History of the Atom Atomic Models – Democritus to Present 400 B.C. Democritus – mental model of the atom → Democritus first suggested the existence of the atom but it took almost two millennia before the atom was placed on a solid foothold as a fundamental chemical object by John Dalton (1766-1844).

Date Model Theory 1700’s John Dalton – Atomic Theory 1800’s J.J. Thompson – plum pudding model; discovered electrons. 1800’s Rutherford – used gold foil experiment to discover nucleus. 1913 Niels Bohr – electrons have fixed energy and circle the nucleus in orbitals called energy levels. 1950’s Wave Model – electrons found in electron cloud.

Additional History 1932 Chadwick solved the problem of the missing mass of the atom and discovered the neutron. Alpha particles were used by Rutherford as bullets in his gold foil experiment. An alpha particle is the nucleus of a Helium atom with all of the electrons stripped off.

Unit on Atoms What is an atom? Structure of an atom Isotopes Calculating atomic mass To Get an A Will be able to determine the composition of isotopes and ions. Will be able to differentiate between atomic mass and mass number. To Get a C Is able to identify the number of subatomic particles in an atom given its atomic and mass number. Will be able to describe an ion/isotope.

What is an Atom? The smallest unit of an element that still has the properties of that element. Example: 1,000,000 atoms of copper represent copper 1,000 atoms of copper represent copper. 1 atom of copper still represents copper I can tear a single copper atom apart, but it wouldn’t represent copper anymore.

WKRP in Cincinnati

Atoms are made of …. Protons (found in the nucleus) with charge p+ Neutrons (nucleus) with no charge n0 Electrons (in a cloud outside the nucleus) e- Protons, neutrons, a & electrons are called subatomic particles.

Different models of the atom All models have a center called the nucleus. The nucleus contains protons (p+) and neutrons (n0). Very lightweight objects fly around the outside called electrons (e-). The nucleus is very dense!!! Black holes are made of these.

Information on how many protons, neutrons, and electrons are in an atom can be found in the periodic table.

Atomic Number…. Is the counting # in the periodic table. We are counting protons (p+) in the nucleus. The # of protons is always the same in an element. Examples: How many protons are in the element copper? 29 How many protons are in the element oxygen? 8

Atomic mass…. Tells us how many things are in the nucleus. Protons + Neutrons = mass The # of protons does not always equal the # of neutrons. Examples: Sodium (draw picture)

What element is this? How many protons does this atom have? How many neutrons? How many electrons? Typically…an atom has the same # of electrons as it does protons. The element is……….________________!

Isotope Video

Isotopes Atoms of the same element that have a different number of neutrons giving each one a different mass. example: Hydrogen has 3 isotopes

Atomic mass vs. Mass Number Found in the periodic table An average mass of that element’s isotopes. Mass number….. Given to you in the symbol written for an individual atom. Examples: carbon – 12 6 protons + 6 neutrons carbon – 13 6 protons + 7 neutrons carbon – 14 6 protons + 8 neutrons

Carbon Isotopes

Symbols used for Isotopes Carbon - 12

Worksheet Examples: Name Atomic # Mass # p+ e- n0 8 Helium – 4 Carbon __ Boron - 11 Atomic # Mass # p+ e- n0 8

Overall video of an atom and isotopes.