Structure of an Atom A brief review of the structure of atoms
History of Atomic Theory • See the Powerpoint on Atomic Theory Scientists for more information and pictures/models. • Review/Summary: • Democritus, 460 BC – proposes particles so small they can not be destroyed or divided, and describes them as “atomos” (uncuttable) • John Dalton, early 1800’s - first to use “atom”; proposes that atoms of the same element are identical, and atoms of different elements have different weights/masses; Solid Sphere Model • JJ Thomson, ~1897 – discovers the first subatomic particle, electrons; Plum Pudding Model • Ernest Rutherford, ~1912 – discovers the proton, and nucleus; Nuclear Model • Niels Bohr, ~1915 – discovers energy levels for the electrons; Planetary Model • Erwin Schrodinger, ~1926 – developed the Electron Cloud Model • James Chadwick, ~1932 – discovers the neutron in the nucleus • Most Modern Model – The electron cloud model with all three subatomic particles
Development of Atomic Theory • A more extensive review of Atomic Theory Development: • http://www.abcte.org/files/previews/chemistry/s1_p1.html Use the “next” button at the bottom of the webpage above to cycle through the different scientists and their theories/models.
What is an atom? • Smallest particle of an element (pure substance) that still has the properties of that element. • Most of the atom is made of EMPTY SPACE • Example: an atom of gold is the smallest particle of gold (an element or pure substance) that still has the properties of gold. • Video: http://www.youtube.com/watch?v=o-3I1JGW-Ck • Brainpop (username: sclee password: cougars): • Atom: http://www.brainpop.com/science/matterandchemistry/atoms/ • Atomic Model: http://www.brainpop.com/science/matterandchemistry/atomicmodel/
Two Main Regions of the Atom • Nucleus • Positive charge • Almost all of the mass of an atom • Tiny, dense center of the atom • Can be broken apart in chain reactions to produce HUGE amounts of energy for use in electricity, medicine, weapons, etc (nuclear energy) • Electron cloud • Negative charge • Almost 0 mass • Energy levels/shells • Max # electrons – 2,8,8* • Valence level – outermost energy level
The Nucleus of an Atom • Tiny and dense • Center of the atom • Contains protons and neutrons • Positive charge (positive protons plus neutral neutrons) • Very little of the volume (space) in an atom, but almost all of the mass • Mass = protons + neutrons • Units for mass are amu (atomic mass unit) • Held together by a strong nuclear force • Can be split apart in controlled chain reactions to release nuclear energy: • http://www.youtube.com/watch?v=ufEAg9h9wU8 • http://www.youtube.com/watch?v=mBdVK4cqiFs • Fermi is the first scientist to conduct controlled chain reactions • Nuclear energy in weapons: http://www.brainpop.com/science/energy/radioactivity/ • Video tutorial: http://www.youtube.com/watch?v=FSyAehMdpyI
Electron Cloud • Most of the volume in the atom, but almost 0 mass • Contains only electrons • Negative charge • Energy levels • Closest to the nucleus is the lowest level • Outermost level is the valence level
Subatomic Particles • Protons • Neutrons • Electrons • Video tutorial: http://www.youtube.com/watch?v=FSyAehMdpyI
Isotopes • Atoms of the same element can have different numbers of neutrons. • This means their mass is also different (mass = protons + neutrons) • Naming isotopes: use the name of the element, followed by the mass number of that atom • Ex: Carbon-12, Carbon-13 • Video Brainpop: • Isotopes: http://www.brainpop.com/science/matterandchemistry/isotopes/ • Radioactivity: http://www.brainpop.com/science/matterandchemistry/periodictableofelements/
Ions • In a neutral atom, the number of protons equals the number of electrons. • Neutral atoms are often unstable, because they do not have a full valence level. • Unstable atoms will gain or lose electrons to become stable. • Atoms which have gained electrons (gained negative particles) are negative ions. They will have more electrons than protons. • Atoms which have lost electrons (lost negative particles) are positive ions. They will have fewer electrons than protons. • To see if it is an ion, compare protons and electrons. • Video Tutorial Brainpop: • Ions: http://www.brainpop.com/science/matterandchemistry/ions/
Periodic Table Arrangement • Dimitri Mendeleev developed the first (near) modern Periodic Table of Elements • Page 71 in your book • Based on physical properties, especially atomic mass (later rearranged by atomic number) • Periods are rows, and tell how many energy levels the neutral atom has; elements increase left to right across the periods and are very different in properties/reactivity; • Groups or families are columns; atoms in the same group have the same number of valence electrons, and similar chemical reactivity.
Chemical Reactivity on the Periodic Table • Metals on the left (2/3 of the table), metalloids on the break, metals on the right (except hydrogen). • Most reactive metals – group 1, with 1 valence electron; • Most reactive nonmetals – group 17, with 7 valence electrons; • Nonreactive (inert) elements – group 18, with full valence level
Periodic Table Trends Atomic Mass Increases Atomic number increases Atomic Mass Increases Atomic number increases Metallic Properties Increase