Atomic Structure

The History Of The Atom

Democritus (460 – 370 BC) Atomism and Atoms

Democritus Lived in the fourth century B.C. Greece (400 BC – 301 BC) He had the thought that : - matter was composed of tiny indivisible particles called atoms.

4th century BC Greece Parthenon built 447 BC– 432 BC

Photo Credit: timerime.com Atoms are invisible, indestructible, fundamental units of matter Ideas not entirely useful because it lacked experimental support.

John Dalton Photo credit: US Library of Congress Dalton was an English Chemist, Physicist, and meteorologist

An atom is an indivisible sphere with a uniform density throughout. Dalton’s Atomic Model An atom is an indivisible sphere with a uniform density throughout. 1807

All elements are composed of submicroscopic, indivisible particles called atoms Atoms of the same element are identical. The atoms are different from those of other elements.

Atoms of different elements can physically mix together or can chemically combine with one another in simple whole-number ratios to form compounds. Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element are never changed into the atoms of another element as a result of chemical reactions.

What was going on in 1807 President: Thomas Jefferson

Electrons

Sir J.J. Thompson Discovered electrons , negatively charged particles, in 1897. Performed the cathode ray experiment to conclude that there are negatively charged particles that are part of the atoms of all elements.

In 1909, Robert Millikan measured the charge of the electron Oil drop experiment https://www.youtube.com/watch?v=XMfYHag7Liw

E. Goldstein Concluded that atoms have positively charged particles called protons. This was done through a similar cathode ray experiment

Sir James Chadwick Confirmed the existence of the neutron

Rutherford

Ernest Rutherford Known for the Gold foil experiment He directed a narrow beam of alpha particles at a very thin piece of gold foil. They expected the beam to pass right through. The particles deflected

A few particle bounced straight back to the source. Based on this experiment , he concluded that there was something in the center of the atom that contained mass. This small region is known as the nucleus.

Rutherford’s Model electrons protons

https://www.youtube.com/watch?v=Zd6_zVdMgJk&feature=related https://www.youtube.com/watch?NR=1&v=kHaR2rsFNhg

Niels Bohr

Niels Bohr Bohr’s theory gets you away from the original thought that all orbital are in one big circle circle His theory says that electrons are arranged in concentric circular paths, or orbits.

BOHR’S MODEL - +

Atomic Number and Isotopes

Atoms are made up of: Protons Electrons Neutrons

Protons Positive charge Located in nucleus Mass of 1 amu Number is always equal to the atomic number of its element

Electrons Negative charge Located outside the nucleus Mass close to zero amu Number is equal to number of protons in a neutral atom

Neutrons No charge – neutral Located in nucleus Mass of 1 amu Number is equal to mass number – atomic number

The Atomic Number of an element is: The number of protons in the nucleus of one of its atoms. This number also equals the number of electrons in the neutral atom.

Mass Number The mass number is the total number of protons and neutrons in the nucleus. Mass Number = p + n

New Notation Mass number Chemical atomic number Symbol

6 Li 34 Cl 3 17 Au 184 158 U 92 79

Atoms of the same element that contain different numbers of Isotopes Atoms of the same element that contain different numbers of neutrons and have different atomic masses. (The number of protons remains the same.)

All atoms have isotopes. Isotopes do not exhibit chemical behavior different from the neutral atom. Some atoms have isotopes that occur in nature, others have isotopes that are artificially created. Because the electrons govern the chemical behavior of an atom, they also govern the chemical behavior of the isotope.

Isotopes 6 Li Lithium-6 3 7 Li Lithium-7 3

Isotopes and atomic mass The atomic mass is the average atomic mass of all the isotopes of an element.

Isotopes and Radioactivity Unstable isotopes emit radiation as they decay. Isotopes decay because they want to become stable. The majority of isotopes are unstable.