ATOMS and MOLECULES

1. ATOMS and MOLECULES Matteris anything that has mass and volume (occupies space). The amount ofmattermeasured by its resistance to acceleration (movement) Massis a measurement ofmatterpresent. Weightis a measurement ofthe gravitational force acting on an object. El Camino College Chemistry 21A Dr. Dragan Marinkovic

2. ATOMS and MOLECULES ELEMENTS Homoatomicmoleculesof individual atomsof the same kind Cannot be chemically subdivided Into simpler substances. PURE SUBSTANCE Matterthat has constant composition and fixed properties COMPOUNDS Heteroatomic molecules of individual atoms (ions) of two or more kinds Can be chemically subdivided into simpler substances Products of the chemical subdivision are either elements or simpler molecules El Camino College Chemistry 21A Dr. Dragan Marinkovic

3. ATOMS and MOLECULES Symbols and Formulas Anelementis a substance that cannot be broken down into a simpler form capable of an independent existence as observable matter. Anelementis homogeneous pure substance made up of identical atoms. 115 differentelementsknown 115 different kinds ofatoms ΑΤΟΜΟΣ 88elementsfound in nature 27elementsman made The word"atom"comes from the Greek a-tomos, meaning "un-cut-able" El Camino College Chemistry 21A Dr. Dragan Marinkovic

4. ATOMS and MOLECULES Symbols and Formulas Elements you already know: Gold Copper Oxygen Helium Iron Mercury Aluminum Hydrogen Lithium Neon Calcium Fluorine Sodium Chlorine Potassium Silver El Camino College Chemistry 21A Dr. Dragan Marinkovic

5. ATOMS and MOLECULES Symbols and Formulas Elements you already know: Gold Au Copper Cu Oxygen O Helium He Iron Fe Mercury Hg Aluminum Al Hydrogen H Lithium Li Neon Ne Calcium Ca Fluorine F Sodium Na Chlorine Cl Potassium K Silver Ag Latin: aurum Latin: cuprum Latin: ferrum quicksilver hydrargyrum Latin natrium Latin: kalium Latin: argentum El Camino College Chemistry 21A Dr. Dragan Marinkovic

6. ATOMS and MOLECULES El Camino College Chemistry 21A Dr. Dragan Marinkovic

7. ATOMS and MOLECULES Symbols and Formulas Compoundis a pure substance consisting of two or more kinds of atoms in the form of heteroatomic molecule of individual atoms. El Camino College Chemistry 21A Dr. Dragan Marinkovic

8. ATOMS and MOLECULES Symbols and Formulas Compoundis a pure substance consisting of two or more kinds of atoms in the form of heteroatomic molecule of individual atoms. Compound formulais a symbol for the molecule of a compound, consisting of the symbols of the atoms found in the molecule. H2O CO CO2 NaCl C3H8 propane C9H8O4 aspirin C5H8NNaO4 monosodium glutamate (MSG) El Camino College Chemistry 21A Dr. Dragan Marinkovic

9. ATOMS and MOLECULES Inside the Atom He, Helium Extensive experimental evidence (since 19th century) indicates that atoms are made up of many smaller particles.More than 100 subatomic particles have been discovered. Most of the chemical behaviour of matter can be explained with the three fundamental particles: PROTON, NEUTRON and ELECTRON. 1Å = 1 x 10-8 m Every atom has two basic parts: nucleus (core) “shell” (“electron cloud”) He 2p+ 2n 2e- El Camino College Chemistry 21A Dr. Dragan Marinkovic

10. ATOMS and MOLECULES Inside the Atom Extensive experimental evidence (since 19th century) indicates that atoms are made up of many smaller particles. More than 100 subatomic particles have been discovered. Most of the chemical behaviour of matter can be explained with the three fundamental particles: PROTON, NEUTRON and ELECTRON. Atoms are composed of protons, neutrons, and electrons, whose properties are shown below: El Camino College Chemistry 21A Dr. Dragan Marinkovic

11. ATOMS and MOLECULES Inside the Atom 9Be (Berillium) 4 protons 4 electrons 5 neutrons U.S. Atomic Energy Commission Shield El Camino College Chemistry 21A Dr. Dragan Marinkovic

