Atomic Structure (P and P ) A.Atomic Structure »Atomic number »Mass Number »Protons, Neutrons, and Electrons B.Isotopes »Average atomic mass »Radioactive Decay

A. Structure of an Atom The atom consists of -protons and neutrons concentrated in the nucleus -electrons constantly moving in a “cloud” around the nucleus

Sub-atomic Particles Particle Mass(amu) Charge Location in Atom Proton Nucleus Neutron Nucleus Electron 1/ Cloud surrounding nucleus 1 atomic mass unit (amu) =1.673x g Defined to be 1/12 of the mass of a carbon atom containing 6 protons and 6 neutrons.

Atomic Number Atomic number = # protons in an atom Uniquely identifies the element Si Silicon Atomic number Symbol Atomic mass

Mass Number Mass Number = # protons + # neutrons Mass Number = atomic mass rounded to nearest integer # neutrons = mass number – atomic number

Protons, Neutrons, and Electrons + 6 C O Cu Carbon: 6 protons and 6 neutrons Oxygen: 8 protons and 8 neutrons Copper: 29 protons and 35 neutrons

Protons, Neutrons, and Electrons In order to have neutrally charged atoms # protons = # electrons

Bohr Model Number of protons and neutrons in nucleus Electrons in cloud Sulfur Energy level Maximum # electrons

Lewis Dot Diagram Valence electrons: electrons in the outermost energy level Use one dot to represent each valence electron

Atomic Structure Summary A=P=E M=A+N Atomic # = # Protons = # Electrons Mass # = # Protons + # Neutrons = Atomic # + # Neutrons

B. Isotopes Atoms of the same element with same number of protons but different numbers of neutrons. Mass # Atomic # Isotope symbol: “Carbon-12”

B. Isotopes © Addison-Wesley Publishing Company, Inc.

B. Isotopes Hydrogen Isotopes

B. Isotopes How many protons, neutrons, and electrons are there in the Uranium isotopes U-235 and U-238?

B. Isotopes Average Atomic Mass –Reported on the Periodic Table –Weighted average of all naturally occurring isotopes of that element Avg. Atomic Mass

Avg. Atomic Mass B. Isotopes About 8 out of 10 chlorine atoms are chlorine-35. Two out of 10 are chlorine-37. Average atomic mass = 35.4 amu

How many isotopes can an element have? Can an atom have any number of neutrons? There are “preferred” combinations of neutrons and protons, at which the forces holding nuclei together seem to balance best. Light elements tend to have about as many neutrons as protons. Heavy elements need more neutrons than protons in order to stick together.

B. Radioactive Decay Atoms with too few or too many neutrons can sometimes exist for a while, but they are unstable. Unstable atoms are radioactive: their nuclei change or decay by spitting out radiation. /carbondating/

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