the amount of time it takes for ½ of a substance to decay. Example - In one half life, 500g of a substance will leave 250g behind. A second half life will leave 125g. A third ½ life will leave 62.5g.  Half Life Sample Problem. A substance has a half life of 10 years. How much substance will remain after 30 years if we start with 100g?