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Chapter 4 Atoms and Elements

Basic Chemistry Copyright © 2011 Pearson Education, Inc.

Atoms: The Building Blocks of Matter

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An atom is the smallest particle that has the characteristics and properties of an element. If you split the atom, you no longer have aluminum.

A brick is the smallest unit that has the characteristics of a brick wall.

Structure Atoms contain sub-atomic particles: •  protons •  electrons •  neutrons Atoms have two regions: •  the nucleus is the small, dense

center that contains protons and neutrons

•  the electron cloud is the large, less-dense outer region that contains electrons and mostly empty space

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Identity

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If a 5th proton is added the atom has a new identity - boron.

A beryllium atom has 4 protons.

The number of protons in an atom determines its identity.

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Atomic Charges •  protons have a positive (+) charge •  electrons have a negative (-) charge •  neutrons are neutral and have no charge •  neutral atoms have an equal number of protons

and electrons •  ions are atoms that have an unequal number of

protons and electrons •  an atom’s net charge = # of protons - # electrons

Basic Chemistry Copyright © 2011 Pearson Education, Inc.

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Forces Acting in Atoms Electrostatic Forces •  like charges repel

and unlike charges attract

•  exerts force in both the nucleus and electron cloud

•  neutrons are not affected by these forces because they are neutral

Basic Chemistry Copyright © 2011 Pearson Education, Inc.

Nuclear Forces •  all the protons and

neutrons in the nucleus are attracted to each other

•  only exerts force in the nucleus

Stability

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•  protons increase the attractive nuclear force, but they also increase electrostatic repulsive force

•  neutrons increase the attractive nuclear force without increasing the repulsive electrostatic force

•  neutrons increase the stability of the nucleus and the atom as long as there are only a few more neutrons than protons

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Stability (cont’d)

•  the electron cloud has a negative charge because it only has electrons

•  the nucleus has a positive charge because it only has protons and neutrons

•  electrostatic forces keep the negatively charged electrons moving around the positively charged nucleus

Basic Chemistry Copyright © 2011 Pearson Education, Inc.

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Atomic Mass & Mass Number Atomic mass and mass number are not the same. Atomic mass is the atom’s actual mass •  1 atomic mass unit (amu) is equal to 1/12 of the mass of

the carbon-12 atom •  a proton has a mass of 1.007 amu •  a neutron has a mass of about 1.008 amu •  an electron has a very small mass of 0.00055 amu Mass number is the atom’s approximate mass •  protons and neutrons have a mass number of 1 •  electrons have a mass number of 0 •  mass number = # of protons + # of neutrons

Basic Chemistry Copyright © 2011 Pearson Education, Inc.

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Particles in the Atom

Basic Chemistry Copyright © 2011 Pearson Education, Inc.