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Models for Atoms Chapter 3 •The atom-building blocks of the elements • • • Atoms are the basis of chemistry and the basis of everything in the universe. Remember all matter is composed of atoms! Atom – smallest particle of an element that still retains the properties of that element the atom is a microscopic structure. It is similar to the cell in biology. Like the cell the atom can be broken down into it simplest parts as well. Atomic Structure Atoms can be broken down into electrons, protons, and neutrons. Electrons are negatively charged subatomic particles. Symbol is e- . (J.J. Thompson) Protons are positively charged subatomic particles. (Rutherford) Neutrons are subatomic particles with no charge. (James Chadwick) • An atom refresher An atom has three parts: • Proton = positive • Neutron = no charge • Electron = negative • The proton & neutron are found in the center of the atom, a place called the nucleus. • The electrons orbit the nucleus. • HELIUM ATOM Shell proton + electron N N + - neutron Protons and Neutrons are found in the nucleus, electrons are found in shells outside the nucleus A quick review What scientists believed about the elements Empedoles (~450 BC) • All matter was made of four elements - Earth, Air, Fire and Water A quick review What scientists believed about the elements Democritus (~400 BC) – Matter was made of tiny particles that cannot be broken down further (called atoms) The same element can make up different kinds of substance. A quick review • What scientist believed about the elements Aristotle (~ 350 BC) – A return to the “4-element” model •A quick review • What scientists believed about the elements Alchemists (500 – 1600 AD) – Still believed in the “4 – element” model! Tried to turn cheap metal into gold •A quick review • What scientists believed about the elements Boyle (1650 AD) – • A return to the atom theory… • Believed in the existence of elements Dalton (1808 AD) Developed the atomic model for matter: - All matter is made up of atoms - Each element has it’s own atom and atomic mass - Compounds can be created when atoms from different elements come together to form molecules - Atoms cannot be created, destroyed, or subdivided in chemical changes Faraday (1800’s) – Discovered that an atom can gain an electric charge and form an ion. • Modified Dalton’s model…. - Matter must contain positive and negative charges - Atoms can combine to form molecules because of electrical attraction Thompson (1904) – Discovered electrons!!!! • Developed the “raisin bun” model - Dough → positive charge - Raisins → negative charge - Overall, atoms have a neutral charge - Dalton’s model is now incorrect because atoms are divisible (but only through a nuclear reaction Rutherford (1911) – Experimented using gold foil and discovered that atoms are 1. not solid 2. most of the atom is made up of space 3. have a small center called a nucleus • • • • Ernest Rutherford, through experimentation, discovered the nucleus. The nucleus is the heaviest part of the atom; it’s comprised of both protons and neutrons. Thus the nucleus has a positive charge. The electrons occupy the outside of the nucleus (the majority of volume of the nucleus) Different elements have different numbers of subatomic particles thus giving different elements different characteristics/properties Basics of the Atom Particle Charge Location in the Atom proton 1+ in nucleus 0 in nucleus neutron Mass ~1 a.m.u. ~1 a.m.u. ~0 a.m.u. orbiting electron 1– nucleus a.m.u.: unit used to measure mass of atoms Review Protons Protons are very important because they help determine the identity of an element • They are inside the nucleus • They have a positive charge + • Atom Identity – the number of protons an atom has will determine its identity. Ex: 8 protons = oxygen 19 protons = potassium • Review Neutrons • Found inside the nucleus • Have a neutral charge • Have a relative mass of 1 atomic unit • Important for creating isotopes (coming soon!) Review electrons • Found outside the nucleus • Forms an orbital cloud (the location where an electron might be found) • Has a negative charge – • Different elements have different numbers of subatomic particles thus giving different elements different characteristics/properties • So how do we know how many subatomic particles an element has? • Grab out.... your periodic tables and let’s find Atomic Number: Number of protons and it is also the number of electrons in an atom of an element. . Chemical Symbol of element Elements Name Atomic Mass/Weight: 8 O Oxygen 16 Number of protons + neutrons. Created by G.Baker www.thesciencequeen.net For each