Science 7 Notes 17

Atom Democritus How has the Dalton model of the atom Thompson changed over Rutherford time? Bohr Electron Cloud Model

HISTORY – The modern model of the atom has evolved over long periods of time through the work of many scientists Who proposed the atom? Earliest Model - 4th Century B.C. • Democritus, a famous Greek teacher proposed the idea of the atom.

What is an atom? • Democritus said that all matter was made of tiny, indivisible particles Atom = Solid

• Called these particles atoms. Sphere

An atom is the smallest unit that retains chemical identity of that element.

What 4 pieces of information did Dalton’s Model – 1800’s Dalton write • John Dalton came up with his atomic theory based on the results of about atoms? his experiments. In what ways did • Dalton published a paper about atoms which stated Dalton and Democritus 1. Atoms are indivisible and indestructible agree? 2. Atoms of the same element are exactly alike

3. Compounds are formed by combining atoms 4. Chemical reactions are rearrangements of atoms 1

Science 7 Notes 17 The Atomic Structure What are the • The smallest particle of an element is an atom. subatomic • The atom is made up of three subatomic particles: particles? • Electrons, protons, and neutrons Which subatomic Electrons: st particles were 1 Discovered in 1897 by J. J. Thomson. identified? • Developed the plum pudding model. This Who discovered said that negatively charged electrons electrons? were stuck on a positively charged ball.

What was the • The electron has a negative charge. Its mass is much name of JJ smaller than the other 2 subatomic particles, therefore its mass is Thompson’s atomic model? usually ignored.

What are the 2 Nuclear Atomic Model parts of the atom? We know today that the atom is made up of 2 parts/sections: Where are they (1) The Nucleus --- (in the center of the atom) located? (2) The Electron Cloud --- (surrounds the nucleus) Who discovered The nucleus was Discovered by Ernest Rutherford in 1911. the proton? • Rutherford’s gold foil experiment proved:

What experiment (1) Most of the atom is more or less empty space was used to (2) The nucleus is very tiny (Stadium Analogy) discover the proton? (3) The nucleus is very dense. (Large Mass ÷ Small Volume) (4) The nucleus is positively charged. How did Rutherford had identified the proton. Rutherford change the 1913 Niels Bohr first explained where the electrons go. atomic model? • According to his model, electrons are arranged like planets around the

Who told where sun in energy levels the electrons go?

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Science 7 Notes 17

What is the Neutrons charge on a neutron?  Neutrons do not have a charge. In other words, they are neutral.  The neutron was discovered in 1932. The neutron has about the What subatomic same mass as the proton. Neutrons keep the nucleus intact. particle has the same mass as a Modern Electron Cloud Model neutron? • Electrons exist in orbitals (areas of probability of finding an electron)

What is the name of the modern atomic model?

In the modern model, where are electrons located?

Describe how the model of the atom has changed over time.

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