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Atomic Structure. www.lab-initio.com. Standards. Modern Atomic Theory. All matter is composed of atoms Atoms cannot be subdivided, created, or destroyed in ordinary chemical reactions . However, these changes CAN occur in nuclear reactions!
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Atomic Structure www.lab-initio.com
Modern Atomic Theory • All matter is composed of atoms • Atoms cannot be subdivided, created, or destroyed in ordinary chemical reactions. However, these changes CAN occur in nuclear reactions! • Atoms of an element have a characteristic average mass which is unique to that element. • Atoms of any one element differ in properties from atoms of another element
Discovery of the Electron In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle. Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.
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
Thomson’s Atomic Model Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.
Rutherford’s Gold Foil Experiment • Alpha () particles are helium nuclei • Particles were fired at a thin sheet of gold foil • Particle hits on the detecting screen (film) are recorded
Rutherford’s Findings • Most of the particles passed right through • A few particles were deflected • VERY FEW were greatly deflected “Like howitzer shells bouncing off of tissue paper!” Conclusions: • The nucleus is small • The nucleus is dense • The nucleus is positively charged
The Bohr Model of the Atom I pictured electrons orbiting the nucleus much like planets orbiting the sun. But I was wrong! They’re more like bees around a hive. Neils Bohr
Quantum Mechanical Model of the Atom Mathematical laws can identify the regions outside of the nucleus where electrons are most likely to be found. These laws are beyond the scope of this class…
Electron Energy Level (Shell) Generally symbolized by n, it denotes the probable distance of the electron from the nucleus. “n” is also known as the Principle Quantum number Number of electrons that can fit in a shell: 2n2
Electron Orbitals An orbital is a region within an energy level where there is a probability of finding an electron. Orbital shapes are defined as the surface that contains 90% of the total electron probability.
s Orbital Shapes The s orbital has a spherical shape centered around the origin of the three axes in space.
p Orbital Shapes There are three dumbbell-shaped porbitals in each energy level above n = 1, each assigned to its own axis (x, y and z) in space.
d Orbital Shapes Things get a bit more complicated with the five d orbitals that are found in the d sublevels beginning with n = 3. To remember the shapes, think of “double dumbells” …and a “dumbell with a donut”!
Electron Spin Electron spin describes the behavior (direction of spin) of an electron within a magnetic field. Possibilities for electron spin:
Pauli Exclusion Principle Two electrons occupying the same orbital must have opposite spins Wolfgang Pauli
Electron Configurations of the elements of the first three series
