# Chapter 17 The Atomic Nature of Matter

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Chapter 17 The Atomic Nature of Matter

Structure of the atom Protons +1 charge Neutrons +0 charge
Electrons charge Protons and Neutrons are found together in the nucleus of the atom Electrons are found in discrete ‘orbital's’ outside the nucleus

Atomic Number = #protons
Mass Number = #protons + #neutrons (for an individual atom) An Isotope of an element occurs by adding or subtracting neutrons from that atom Atomic Mass = is the mass average of the naturally occuring isotopes of that element

If an atom has more protons than electrons it has a net positive charge. These atoms are called cations If an atom has more electrons than protons it has a net negative charge. These atoms are called anions

How big are protons, neutrons, and electrons
How big are protons, neutrons, and electrons? It turns out that protons and neutrons are much heavier than electrons. Protons and neutrons weigh 1.67 x grams. Electrons weigh 9.11 x grams. Or put another way, protons and neutrons are 1800 times more massive than electrons. Therefore, most of mass of an atom comes from protons and neutrons.

Discovery of Atomic Structure
It was discovered in 1911 that most of the atom is empty space. If an atom were the size of a football stadium, the nucleus would be the size of a marble in center field, with electrons orbiting on the edge of the stadium. How was this discovered?

Ernest Rutherford

The Rutherford Experiment
In this experiment helium nuclei (alpha particles) are beamed at a thin piece of gold foil. Since atoms are mostly empty space, most of the helium nuclei pass through the foil. Occasionally, a helium atom will hit a gold nuclei and deflect it at different angles where it is detected by a fluorescent screen.

Columns on the periodic table are called groups.
Groups of elements have similar chemical properties:

Phases of Matter Liquid Gas Plasma Can you identify examples of each?
Solid Liquid Gas Plasma Can you identify examples of each? Solid: Desk, Floor, Textbook Liquid: Water Gas: Air we Breathe Plasma: Fluorescent Lights, Sun