1 The History of Atomic Theory A long and winding road Section 4.1

2

3 The “Atom” The smallest particle of an element that retains its identity in a chemical reaction. The smallest particle of an element that retains its identity in a chemical reaction.

Aristotle Matter is in continuous motion Matter is in continuous motion 4

5 Democritus Greek Philosopher (460 BC- 370 BC); the “laughing philosopher” Greek Philosopher (460 BC- 370 BC); the “laughing philosopher” Atoms are indivisible and indestructible Atoms are indivisible and indestructible Did not explain chemical behavior and lacked experimental support. Did not explain chemical behavior and lacked experimental support.

6 Dalton’s Atomic Theory John Dalton ( ) 1. All elements are composed of tiny indivisible particles called atoms. 2. Atoms of the same element are identical. The atoms of any one element are different from the atoms of any other element. 3. Atoms of different elements can physically mix together or can chemically combine in whole-number ratios to form compounds. 4. Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element, however, are never changed into atoms of another element as a result of a chemical reaction.

7 Joseph John Thomson ( ) Adds detail to Dalton’s theory. Adds detail to Dalton’s theory. “Plum pudding model”- negatively charged subatomic particles within positively charged atom. “Plum pudding model”- negatively charged subatomic particles within positively charged atom. Called particles “corpuscles” Called particles “corpuscles” Cathode ray tube Cathode ray tube electrons electrons

8 Robert A. Millikan Nobel Prize in 1923 Conducted an experiment to find the quantity of charge carried by an electron Conducted an experiment to find the quantity of charge carried by an electron Oil-drop experiment Oil-drop experiment

9 Rutherford Model (1911) “the father of nuclear physics” Gold Foil Experiment- positively charged alpha particles aimed at thin sheet of gold foil. Gold Foil Experiment- positively charged alpha particles aimed at thin sheet of gold foil. Proposed that atom is mostly empty space, with positive charge concentrated in the center. Electrons move around the nucleus. Proposed that atom is mostly empty space, with positive charge concentrated in the center. Electrons move around the nucleus.

10 To sum up Rutherford’s Gold Foil Experiment Most of the alpha particles pass through the gold foil Most of the alpha particles pass through the gold foil Few alpha particles were deflected (went off at an angle) Few alpha particles were deflected (went off at an angle) Very few alpha particles were reflected right back Very few alpha particles were reflected right back  Not a lot of charged particles located throughout atom  Particles ran into small amount of charge  Strong concentration of positively charged particles (nucleus)  Not a lot of charged particles located throughout atom  Particles ran into small amount of charge  Strong concentration of positively charged particles (nucleus) RESULTSCONCLUSIONS

11 James Chadwick ( ) February “The possible existence of a neutron” February “The possible existence of a neutron” May “The existence of a neutron” May “The existence of a neutron”

12 The Bohr Model Neils Bohr Proposed that an electron is found in specific circular paths, or orbits, around the nucleus. Proposed that an electron is found in specific circular paths, or orbits, around the nucleus. Each orbit has a fixed energy. Each orbit has a fixed energy. A quantum of energy is the amount of energy required to move an electron from one energy level to another. A quantum of energy is the amount of energy required to move an electron from one energy level to another.

13 Quantum Mechanical Model ( WAVE-MECHANICAL ) Erwin Schrodinger: mathematical equation describing the behavior of the electron in a hydrogen atom Erwin Schrodinger: mathematical equation describing the behavior of the electron in a hydrogen atom. This model determines the allowed energies an electron can have and how likely it is to find the electron in various locations around the nucleus. This model determines the allowed energies an electron can have and how likely it is to find the electron in various locations around the nucleus. “electron cloud” “electron cloud”

14 Atomic Orbitals A region of space in which there is a high probability of finding an electron. A region of space in which there is a high probability of finding an electron. Each energy sublevel corresponds to an orbital of different shape describing where the electron is likely to be found. Each energy sublevel corresponds to an orbital of different shape describing where the electron is likely to be found. More on this later…. More on this later….

