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Chapter 4 Atoms and Elements 1 Basic Chemistry Copyright © 2011 Pearson Education, Inc. Atoms: The Building Blocks of Matter A brick is the smallest unit that has the characteristics of a brick wall. 2 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. 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 3 Basic Chemistry Copyright © 2011 Pearson Education, Inc. Identity The number of protons in an atom determines its identity. A beryllium atom has 4 protons. 4 If a 5th proton is added the atom has a new identity boron. 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 5 Basic Chemistry Copyright © 2011 Pearson Education, Inc. 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 6 Basic Chemistry Nuclear Forces • all the protons and neutrons in the nucleus are attracted to each other • only exerts force in the nucleus Copyright © 2011 Pearson Education, Inc. Stability • 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 7 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 8 Basic Chemistry Copyright © 2011 Pearson Education, Inc. 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 9 Basic Chemistry Copyright © 2011 Pearson Education, Inc. Particles in the Atom 10 Basic Chemistry Copyright © 2011 Pearson Education, Inc.