12. ATOMS and MOLECULES Isotopes He, Helium ATOMIC NUMBER, Z,is equal to the number of protons in the nucleus of an atom. MASS NUMBER, A,is equal to the sum of protons and neutrons in an atom. 1Å = 1 x 10-8 m p = ? n = ? e = ? He 2p+ 2n 2e- Z = 2 A = 4 (2p + 2n) El Camino College Chemistry 21A Dr. Dragan Marinkovic

13. ATOMS and MOLECULES Isotopes The mass spectrum of magnesium shows three isotopes of masses 24 through 26 having the natural abundances shown on the plot. Of the 92 natural elements, 23 have only a single isotope, and are said to be monoisotopic. ATOMIC NUMBER, Z,is equal to the number of protons in the nucleus of an atom. MASS NUMBER, A,is equal to the sum of protons and neutrons in an atom. ISOTOPES are atoms that have the same atomic number (number of protons) but different mass numbers (number of neutrons). Greek isos = "equal", tópos = "site, place" El Camino College Chemistry 21A Dr. Dragan Marinkovic

14. ATOMS and MOLECULES Isotopes El Camino College Chemistry 21A Dr. Dragan Marinkovic

15. ATOMS and MOLECULES Relative Masses of Atoms and Molecules Relative massesprovide a simple way of comparing the masses of atoms. µ The unified atomic mass unit (µ)is defined as 1/12 of the mass of one atom of carbon-12, (1.661 x 10-24 g) The atomic masses of other elements can be converted in a similar way The measured atomic mass of a 31P atom is 5.143 x 10-23 g 5.143 x 10-23 g Conversion to atomic mass units gives -------------------------- = 30.974 µ 1.661 x 10-24 g 31P atom is 30.974 µ El Camino College Chemistry 21A Dr. Dragan Marinkovic

16. ATOMS and MOLECULES Relative Masses of Atoms and Molecules Moleculesare made of atoms, so the relative mass of moleculecan be calculated by adding together the atomic weights of the atoms that make up the molecule. MOLECULAR WEIGHT is the relative mass of a molecule expressed in atomic mass units (µ) and calculated byadding together theatomic weights of the atoms that make up the molecule. El Camino College Chemistry 21A Dr. Dragan Marinkovic

17. ATOMS and MOLECULES Relative Masses of Atoms and Molecules Moleculesare made of atoms, so the relative mass of moleculecan be calculated by adding together the atomic weights of the atoms that make up the molecule. MOLECULAR WEIGHT is the relative mass of a molecule expressed in atomic mass units (µ) and calculated byadding together theatomic weights of the atoms that make up the molecule. What is the molecular weight of sulfuric acid, H2SO4? Solution: The atomic weights of hydrogen and of oxygen are 1.01 and 16.00, respectively From a table, you can find that the atomic weight of sulfur is 32.06. Adding everything up, we have 2 x H S 4 x O (2 x 1.01) + 32.06 + (4 x 16.00) = 98.08 The molecular weight of sulfuric acid, H2SO4 is 98.08 El Camino College Chemistry 21A Dr. Dragan Marinkovic

18. ATOMS and MOLECULES Isotopes and Atomic Weights For elements with more than oneisotope, theatomic mass (atomic weight)reported in the periodic table represents a weighted average of the atomic massesof the naturally occurringisotopes In other words, it’s the weight of an average atom For example: 35Cl (75.53 % abundance) has atomic mass = 34.97 µ 37Cl (24.47 % abundance) has atomic mass = 36.97 µ (75.53)(34.97 µ) + (24.47)(36.97 µ) Atomic mass = ------------------------------------------------- = 35.45 µ 100 El Camino College Chemistry 21A Dr. Dragan Marinkovic

19. ATOMS and MOLECULES Isotopes and Atomic Weights Magnesium: 24Mg (78.991 %); 23.9850 µ 25Mg (10.002 %); 24.9858 µ 26Mg (11.008 %); 25.9826 µ (78.991%)(23.9850 µ) + (10.002%)(24.9858 µ) + (11.008%); 25.9826 µ Atomic mass = ------------------------------------------------------------------------------- = 100 Atomic mass of Mg = 24.3052 µ El Camino College Chemistry 21A Dr. Dragan Marinkovic

