A Brief History of Atomic Theory This Section will focus on Scientists who have had an impact on the study of the atom. Ready To Meet Them?

Democritus Democritus proposed that matter cannot be broken down indefinitely. At some point you end up with a piece that can’t be divided. That smallest piece he called an atom, from the Greek word atomos, which means “indivisible”. 470-380 B.C. Next Slide

Democritus’ Model “Atomos” ATOMOS was the word Democritus used the point, or stage where matter cannot be broken down any further. ATOMOS literally means “indivisible”

John Dalton John Dalton was a British chemist.He was the first modern scientist to propose the existence of atoms.He described an atom as an invisible indestructible, solid sphere, like a billiard ball. 1766 - 1844

Dalton’s Model The “Indivisible Sphere”

Sir Joseph “J.J.” Thomson J.J. Thomson was a British physicist who was the first scientist to propose the plum pudding model. He was also the first to propose the theory of the negatively charged electron. He was credited for the discovery. He also proposed the isotope. 1856 - 1940

Thomson “plum pudding” model + -

Ernest Rutherford Ernest Rutherford experiments proved that atoms are mostly empty space Discovered the nuclear, which contains positively charged particles .Was the first to suggest that electrons circle the dense nucleus. 1871- 1937

Rutherford’s Model Nucleus It has a + charge Electron Negative charge

Niels Bohr Niels Bohr stated that electrons move in different orbits, or energy levels, around the nucleus like planets orbit the sun. Each energy level is located a specific distance from the nucleus and contains a certain number of electrons. 1885 - 1962 Next Slide

Bohr Model Nucleus Energy levels Electron- negative charge

Current Model This model is based upon Bohr’s model, except that electrons orbit the nucleus in random patterns. The region where these particles are found is referred to as the electron cloud. Electron Clouds Nucleus Next Slide

Demitri Mendeleev Demitri Mendeleev was a Russian scientist. He was the first person to make the periodic table. He also wrote a chemistry book.The periodic table is a tool to help read elements.

Mendeleev’s Table

Modern Periodic Table

How to Read the Table 6 2 C 4 carbon 12.01115 Atomic Number 6 2 C 4 carbon 12.01115 Chemical Symbol Number of Electrons in each energy field Element Name Atomic Mass

Calculating Atomic Mass 6 C Carbon 12.01115 To find the Atomic Mass, you look at the bottom number in the Atom tile and round it to the nearest whole number. Atomic Mass

How to find number of protons 6 C CARBON 12.01115 Atomic Number/ number of Protons The number of Protons in atom is also the Atomic number, so therefore the Atomic number also represents the amount of Protons in the nucleus of that Atom.

How to Find the Amount of Neutrons in an Atom To find the amount of neutrons in an atom you have to subtract the atomic mass from the atomic number . 16 S sulfur 32.064 2 8 6 Atomic Mass Number of protons 32 Atomic Number 16 16

To determine amount of electrons To find the number of electrons in an atom, look at the numbers in the right side. To determine the number of energy levels count the number of side numbers and there you go!!! 6 C CARBON 12.01115 2 4

How to tell what period the element is listed 33 AS ARSENIC 74.9160 2 8 18 5 APPEARS IN THE 4TH PERIOD 4 ENERGY LEVELS

What's the BIG difference The big difference between an element, compound, and a molecule is…… Element: a substance made up of only 1 kind of atom Compound: a substance made up of 2 or more DIFFERENT elements Molecule: a grouping of 2 or more atoms joined together

For instance . . . This is an atom of an element. As you can see, it is made up of only one kind of atom . . In this case Hydrogen H

Now, with a compound O H O When two or more atoms are put together it makes a compound.

How elements are formed Elements are formed with a nucleus, and inside the nucleus is protons and neutrons. Around the nucleus are energy fields, and inside the energy fields are are electrons. Sulfur

Vocabulary Electron Electron fields Neutron Proton Mass Nucleus Compound Atom Isotope element Electron fields Mass Atomic structure Periodic table Chemical symbol

Suggested Vocabulary Element name chemist properties matter electron cloud synthetic metal metalloid Solid liquid gas noble gasses subatomic Nonmetals particles

Check out some of these helpful websites to do some research of your own . . . A periodic table of Elements Periodic Table Adventure Science Education at Jefferson Lab

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