1. Writing Elemental Symbols Distinguishing Among Atoms

2. Introduction We all know that individual elements are different from all other elements. Oxygen is different from carbon. Carbon is different from gold. John Dalton’s Atomic Theory stated that: “Atoms of the same element are identical and the atoms of any one element are different from the atoms of any other element.”

3. Atomic Number We now know that elements are different from each other because they contain different numbers of protons. The number of protons, p +, in the nucleus of an atom is called the atomic number. Each element has its own atomic number: hydrogen has an atomic number of 1 helium has an atomic number of 2 lithium has an atomic number of 3 uranium has an atomic number of 92

4. Mass Number In addition to the atomic number, each atom of an element has a mass number. Most of the mass of an atom is in the nucleus. The nucleus is composed of protons and neutrons, n 0. The mass number of an atom is the sum of the protons and the neutrons in the nucleus. hydrogen usually has 1 p + and 0 n 0 : ➠ mass number = 1 carbon usually has 6 p + and 6 n 0 : ➠ mass number = 12

5. Element Symbols The symbols are a shorthand way of telling us what element we are dealing with. Most symbols are based on the common name of the element: H for hydrogen He for helium O for oxygen C for carbon N for nitrogen

6. Element Symbols Some of the symbols are based on historical, usually latin, names: Pb for lead (from plumbum) Fe for iron (from ferrum) Au for gold (from aurum) Na for sodium (from natrium) K for potassium (from kalium) W for tungsten (from wolfram [german])

7. Element Symbols When we are using the symbols to talk about an element in a chemical reaction, we most often use the symbol by itself. Sometimes, we want to know the atomic number and the mass number of the element we are working with. In that case, we use the symbol with the atomic number and the mass number.

8. Element Symbols First, we write the symbol. C Next, we write the atomic number in the lower left. 6 Next, we write the mass number in the upper left. 12 This is the complete element symbol

9. Isotopes John Dalton felt that all atoms of a particular element were “identical.” He was wrong. Individual atoms of an element all have the same atomic number, the same number of protons. Individual atoms of an element sometimes have different mass numbers. These are called isotopes. Isotopes of an element have the same number of protons but a different number of neutrons.

10. Isotopes For example, most carbon atoms have 6 protons and 6 neutrons; ➠ mass number = 12. C 6 12

11. Isotopes C 6 12 However, some carbon atoms have 6 protons and 7 neutrons; ➠ mass number = 13. C 6 13

12. Isotopes For example, most carbon atoms have 6 protons and 6 neutrons; ➠ mass number = 12. C 6 12 However, some carbon atoms have 6 protons and 7 neutrons; ➠ mass number = 13. C 6 13 And, other carbon atoms have 6 protons and 8 neutrons; ➠ mass number = 14. C 6 14

13. Counting When we use the elemental symbols, we can count the number of protons, neutrons, and electrons in an atom. The number of protons is the same as the atomic number. The number of neutrons is the same as the mass number minus the atomic number. The number of electrons is the same as the number of protons in a neutral atom. Ions have a different number of electrons than protons and we will cover those later.

14. Counting So, the number of p + is the atomic number. C The mass number is the number of p + plus the number of n 0. 6 The number of e - is the same as the number of p +. 12 # of p + # of p + plus # of n 0 # of e -

15. Counting Let’s do an example. P Find the number of p +, n 0, and e - in phosphorus-31. 15 The number of p + is the same as the atomic number. 31 # p + = 15 The number of e - is the same as the atomic number. # e - = 15 The number of n 0 is the mass number minus the atomic number. # n 0 = 31 - 15 = 16

16. Counting Let’s do another example. Pt Find the number of p +, n 0, and e - in platinum-195. 78 The number of p + is the same as the atomic number. 195 # p + = 78 The number of e - is the same as the atomic number. # e - = 78 The number of n 0 is the mass number minus the atomic number. # n 0 = 195 - 78 = 117

17. Counting Let’s do another example. U Find the number of p +, n 0, and e - in uranium-238. 92 The number of p + is the same as the atomic number. 238 # p + = 92 The number of e - is the same as the atomic number. # e - = 92 The number of n 0 is the mass number minus the atomic number. # n 0 = 238 - 92 = 146

18. Counting Let’s do another example. K Find the number of p +, n 0, and e - in potassium-40. 19 The number of p + is the same as the atomic number. 40 # p + = 19 The number of e - is the same as the atomic number. # e - = 19 The number of n 0 is the mass number minus the atomic number. # n 0 = 40 - 19 = 21