Atoms RPI - ERTH 2330 The Basis of All Materials

Neutron - neutral particle Proton - positively charged particle Electron - negatively charged particle E.B. Watson The Atom

Visible light – part of the EM spectrum Longer v = f, and v = 299,792,458 m / s (~3 E 8) in a vacuum  is length of a cycle in m, f is Hz (cycles per second) Higher f Our story begins with light…

FREQUENCY (f) number of cycles per unit time [units = Hertz (Hz)] 1 Hz = 1 cycle/s T = 1/f; f = 1/T; T f = 1 Wave Terms

Dispersion High f – more “bending” in prism Refraction – producing constituent wavelengths

Refraction Waves are bent as the move through materials with different wave propagating properties E.B. Watson

The spectrum of the hydrogen atom E.B. Watson

Violet 17.3E14 Hz Violet 26.9E14 Hz Green6.2E14 Hz Red 4.6E14 Hz What are the wavelengths (v=c)?  = v / f

Photo-Electric Effect Incident light causes movement of charge. Electrons move from the surface of the negatively-charged metal plate The intensity of the light determines how many electrons are produced, but not their kinetic energy. Wavelength changes kinetic energy

Small packets, or quanta, possessing specific amounts of energy. An incident photon is either totally absorbed by "target" matter or not absorbed at all* The energy of the photons in a monochromatic beam of light E = h f h is (Planck’s constant E -34 Js) *Quantization of photons is like counting children - you can’t have a partial child PHOTONS

E photon = E H - E L = h f Energy transitions in Hydrogen

Linen Violet6 Violet5 Blue-green4 Red3 Balmer relationships Any hot gas at low pressure will produce a line spectrum, although the spectra of other gases are more complex than that of hydrogen. Hydrogen spectra

Bohr model - a "solar-system" model. assumptions: an electron is in specific "allowed" orbits the allowed orbits are described by m v r = n h / 2  Angular momentum Planck’s const. F = m v 2 / randF = k q 1 q 2 / r 2 Bohr saw this as charged particles on circular path set the forces equal

Bohr specific radii for electrons j k l E.B. Watson

It requires energy to remove electrons Energy increases with orbital distance. Lowest energy configuration – ground state Quantum number Energy level at quantum number Energy level at ground state

You can use this to determine changes in the amount of energy Describes the change in energy of an electron moved from 1st orbital (the ground state in H) to the n orbital

E.B. Watson

Electrons are attracted to the protons in the nucleus. Great, how big is the nucleus? E.B. Watson

Elements differ from one another by the number of protons they contain (Z) Hydrogen (H) – 1 proton Helium (He) – 2 protons Sodium (Na) – 11 protons Francium (Fr) – 87 protons neutral A neutral atom will have as many electrons as protons. Many atoms for each element will have the same number of neutrons as protons isotopes Elements of the same Z that differ in the number of neutrons are isotopes.

AtomsElectronNeutronsProtonMass E -31 kg E -27 kg Charge (-1) E -19 coul. (0) None (+1) E -19 coul. Z = number of protons N = number of neutrons A = atomic mass number (N + Z) Element = unique Z Isotope = unique Z, different N What happens to A? 14 C

Isotope Same Z, different N Carbon (C ) Z = 6 Carbon C6 protons, 6 neutrons98.89% Carbon C6 protons, 7 neutrons1.11% Carbon C6 protons, 8 neutronstrace Atomic mass units (u) Every isotope is scaled to 12 C ( u)

The total mass (“atomic weight”) of any element is the sum of the weighted mass of its isotopes. For example, Oxygen has three isotopes mass (rel 12 C)Abundance 16 O % 17 O % 18 O % ( x ) + ( x ) + ( x ) = u

The interactions of electromagnetic radiation and electrons reveal the energy structure of the atoms The interaction of charged nuclei reveal the size of atomic nuclei The number of protons(+) determines the element’s identity It also determines the number of electrons (-) The number of electrons controls the behavior of the atom The number of neutrons may vary - isotopes Atomic mass is normalized to 12C Atomic mass for an element is a function of isotope abundance and mass.