Atoms Atom- the smallest part of an element Atoms of one element are different from another element. contains protons, electrons, and neutrons Atomic Mass Unit – standard unit of mass for an atom & equivalent to the mass of a proton. 1 amu = 1.66 x g (very tiny!) abbreviated with amu or u. Protons and neutrons are approximately 1 amu each. electrons are approximately 1/2000 the mass of a proton- so small it is considered negligible.

Bohr Model of Helium Helium has 2 protons, 2 neutrons, and 2 electrons. Where do they go? nucleus electron orbit + + n n n + neutron Proton electron

Atomic Properties Nucleus Cloud 0 Nucleus Subatomic Particle Charge Mass (amu) Location Proton Electron Neutron

Elements Element- the simplest form of matter with a unique set of properties and cannot be broken down into smaller components by chemical means. Atomic Number- the number of protons in the nucleus of an atom. Every element has a unique number of protons Elements are arranged on The Periodic Table by their atomic numbers Atomic Charge- The balance between the number of protons (positive) and electrons (negative) Atoms are neutral. # of protons = # of electrons Mass number- the number of protons and neutrons added together in an atom. Protons and neutrons are both approximately 1 amu. Electrons are too small, not added. Hydrogen (H) has 1 proton and 1 electron. What is its atomic mass? Beryllium has 4 protons, 4 electrons, and 5 neutrons. 1 amu + 0 amu = 1 amu 4 amu+ 0 amu + 5 amu = 9 amu

Nuclear Symbols Nuclear Symbols are used to represent atoms and their isotopes Isotopes – atoms with the same atomic number but different atomic masses due to different number of neutrons Example: uranium-238 A nuclear symbol tell us: The element (ex. uranium) top number is the mass number # protons + # neutrons bottom number is the atomic number # protons You can determine the number of neutrons by subtraction What are the nuclear symbols for uranium-235, iodine-181, and Xe-135? mass number → atomic number → 238 (mass #) - 92 (atomic #) = 146 (# of neutrons)

Isotopes Isotopes are atoms of the same element but with different atomic masses –number of protons are the same (therefore same element) –number of neutrons are different Example: The element carbon has an atomic number of 6. –it has 6 protons. –Some carbon atoms have 6 neutrons and some have 8 neutrons. Carbon-12 Carbon-14 6 protons + 6 neutrons 12 atomic mass units 6 protons + 8 neutrons 14 atomic mass units