1. Subatomic Particles Atoms are composed of three particles Protons Neutrons Electrons Particles can be distinguished by their charge, mass, and location within the atom

2. Subatomic Particles ParticleSymbolChargeMassLocation Protonp+p+ +11Nucleus Neutronn01Nucleus Electrone-e- 1 / 1836 Around nucleus

3. Atomic Number Each element has a different number of protons in its nucleus The number of protons determines which element it it The “atomic number” of an element is the number of protons in its nucleus Because atoms are neutral, the number of electrons is equal to the atomic number

4. Mass Number & Isotopes The mass of an atom depends on the number of protons plus the number of neutrons (p + + n) This is called the “mass number” Isotopes are atoms of the same element (same number of protons in nucleus) that have different numbers of neutrons