Atoms and Their Isotopes Why? Atoms and isotopes are identified by the number of protons, neutrons, and electrons that they contain. Before you can understand the properties of atoms, how atoms combine to form molecules, and the properties of molecules, you must be familiar with the number of protons, neutrons and electrons associated with atoms. Information From the perspective of a chemist, the entire world is composed of atoms, and atoms are composed of protons, neutrons, and electrons. Protons and neutrons are 2000 times heavier than an electron. A proton has a charge of +1, a neutron has no charge and an electron has a charge of -1. The nucleus is very dense and very small compared to the entire atom. The properties of atoms are determined by the number of protons, neutrons, and electrons that they contain. Atoms with the same number of protons but different number of neutrons are called isotopes of an element. The isotopic notation for an atom includes the following information: symbol of the element the element s atomic number (Z) which specifies the number of protons in the nucleus, and the mass number (A) which indicates the number of protons plus neutrons in the nucleus. [The number of electrons in a neutral atom is equal to the number of protons in the nucleus of the atom. The mass contributed by the electrons in an atom is very small, so it is not included when calculating the mass number.] A Z X Atomic Symbol Notation Mass Number Atomic Symbol Atomic Number Subatomic Particles Particle Symbol Relative Charge Absolute Mass Relative Mass Electron e - -1 9.109 x 10-31 kg 0 Proton P + +1 1.673 x 10-27 kg 1 Neutron n 0 0 1.675 x 10-27 kg 1
Model 1: Two Isotopes of Sodium The diagrams below show representations of sodium isotopes. [Note: the diameter of an atom is about 10,000 times larger than the diameter of the atomic nucleus so the relative sizes of the atom and the nucleus are not accurately depicted in these diagrams.] Isotope 1 23 Nucleus a tiny dot Na (11 protons, 12 neutrons) 11 11 electrons Isotope 2 24 Nucleus a tiny dot Na (11 protons, 13 neutrons) 11 11 electrons Key Questions 1. What information is provided by the atomic number, Z? 2. What information is provided by the mass number, A? 3. What is the relationship between the number of protons and the number of electrons in an atom? 4. Because of the relationship between the number of protons and number of electrons in an atom, what is the electrical charge of an atom? 5. Where are the protons and neutrons located in an atom? 6. What do the two sodium isotopes shown in the model on page 2 have in common with each other?
7. How do the two sodium isotopes shown in the model differ from each other? 8. What distinguishes an atom of one element from an atom of another element? Further Information Up until now we have only dealt with atoms that are neutrally charged as the result of their having the same the same number of protons (+ charge) and electrons ( charge). But when certain atoms interact with each other or enough energy is added to an atom, the atom may form an ion. Ions are atoms that either have a positive or a negative charge. The atom becomes electrically charged without changing the type of atom. As a result, ions can be formed only by gaining or losing electrons, which have a negative charge. If you changed the number of positive charges, protons, in an atom you would form a different type of atom (element). Since neutrons have no overall charge, changing their number would not form an ion, only a different isotope. Depending on how many electrons are lost or gained, will determine the size of the ionic charge which is based upon the difference between the number of protons and electrons. The ionic charge is shown in the upper right corner of the Atomic Symbol Notation. Atom 1: 59 Protons (p + ) = 27 Atom 2: Proton (p + ) = 9 Co Neutrons (n 0 ) = 32 19 F Neutrons (n 0 ) = 10 Electrons (e - ) = 27 Electrons (e - ) = 9 Ion 1: 59 Protons (p + ) = 27 Ion 2: Protons (p + ) = 9 Co 3+ Neutrons (n 0 ) = 32 19 F 1- Neutrons (n 0 ) = 10 Electrons (e - ) = 24 Electrons (e - ) = 10 Key Questions 9. Ions are only formed by changing the number of. 10. What happened to number of electrons in an atom when a negatively charged ion is formed? 11. What happened to number of electrons in an atom when a positively charged ion is formed? 12. An ion is found to contain 15 protons and 18 electrons. Will the ion have a positive or negative charge? Explain.
1. Describe the similarities between 35 Cl, and 37 Cl. 2. Describe the differences between 35 Cl, and 37 Cl. Atoms and Their Isotopes and Ions 3. Write the atomic symbols for two isotopes of uranium, U, one with 143 neutrons and the other with 146 neutrons. 4. Answer the questions below about the isotope to the right. Note: This is only the nucleus of the isotope. a) What is the atomic number? c) What element is it? b) What is the mass #? d) Draw the symbol for the isotope. 5. In the circles provided draw the nucleus of each of these isotopes a) Helium-4 b) carbon-12 c) neon-19 d) Lithium-7 e) oxygen 16 f) Boron-10 6. Using the circles to the right, show how the carbon-12 isotope differs from the carbon-13 isotope. 7. a) How does the mass of a proton compare to the mass of an electron? b) How does the mass of a proton compare to the mass of a neutron? c) What parts of an atom combine to give you the atomic mass? d) What is the atomic mass of carbon?
8. Use your periodic table to fill in the missing information in the following table. Name Symbol Number of Protons Mass Number Number of Neutrons Number of Electrons Ionic Charge oxygen 16 O 8 16 8 8 0 7 7-3 34 S 18 18 1 1 0 hydrogen 3 0 12 24 10 12 25 +2 197 76 +3 84 Kr 84 36