 The smallest part of an element that retains the properties of that element  How Small is an atom? 20,000,000,000,000,000,000,000 atoms of copper and zinc in a penny  What is an atom made of? Protons:  Positively charged particles in the nucleus  The atomic number is = to the number of protons Neutron:  Particles of the nucleus that have no electrical charge Electrons:  Negatively charged particles outside the nucleus  Orbiting around the nucleus in energy levels, shells, or clouds  Electrons have smallest amount of mass Atomic Mass Unit:  Used to express the masses of particles (protons + neutrons) in atoms

Charges of protons and electrons are opposite but equal, so their charges cancel out. Atoms have no overall charge  Unless the number of electrons and protons are unequal Ion:  Charged particle formed when electrons and protons are not equal More electrons / negatively charged ion Less electrons/ positively charged ion  Remember: Electrons and Ions go together!

 All atoms contain protons and electrons  Most atoms contain neutrons, but not all ex. hydrogen  An atom does not have to have an = number of protons and neutrons  Atomic Number: The # of protons in the nucleus

 Isotope: Have the same number of protons but have different numbers of neutrons Atoms that are isotopes of each other are always the same element because isotopes always have the same number of protons Have different # of neutrons which gives them different masses Remember: Neutrons and Isotopes go together!!

 Some isotopes are unstable  An unstable atom is an atom with a nucleus that will change over time – radioactive  Radioactive atoms spontaneously fall apart over time  Telling Isotopes Apart: You can tell isotopes of an element apart by its mass number  protons + neutrons =mass number (figure 5 page 92) + H ionH atom- H ion Electrons are not included in the mass number because they are so small that they have little effect on the elements atomic mass. Atomic Mass:  the mass of an atom expressed in atomic mass units  The weighted average of the masses of all the naturally occurring isotopes of that element.

 Each atom has forces that act between the particles Gravitational force Electromagnetic force Strong force Weak force (page 94 figure 7)