Atomic Structure (History & Background

ATOMS MAKE UP ELEMENTS!! ATOMS ARE MADE OF PROTONS, NEUTRONS, & ELECTRONS!! SO…WHERE DID THE IDEA OF ATOMS ORIGINATE????

Early Greeks Democritus, Aristotle first to devise idea of “atoms”

Dalton’s Model Noticed compounds always have a fixed composition Ex: H2O John Dalton

Dalton’s Atomic Theory Elements are made of atoms that cannot be divided Atoms of same element have same mass, atoms of different elements have different masses Compounds contain atoms of more than one element In compounds, atoms of different elements always combine in the same way

Dalton’s Atomic Model - Solid Sphere cannot be divided (No protons, neutrons, electrons)

JJ Thomson’s Experiments Cathode ray tube (sealed glass with gas inside, electric current passed through) Current caused beam to glow

Thomson’s Experiments Placed 2 charged plates (+ and -) on either side of tube Beam Bent towards (+) plate + Beam - Electric current

Thomson’s Conclusion Beam was negative charged particles from inside atoms First evidence of subatomic particles Discovered electron

Thomson’s “Plum Pudding” Model Electrons evenly mixed in a sphere of (+) charge

Mass of the Electron The oil drop apparatus 1916 – Robert Millikan determines the mass of the electron: 1/1840 the mass of a hydrogen atom; has one unit of negative charge

Conclusions from the Study of the Electron: Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons. Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons Electrons have so little mass that atoms must contain other particles that account for most of the mass

Conclusions from the Study of the Electron: Eugen Goldstein in 1886 observed what is now called the “proton” - particles with a positive charge, and a relative mass of 1 (or 1840 times that of an electron) 1932 – James Chadwick confirmed the existence of the “neutron” – a particle with no charge, but a mass nearly equal to a proton

Rutherford’s Gold Foil Experiment Passed (+) charged beam of alpha particles at gold foil Expected only slight deflection

Results showed more deflection than expected

Rutherford’s Model Atoms have a small, dense, positive center: (Nucleus) surrounded by mostly empty space containing electrons Small dense nucleus Empty space ( electrons)

Bohr Model Bohr was correct in saying electrons were located in energy levels He was wrong about electrons moving in predictable orbits around nucleus

Electron Cloud Model Heisenberg uncertainty principle: the exact speed and position of an electron cannot be determined Can only determine the probability of finding electrons at possible locations outside nucleus

Electron Cloud Model Propeller in motion Propeller at rest (cannot see exact location- Only blur) Propeller at rest (can see exact location)

Highest probability of finding electron (near the nucleus)

Electron cloud Cloud is denser where probability of finding electron is high Cloud is less dense where probability of finding an electron is low

Orbital: 3-D Region of space around nucleus that indicates probable location of electron. Each orbital can have maximum of 2 electrons

Subatomic Particles Particle Charge Mass (g) Location -1 9.11 x 10-28 Electron (e-) -1 9.11 x 10-28 Electron cloud Proton (p+) +1 1.67 x 10-24 Nucleus Neutron (no) Most of the mass of an atom is in the nucleus; Relative mass (p=1, n=1, e=1/2000) Most of the volume of an atom is in the e- cloud;