15 Atomic Models…a recap

16 Atomic Details Sections 4.2 and 4.3 Sections 4.2 and 4.3

Subatomic Particles ParticleSymbolRelative Charge Relative Mass Actual Mass (g) Electrone-e / x Protonp+p x Neutronn° x

18 Atomic Number Elements are different because they contain a different number of protons Elements are different because they contain a different number of protons Atomic # = # of p + in the nucleus of an atom of that element Atomic # = # of p + in the nucleus of an atom of that element

19 Atoms are ELECTRICALLY NEUTRAL ELECTRICALLY NEUTRAL # of protons = # of electrons # of protons = # of electrons In an electrically neutral atom: In an electrically neutral atom: Atomic # = # of electrons = # of protons Atomic # = # of electrons = # of protons Protons = Electrons = Atomic number Protons = Electrons = Atomic number

20 Mass Number Most of the mass of an atom is concentrated in the nucleus Most of the mass of an atom is concentrated in the nucleus Mass Number = # of p + + # of n o Mass Number = # of p + + # of n o Number of Neutrons= Mass # - Atomic # Number of Neutrons= Mass # - Atomic #

21 What does the Periodic Table tell us about subatomic particles? C C 12 6 Element Symbol- one or two letter abbreviation of element name Mass Number Atomic Number = # of protons

22

23 Isotopes Atoms that have the same number of protons but different numbers of neutrons. Atoms that have the same number of protons but different numbers of neutrons. Because isotopes of an element have different numbers of neutrons, they also have different mass numbers Because isotopes of an element have different numbers of neutrons, they also have different mass numbers Ex. C-12 and C-14 Ex. C-12 and C-14

Atomic Mass Most elements occur as a mixture of two or more isotopes, each with a fixed mass and a natural % abundance. Most elements occur as a mixture of two or more isotopes, each with a fixed mass and a natural % abundance. ATOMIC MASS= the weighted average mass of the atoms in a naturally occurring sample of the element. ATOMIC MASS= the weighted average mass of the atoms in a naturally occurring sample of the element. To calculate the atomic mass of an element, multiply the mass of each isotope by its natural abundance (expressed as a decimal) and then add the products. To calculate the atomic mass of an element, multiply the mass of each isotope by its natural abundance (expressed as a decimal) and then add the products.

25 Calculate the average atomic mass of Sc if: Sc-44: 50% abundance Sc-44: 50% abundance Sc-45: 15% abundance Sc-45: 15% abundance Sc-46: 35% abundance Sc-46: 35% abundance

Calculate the Average Atomic Mass of copper: 69.1% Cu % Cu % Cu % Cu-65 26

Calculate the Average Atomic Mass of boron: 19.9% B % B % B % B-11 27

MOLE SI Unit SI Unit Abbreviated mol Abbreviated mol Amount of a substance that contains the same number of particles as the number of atoms in exactly 12 g of C-12 Amount of a substance that contains the same number of particles as the number of atoms in exactly 12 g of C-12 28

6.02 x Avogadro’s Number Avogadro’s Number Number of particles in exactly one mole of a pure substance Number of particles in exactly one mole of a pure substance 29

Molar Mass Gram Atomic Mass (g.a.m.) Gram Atomic Mass (g.a.m.) Mass (g) of one mole of a pure substance Mass (g) of one mole of a pure substance Mass of 1 mole atoms (g) = atomic mass (amu) Mass of 1 mole atoms (g) = atomic mass (amu) 30

Conversion Factors 1 mol = 6.02 x mol = 6.02 x mol = mass (g) from P. Table 1 mol = mass (g) from P. Table Most atomic masses are known to 4 or more sig figs Most atomic masses are known to 4 or more sig figs For calculations, we will round the atomic masses to the tenths place For calculations, we will round the atomic masses to the tenths place 1 mole C = 6.02 x atoms of C 1 mole C = 12.0 g 1 mole Cu = 6.02 x atoms of Cu 1 mole Cu = 63.5 g 31

Conversion Problems Grams  MOLE  Atoms Grams  MOLE  Atoms What is the mass of 2.8 mol He? What is the mass of 2.8 mol He? How many moles are represented by 7.11 x atoms of Hg? How many moles are represented by 7.11 x atoms of Hg? A sample containing 59.2 g of Al contains how many atoms? A sample containing 59.2 g of Al contains how many atoms? 32