AP Chem Today: Atoms Review You’re welcome to start coming in during help session to do test corrections Next test in ~ 2 weeks

Evolution of the Atomic Model Since atoms are too small to see even with a very powerful microscope, scientists rely upon indirect evidence and models to help them understand and predict the structure of an atom.

Democritus & Leucippus (~400 BC) Greek philosophers first to propose that matter made up of tiny, indivisible particles called atomos, the Greek word for atoms

John Dalton (1803) Billiard Ball Model: Dalton theorized that the basic unit of matter is a tiny particle called an atom Dalton’s Theory: All elements are composed of indivisible (can’t be broken down) atoms All atoms of a given element are identical Atoms of different elements are different; Compounds are formed by the combination of atoms of different elements Billiard Ball Model: An atom is represented by a hard sphere

JJ Thomson (1897) Used a cathode ray tube to show one of the smaller units that make up an atom Because the cathode ray deflected towards the + charged plate when an electric field was applied, Thomson concluded that the ray was formed by particles and the particles were negatively charged

JJ Thomson (1897) Thomson discovered that the atom is made up of small, negatively charged particles which he called electrons Developed the Plum Pudding Model of the atom

Robert Millikan (1909) Millikan’s Oil drop experiment allowed him to determine the mass and charge of an electron

Rutherford: Gold Foil Experiment

Ernest Rutherford (1911) Conducted the gold-foil experiment: Directed alpha particles (small, positively charged particles), at a thin piece of gold foil Most of the alpha particles passed straight through the foil and a few were slightly deflected Some of the alpha particles were greatly deflected and bounced back  

Ernest Rutherford (1911) Rutherford concluded that atoms have a dense, central core called the nucleus, while the remainder of the atom is essentially empty space Positively charged particles known as the protons are found in the nucleus

James Chadwick First to prove the existence of the neutron Provided explanation as to why the positively charged protons in the nucleus stayed intact and did not repel each other.

Neils Bohr (1913) Bohr Model The nucleus contained the protons and neutrons The electrons orbited around the nucleus (like planets orbiting the sun)   Electrons are shown in concentric circles or shells around the nucleus The 1st shell can hold a maximum of 2 electrons The 2nd shell can hold a maximum of 8 electrons The 3rd shell can hold a maximum of 18 electrons Electrons in the outermost shell are called the valence electrons

Wave-Mechanical/Cloud Model (modern, present-day model) Developed after the famous discovery that electrons can be viewed as both a wave and particle Like planetary model, atom is pictured as having dense, positively charged nucleus The difference in this model is how the electrons are pictured

Wave-Mechanical Model Electrons have distinct amounts of energy and move in areas called orbitals (clouds) An orbital is a region in which an electron is most likely (most probable) to be located Value of l Orbital (subshell) Orbital Shape Name* s sharp 1 p principal 2 d diffuse 3 f fine

What is important about the atomic number? Unique for each element = # protons in nucleus

How do you figure out the number of: Protons in an atom? # protons = the atomic number Electrons in an atom? # electrons = # protons FOR NEUTRAL ATOMS, only! Neutrons in an atom? # neutrons = mass – # protons

What are Isotopes? Isotopes are different forms of the same element that have a different mass Isotopes have the same # protons but different # neutrons

What are Ions? Ions are atoms that have a positive or negative charge In an Ion, the number of protons and electrons are not equal

Standard Nuclear Notation Note: for ions, the charge (+ or -) is indicated in the upper right corner

What is the ONE THING that determines the identity of an atom? # of protons (atomic #)

Practice

Average atomic mass on the periodic table is a weighted average of the masses of all naturally occurring isotopes

Average Atomic Mass 28.085 amu , 14 28 𝑆𝑖 151.9641 amu, 63 152 𝐸𝑢 75.91 % Cl-35, 24.08% Cl-37