element, indicate the: 1. Mass number 2. atomic number 3. # of protons 4. # of electrons a. sulfur b. Calcium c. Iron •Charged Atoms (ions) Why is it dangerous to work with electrical appliances around water? Tap water and rain water (not pure) contain charged atoms that can move in a solution and carry electric current. • These charged atoms are called ions (atoms that have become charged by gaining or losing one or more electrons). • They have a charge because the number of electrons is NOT equal to the number of protons • The first “modern” model of the atom What do we believe now Neils Bohr (1913) – • Electrons move around the nucleus in paths called orbitals • Each electron in an orbit has a definite amount of energy ; further away from nucleus = greater energy • Electrons cannot exist between orbits, but they can move up or down from one orbit to another • Electrons are more stable when they are at lower energy (closer to the nucleus) Excited state – when electrons jump to a higher orbit due to being energized by light, heat or electricity. Ground state – When electrons fall to lower orbits and a more stable state •Bohr Model • The Bohr Model shows all of the particles in the atom. • In the center = write the # of protons and neutrons in the nucleus, using “p” and “n” • In a circle around the nucleus are the electrons. Represent electrons with dots. Created by G.Baker www.thesciencequeen.net •Electrons have special rules…. • You can’t just shove all of the electrons into the first orbit of an electron. • Electrons live in something called shells or energy levels. • Only so many can be in any certain shell. • The order of filling of electrons for the first 4 orbits: 2, 8, 8, 18 Created by G.Baker www.thesciencequeen.net ATOMIC STRUCTURE Electrons are arranged in Energy Levels or Shells around the nucleus of an atom. • first shell a maximum of 2 electrons • second shell a maximum of 8 electrons • third shell a maximum of 8 electrons •So let’s try it…. How to draw a Lithium atom • First, look at the Periodic Table • Second, determine the number of protons (Look @ the atomic number) • Then determine the number of neutrons (Atomic mass – atomic number) • Then determine the number of electrons (Look @ the atomic number) • 3 Li Lithium 7 Protons = 3 3 Li Lithium 7 Electrons = 3 2 in the 1st shell, 1 in the 2nd shell Neutrons = 4 (7-3=4) Created by G.Baker www.thesciencequeen.net ELECTRONIC CONFIGURATION With electronic configuration elements are represented numerically by the number of electrons in their shells and number of shells. For example; Nitrogen 2 in 1st shell 5 in 2nd shell configuration = 2 , 5 2 + 5 = 7 14 7 N ELECTRONIC CONFIGURATION Write the electronic configuration for the following elements; a) 40 20 Ca b) 2,8,8,2 d) 35 17 Cl 2,8,7 23 11 Na c) 16 2,8,1 e) 28 14 Si 2,8,4 O 8 2,6 f) 11 5 B 2,3 Isotopes and Radioisotopes • Isotope – any of two or more forms of an element, each having the same number of protons but having different mass due to a number of neutrons. • Isotopes of the same element have the same physical properties and the same chemical properties • Hydrogen has three common isotopes: Chlorine has two common isotopes: • Cl- 35 (atomic number 17 and mass number 35; 17 protons plus 18 neutrons) • Cl – 37 (atomic number 17 and mass number 37; 17 protons plus 20 neutrons) • • There are three isotopes for Carbon. Let’s draw the isotopes for Carbon using Bohr diagrams: Remember all that has changed is the neutrons and thus the atomic mass. (The number beside Carbon is the new atomic mass). Carbon – 12, Carbon- 13 and Carbon -14 Now write the standard atomic structure for these: Carbon – 12 Carbon- 13 Carbon -14 • • • Some isotopes are unstable, or radioactive, which means that the nucleus has a tendency to break apart and eject very-high-energy particles into its surroundings. The huge amount of energy these particles have can be both dangerous and useful. An atom with an unstable nucleus is called a radioisotope. Decay – splitting apart of unstable nuclei to produce radioactive particles • • • • Around the year 1900, three different kinds of radioactivity were identified: alpha ( ) particles were found to be helium nuclei, containing 2 protons and 2 neutrons beta ( ) particles are high energy electrons gamma rays (Ƴ )- radiation is high – energy electromagnetic radiation with no mass. • • • Radioisotopes can damage living tissue; they may alter the DNA, which affects how cells divide. This can cause serious diseases including cancer and birth defects. However, radioisotopes can be very useful and are useful and are used in smoke detectors and also to diagnose medical problems such as thyroid disease.