JJ Thomson discovered the electron using cathode ray experiment
Electron – smallest particle in an atom Negatively charged Directly involved in chemical reactions Hangs out in the Electron Cloud Electron Cloud – area around the nucleus where the electrons exist; mostly empty space; denser in areas where electrons are likely to exist
Ion – An atom that gains or loses electrons Nucleus
Rutherford & the Gold Foil Experiment
Discovered the nucleus
Nucleus – densely packed group of protons and neutrons in the center of the atom
Very tiny compared to whole atom
Electron cloud occupies most of the volume of the atom Nucleus – Protons & Neutrons
Within the nucleus:
Proton - positively charged subatomic particle Very heavy compared to the electron Located in the nucleus
Neutron – neutral subatomic particle About the same mass as a proton Adds mass and stability to the nucleus Discovered by Chadwick Relative Masses
Particle Symbol Charge Relative Mass Actual Mass
Electron e- 1- 1/1840 9.11 x 10-28
Proton p+ 1+ 1 1.67 x 10-24
Neutron n0 0 1 1.67 x 10-24 The Atom Atomic Number
Atomic Number - number on the periodic table; gives the number of protons in the nucleus of the atom
# Protons = Atomic Number # Protons = # Electrons (in a neutral element)
Carbon – Atomic Number = 6 : 6 protons in the nucleus Barium – Atomic Number = 56 : 56 protons in the nucleus Neutrons & Mass Number
Mass Number – the total number of protons and neutrons in an atom.
Shorthand notation: Mass Number
Element Symbol
Atomic Number
Mass # = # Neutrons + # Protons
# Neutrons = Mass # - Atomic # (# Protons) Neutrons
Find the number of neutrons using the atomic number and mass number.
Element Atomic Number Mass Number Neutrons
Beryllium (Be) 4 9
Neon (Ne) 10 20
Sodium (Na) 11 23 Isotope
Isotope – An atom that gains or loses a neutron.
Same number of protons, but different numbers of neutrons
Different neutrons = different masses
Same chemical, just a different mass
Neon-20 – 10 protons, 10 neutrons, 10 electrons
Neon-21 – 10 protons, 11 neutrons, 10 electrons
Neon-22 – 10 protons, 12 neutrons, 10 electrons Atomic Mass
AMU – Atomic Mass Unit Unit used for measuring the mass of an atom
Atomic Mass Number – weighted average mass of the atoms in a naturally occurring sample of the element.
Masses on the periodic table are decimals because they are an AVERAGE of the abundances of the isotopes.
Cl-35: 75% Cl-37: 25% Chlorine on the periodic table: 35.4527g/mol Practice
Number of Number of Number of Symbol of Atomic Number Mass Number Protons Neutrons Electrons Element 4 10
25 12
137 56
13 15