20. ATOMS and MOLECULES Avogadro’s Number: the Mole Atoms and molecules cannot be counted by direct observation because of their tiny size. If we consider a weight of substance that is the same as its formula (molecular) weight expressed in grams, we have only one number to know: Avogadro's number, 6.022137 × 1023, usually designated by NA. Themole(abbreviatedmol)is the the SI measure of quantity of a "chemical entity", which can be anatom, molecule, formula unit, electron or photon. One molof anything is just Avogadro's number of that something. Or, if you think like a lawyer, you might prefer the official SI definition: The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12. El Camino College Chemistry 21A Dr. Dragan Marinkovic

21. ATOMS and MOLECULES Avogadro’s Number: the Mole • The Avogadro constant can be applied to any substance. • It corresponds to the number of atoms or molecules • needed to make up a mass equal to the substance's • atomic or molecular mass, in grams. • For example, the atomic mass of iron is 55.847 g/mol, • so NA iron atoms (i.e., one mole of iron atoms) have a • mass of 55.847 g. • Conversely, 55.847 g of iron contains NA iron atoms. • The Avogadro constant also enters into the definition • of the unified atomic mass unit, µ: • 1 1 • 1 µ = ------ = ----------------------- = 1.661 x 10-24 g • NA6.022 × 1023 El Camino College Chemistry 21A Dr. Dragan Marinkovic

22. ATOMS and MOLECULES Avogadro’s Number: the Mole 1 mol S atoms = 6.02 x 1023 S atoms = 32.1 g S 1 mol Cu atoms = 6.02 x 1023 Cu atoms = 63.5 g Cu El Camino College Chemistry 21A Dr. Dragan Marinkovic

23. ATOMS and MOLECULES Avogadro’s Number: the Mole The mole concept can be applied to molecules instead of atoms. • What is the number of: • Moles, and • b) molecules • in 3 oz of table salt, sodium chloride (NaCl)? El Camino College Chemistry 21A Dr. Dragan Marinkovic

24. ATOMS and MOLECULES Avogadro’s Number: the Mole The mole concept can be applied to molecules instead of atoms. • What is the number of: • Moles, and • b) molecules • in 3 oz of table salt, sodium chloride(NaCl)? Molecular mass (weight) of NaCl is 22.99 + 35.45 = 59.44 µ 1 mole of NaCl = 59.44 g 3 oz NaCl = 3 x 28.35 = 85.05 g a) 85.05 g of NaCl = 85.05 g/59.44 g = 1.4553 moles of NaCl b) Number of molecules of NaCl = 1.455 x 6.022x1023 = 8.762x1023 El Camino College Chemistry 21A Dr. Dragan Marinkovic

25. ATOMS and MOLECULES The Mole and Chemical Formulas Chemical formulasrepresent the numerical relationships among theatoms in a compound. 2CO2 = 2C + 4O 44.0 g of CO2 contains 12.0 g of C and 32.0 (2x16.0) g of O. Find the masses of carbon, hydrogen and oxygen in one mole of ethanol (C2H5OH). Solution: Using the atomic weights (molar masses) of these three elements, we have carbon: (2 mol)(12.0 g mol–1) = 24.0 g of C hydrogen: (6 mol)(1.01 g mol–1) = 6.0 g of H oxygen: (1 mol)(16.0 g mol–1) = 16.0 g of O El Camino College Chemistry 21A Dr. Dragan Marinkovic

26. ATOMS and MOLECULES The Mole and Chemical Formulas Find the mass fraction and mass percent oxygen in ethanol (C2H5OH). Solution: Using the information developed in the preceding example, the molar mass of ethanol is (24.0 + 6.0+ 16.0)g mol–1 = 46.0 g mol–1. Of this, 16.0 g is due to oxygen, so its mass fraction in the compound is (16.0 g)/(46.0 g) = 0.35 which corresponds to 35%. El Camino College Chemistry 21A Dr. Dragan Marinkovic

27. ATOMS and MOLECULES El Camino College Chemistry 21A Dr. Dragan Marinkovic

28. MATTER El Camino College Chemistry 21A Dr. Dragan Marinkovic