Chapter 4 Atomic Structure

Defining the Atom  All elements are composed of particles called atoms  All atoms of the same element are identical  Atoms of different elements combine to form compounds  Atoms of one element can never be changed into another element

History of the Atom   Democritus (Greek BC) 1 st to use the word atom Believed that atoms were indivisible & indestructible No scientific support  Aristotle (Greek BC) Rejected the theory of atoms Believed in 4 core elements fire, air, water, earth

 Antoine Lavoisier (French ) Law of Conservation of Matter –Law of Conservation of Matter – Matter is not created or destroyed during a chemical reaction

 Joseph Proust (French ) Law of Definite Proportions -Law of Definite Proportions - A compound is always composed of the same elements, in the same proportion by mass

 John Dalton (English ) Studied the ratio in which elements combineStudied the ratio in which elements combine Quantitative data – massQuantitative data – mass -Law of Multiple Proportions - When 2 elements form more than one compound, they do so in a ratio of whole numbers When 2 elements form more than one compound, they do so in a ratio of whole numbers

Dalton’s Atomic Theory 1.All elements are composed of indivisible ‘solid sphere’ atoms. 2.All atoms of a given element are identical. 3.Atoms of different elements differ in their masses. 4.Different atoms combine in simple whole number ratios to form compounds.

Structure of the Atom

Subatomic Particles  Today… Dalton’s Atomic Theory has one important change  Atoms are divisible…  3 types of subatomic particles ProtonsProtons NeutronsNeutrons ElectronsElectrons

Electrons  J.J. Thomson (English 1897) – Identified the 1 st subatomic particleIdentified the 1 st subatomic particle Cathode-ray tubeCathode-ray tube

Thomson’s Electron Model  Named electrons  Symbol: e -  Charge: (-1)  Mass: ~ 0 amu  Plum Pudding/ Chocolate Chip Cookie

What about the protons & neutrons?  Protons Symbol: p +Symbol: p + Charge: (+1)Charge: (+1) Mass: 1 amuMass: 1 amu  Neutons Symbol: n 0 Charge: (0) neutral Mass: 1 amu

The Nucleus  Ernest Rutherford (English 1911) - Discovered the ‘structure’ of the atomDiscovered the ‘structure’ of the atom Gold foil experimentGold foil experiment Rutherford’s Gold Foil Experiment

Rutherford’s Nuclear Model  Atom is mostly empty space  Atoms have a dense center – nucleus  Most of the mass is in the nucleus  Nucleus has a positive charge  Protons & neutrons are located in the nucleus  Electrons are outside of the nucleus & occupy most of the atom’s volume

Distinguishing Among Atoms

What makes atoms different from one another?  # of protons  Atomic number = # of protons  Usually written as a subscript Mg 12 or 12 Mg What element has 11 protons? How many protons does potassium have? Sodium 19

How do you determine the # of neutrons?  Atomic mass = # protons + # neutrons  Usually written as a superscript 12 C or C 12 or C -12  # of neutrons is the difference between the atomic mass & the atomic number # of neutrons = atomic mass - atomic #

What is the atomic mass of helium? What element has an atomic mass of 27? How many protons does this element contain? How many neutrons does this element contain? … round to 4 Aluminum – 13 = 14

What about electrons?  # electrons = # of protons  Why? Atoms are electrically neutralAtoms are electrically neutral How many electrons does sulfur contain? Which element contains 10 electrons? 16 Neon

Sample Problems  How many protons, neutrons and electrons are in each atom? Beryllium (Be) - Fluorine (F) - Hydrogen (H) - 4, 5, 4 9, 10, 9 1, 0, 1

 How many neutrons are in each atom? 8 O Ag Pb – 8 = – 47 = – 82 = 125

 Express the composition of each atom in shorthand form. Nitrogen -14 Sodium - 23 Phosphorus -31 p + = 7, n 0 = 7, e - = 7 p + = 11, n 0 = 12, e - = 11 p + = 15, n 0 = 16, e - = 15

Isotopes  Atoms that contain the same number of protons but have different numbers of neutrons  Different atomic masses  Does not change the atom’s identity  Are naturally occurring  Symbols: 11 Na 23 or 11 Na 24 6 C 12 or 6 C 13 or 6 C 14

 Determine the number of protons, neutrons and electrons in the following: 1 H 1 1 H 2 1 H 3 p + = 1, n 0 = 0, e - = 1 p + = 1, n 0 = 1, e - = 1 p + = 1, n 0 = 2, e - = 1

Determining the Atomic Mass  Atomic mass # is an average of atom’s naturally occurring isotopes Copper has 2 isotopes Cu-63 and Cu-65 Which isotope is most abundant? There are 3 isotopes of Silicon with mass numbers of 28, 29, and 30. Which is more abundant? 63 Si -28

Calculating the Atomic Mass  Multiply the mass of each isotope by its natural abundance (expressed as a decimal), then add the products  Practice…

Ions  An atom with an electrical charge  Occurs 2 ways: Oxidation – loss of an e -Oxidation – loss of an e - results in a (+) charge results in a (+) charge Reduction – gain of an e -Reduction – gain of an e - results in a (-) charge results in a (-) charge Mnemonic: ‘Leo says Ger’ Loss of electrons = oxidation Gain of electrons = reduction

Examples of Ions H+H+H+H+  Cl -  As 3-  Mg 2+  Oxidation State – Number found on the periodic tableNumber found on the periodic table Shows the number of e - gained or lostShows the number of e - gained or lost Many elements have more than one numberMany elements have more than one number Lost 1e - Gained 1e - Gained 3e - Lost 2e -