1. Chapter 3: Chemical Foundations, Elements, Atoms, and Ions The atom, Compounds and Average Atomic Mass

2. Learning Targets You will learn about Dalton’s theory of atoms. You will learn about important features and experiments discovering the atomic structure. You will learn how chemical formulas represent compounds. You will learn how to calculate the average atomic mass of an element.

3. Dalton’s Atomic Theory Most natural materials are mixtures of pure substances (either elements or compounds) Given compound always contains same proportions (by mass) of elements - “Law of constant composition”. Explanation of these observations known as Dalton’s atomic theory.

4. Dalton’s Atomic Th. Cont. 1. Elements are made of tiny particles called atoms 2. All atoms of a given element are identical 3. Atoms of one element differ from atoms of another element

5. Dalton’s Atomic Th. Cont. 4. Atoms of one element can combine with atoms of other elements to form compounds. A given compound always has the same relative numbers and types of atoms 5. Atoms are indivisible in chemical processes. CANNOT be created or destroyed in reactions, only grouped together differently

6. Formulas of Compounds A chemical formula represents the types of atoms and the number of each type of atom in a compound. (H 2 O, CaCl 2, NaNO 3 ) When a compound is formed between ions, the compound must be neutral in charge. This affects the numbers of each type of atom.

7. Rules for Writing Formulas 1. Each atom present is represented by its element symbol. 2. The number of each type of atom is indicated by a subscript written to the right of the element symbol. 3. When only one atom of a given type is present, the subscript 1 is not written.

8. Formulas of Compounds-Practice Write the formula for each of the following compounds, listing the elements in the order given: –Each molecule of a compound that has been implicated in the formation of acid rain contains one atom of sulfur and three atoms of oxygen. Each molecule of a certain compound contains two atoms of a nitrogen and five atoms of oxygen.Answers: SO 3 N2O5N2O5N2O5N2O5

9. Formulas of Compounds-Practice The pairs of ions contained in several ionic compounds are listed below. Give the formula for each compound: –Ca 2+ and Cl - –Na + and S 2- –Ca 2+ and P 3- Answers: CaCl 2 Na 2 S Ca 3 P 2

10. Formulas of Compounds-Try on own Write the formula for each of the following compounds, listing the elements in the order given. –A molecule contains four phosphorus atoms and ten oxygen atoms. –A molecule contains one uranium atom and six fluorine atoms. – A molecule contains one aluminum atom and three chlorine atoms.Answers: P 4 O 10 UF 6 AlCl 3

11. Formulas of Compounds-Try on own Use the pairs of ions below to give the formula for the compound containing these ions. –Ba 2+ and O 2- –Al 3+ and S 2- –K + and P 3- Answers:BaO Al 2 S 3 K3PK3PK3PK3P

12. The atom Electron: negatively charged subatomic particle Nucleus: dense, positively charged center of atom Neutron: subatomic particle with zero charge If talking about atom IT IS NEUTRAL! If talking about ions, compounds are being formed.

13. Scientist #1 – J.J. Thomson –Atoms must contain negative particles called electrons (repelled by negative part of electric field) –Atoms must contain positive particles to balance the atom to make the atom a zero overall charge

14. Scientist #2 – William Thomson –The Plum Pudding Model –Uniform positive charge with negative electrons scattered through to neutralize charge. –Think of a batch of chocolate chip cookies - batter similar to the pudding (uniform positive charge) –Chocolate chips like raisins in pudding - negative electrons scattered through to neutralize the charge

15. Scientist #3 – Ernest Rutherford –Changed previous ideas about the atom dramatically –Alpha particles –Some of these particles were deflected during their flight through air. This deflection proved there was a center of positive charge (the nucleus) and it must contain protons –Protons and neutrons believed to be same weight, electrons MUCH smaller

16. Calculating average atomic mass Isotopes exist in different abundances. For example, an element may have 3 isotopes but one exists over 90%, another less than one % and another less than 10% To continue with that example, each isotope has a respective mass with each percent it represents.

17. Calculating average atomic mass So how do we calculate an average atomic mass to put on the periodic table? A problem like this would look like the following: –An element consists of 90.51% of an isotope with a mass of 19.922 amu, 0.27% of an isotope with a mass of 20.994 amu and 9.22% of an isotope with a mass of 21.990 amu. Calculate the average atomic mass and identify the element.

18. Calculating average atomic mass How do you solve this problem to get an average mass? –1. You change your percents to decimals. –2. You multiply the decimal number and its respective mass for each pair of numbers they give. –3. You add those multiplied numbers together, round off 3-4 places to be safe and match up the mass you calculated to an element.

19. Calculating average atomic mass So let’s look at the problem from earlier: –An element consists of 90.51% of an isotope with a mass of 19.922 amu, 0.27% of an isotope with a mass of 20.994 amu and 9.22% of an isotope with a mass of 21.990 amu. Calculate the average atomic mass and identify the element.

20. Calculating average atomic mass 1. You change percents to decimal form 90.51% .9051 0.27% .0027 9.22% .0922 2. You multiply the decimal number and its respective mass for each pair of numbers they give. 90.51% .9051 x 19.922 amu 0.27% .0027 x 20.994 amu 9.22% .0922 x 21.990 amu

21. Calculating average atomic mass 3. You add those multiplied numbers together, round off 3-4 places to be safe and match up the mass you calculated to an element. (.9051 x 19.922 amu = ) +(.0027 x 20.994 amu = ) +(.0922 x 21.990 amu = ) ELEMENT?

22. Calculating average atomic mass It should be Neon (Ne) 20.115564

23. Try on your own The element magnesium has three stable isotopes with the following masses and abundances: IsotopesMass (amu)Abundance Mg-2423.985078.99% Mg-2524.985810.00% Mg-2625.982611.01% Calculate the average atomic mass of Mg.

24. Answer 24.30501576 Can compare with mass on periodic table to make sure correct.

25. Recap What are the components of Dalton’s law of atomic theory? Who contributed to the thoughts about atom structure? What is an atom comprised of? What does a chemical formula represent? How do you calculate the average atomic mass of an element?