Atomic Structure Chapter 4

Dalton’s Atomic Theory 4.1 John Dalton – 18 th century All matter is composed of atoms All atoms of a given element are identical Atoms cannot be created, divided or destroyed Different atoms combine to form compounds In chemical reactions atoms are rearranged.

Subatomic Particles 4-2 Discovering the Electron- Thomson identified the electron through use of a cathode ray tube. Electrons carry a negative charge Electrons have an extremely small mass Thomson created the plum pudding model (chocolate chip cookie dough model) Electrons --- charge = -1 e -

Plum Pudding Model Negatively charged electrons were distributed throughout a uniform positive charge

The Nuclear Atom 4.2 Rutherford’s Gold Foil Experiment Discovered the nucleus Atoms consist of mostly empty space through which the electrons move There is a tiny dense region (nucleus) which contains all of the atom’s positive charge and almost all its mass

Gold Foil Experiment

Plum Pudding Model Nuclear Model Because some of the alpha particles bounced back – they must have hit something with a positive charge

What’s in the nucleus Protons – subatomic particle with a charge equal and opposite to an electron Proton --- charge = +1 p + Neutrons – subatomic particle with a mass almost equal to that of a proton but it has no charge – neutral Neutron --- charge = 0 n 0

How atoms differ 4.3 Atomic Number: The number of protons in an atom is the elements atomic number The number of protons in an atom identifies it as an atom of a particular element Atoms are neutral so # protons = # electrons Example: Helium - He Atomic # = 2 so it has 2 p + and 2 e -

How atoms differ 4.3 Isotopes and Mass Number Not all atoms of a particular element are identical. They all have the same atomic number (protons), but they can have a different number of neutrons Isotopes - same # of protons different # of neutrons

How atoms differ 4.3 Example: 3 types of potassium - all have 19 protons (that’s what makes them K) - 20 neutrons; 21 neutrons; 22 neutrons Differ in mass, but they have the same chemical properties Mass # = protons + neutrons

How atoms differ 4.3 Example cont. 3 isotopes of Potassium (same protons, different neutrons) Potassium – 39 ( ) Potassium – 40 ( ) Potassium – 41 ( )

Mass of Individual Atoms 4.3 Atomic Mass Units (AMU) Table amu is almost equal to the mass of one proton or one neutron The atomic mass of an element is the average mass of the isotopes of that element

Mass of Individual Atoms 4.3 Example: Chlorine - two isotopes Chlorine – 35 and Chlorine – 37 How many protons and neutrons does each have? IsotopesMass% abundance Chlorine amu75.0% Chlorine amu25.0% Atomic Mass = x 0.75 = x 0.25 = = 35.5 amu

Mass of Individual Atoms 4.3 Practice Problem: IsotopesMass% abundance Boron – amu19.8% Boron – amu80.2% x = x = = amu

Mass of Individual Atoms 4.3 IsotopesMass% abundance X – amu7.50 % X amu92.5 % You Try: x = x = = amu What element has a average atomic mass of 6.94 amu? Lithium (Li)

Radioactive Decay 4.4 Nuclear Reactions – a rxn that involves a change in the atoms nucleus change in the # of protons atoms of one element turn into atoms of another element

Radioactive Decay 4.4 Radioactivity – the process by which some substances spontaneously emit radiation They do this because they are unstable – they have a unstable ratio of protons to neutrons in their nucleus. They lose energy and undergo radioactive decay to become stable, non-radioactive atoms

Radioactive Decay types of radiation: Alpha, Beta, & Gamma Alpha Radiation- Alpha Particle - 2 p + and 2 n 0 - has a charge of 2+ - equivalent to a Helium -4 nucleus - symbol = α or 4 2 He Ex:

Radioactive Decay 4.4 Beta Radiation- Beta Particle – fast moving electrons - negative charge ( -1) - symbol = 0 -1 β Ex:

Radioactive Decay 4.4 Gamma Radiation Gamma Rays γ - high energy - have no mass, no charge - usually accompany alpha or beta - don’t make a new atom by themselves Ex: