CHEM1001 Worksheet 2: Atoms and Ions Model 1: Composition of atoms and atomic symbols An atom is comprised of protons, neutrons and electrons. The nucleus is the centre of the atom, and holds the protons and neutrons. The electrons surround the nucleus in a diffuse cloud. The outermost electrons are called the valence electrons and can be determined by the elements group number (column on the periodic table). Group 1 has 1 valence electron, group 2 has 2 valence electrons, group 17 has 7 valence electrons etc. This picture of a lithium atom is not to scale β if the nucleus were the size of a ping-pong ball, the outer limits of the electron cloud would be the size of a football stadium. Valence electrons P N N N P P N Inner electrons Nucleus Protons have a positive charge (+1) and an atomic mass of 1.0073. Neutrons have 0 charge, and have an atomic mass of 1.0087. Electrons have a negative charge (-1) and an atomic mass of 0.0005. Like charges repel each other, and unlike charges attract each other. In an atom, the charges of the protons and electrons must balance to make it neutral overall. The number of protons, neutrons and electrons in an atom can be determined from the atomic symbol notation, the information for which can be found on the periodic table. A is the mass number: the number of protons and neutrons in the nucleus. Z is the atomic number: the number of protons. X is the atomic symbol. π¨ ππ Critical thinking questions 1. Which particles contribute most to the mass of the atom? 2. What is the A-Z-X notation for the lithium atom in the figure above? 3. Is the Periodic Table ordered by atomic or mass number? Which gives an element its identity? 4. How do you work out the total number of electrons in an atom? 5. How many valence electrons does lithium have? 6. Which do you think is more important for an atoms chemical reactivity β the total number of electrons, or the number of valence electrons? Why? Model 2: Isotopes Atoms of the same element have the same number of protons but not necessarily the same number of neutrons. These are called isotopes. This is why on the periodic table, the mass numbers are not integers β they are an average of all the different isotopes of a given element. Different isotopes have different stabilities, and different abundances on earth. For example, there are 5 isotopes of C, but not all are stable. The most common/useful ones are: ππ ππ is the βnormalβ carbon, ie: the most abundant at ca. 98.93%. ππ ππ is most of the rest at ca. 1.07%. ππ ππ is used in carbon dating, but is only 0.0000000001% abundant! Critical thinking questions 1. How many neutrons do each of the three carbon isotopes above have? 2. Given 10 million carbon atoms, how many of each isotope will there be? 3. What would the average mass of a carbon atom be? 4. Do you think that different isotopes would have different chemical reactivity? Why or why not? 5. Do you think there is a limit on the number of isotopes an element can have? Why or why not? 6. An element has as many electrons as !"# !"I has neutrons. What is it? Where can you find it in everyday life? Model 3: Ions When an atom loses or gains an electron it becomes charged and is called an ion. A positively charged ion is called a cation. A negatively charged ion is called an anion. The electrons are either lost from, or gained into the valence shell which is the outer region of the atom. The charge of an ion can be added to the atomic symbol notation from section 1. ππ ! ππ π! πππ ππ Critical thinking questions 1. Recalling Model 1, if an atom loses an electron, does it become a cation or an anion? 2. How many electrons have been gained or lost in each of the ions above? 3. How many electrons overall are in each of the ions above? 4. How many valence electrons in each? 5. A particle has 52 protons, 54 electrons and 76 neutrons. Draw the complete atomic symbol notation. Is it a cation or anion? 6. Do you think that an ion would display different chemical reactivity to that of its parent atom? Why? Do you think ions with different charges would display different chemical reactivity? 7. Complete the table. Comment on the number of valence electrons in the ions. Symbol Protons Neutrons Electrons 31 39 28 !! !! !"Sr !" ! !F 11 !" !! !"P 10 Valence Electrons
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