1. Physical Science: Ch 4 Notes
A. The Structure of the Atom
Protons, electrons, and neutrons are subatomic particles
Proton – a positively charged subatomic particle that is found in the nucleus of an atom.
Each proton is assigned a charge of +1.
Electron – a negatively charged subatomic particle that is found in the space outside the nucleus.
Each electron has a charge of -1.
Neutron – a subatomic particle that is found in the nucleus of an atom.
It has a charge of 0 (uncharged) and has a mass almost exactly equal to that of a proton.
Protons, electrons, and neutrons can be distinguished by mass, charge, and location in an atom.

2. Physical Science: Ch 4 Notes
Particle
Symbol
Relative Charge
Relative Mass (proton = 1)
Actual Mass (g)
Electron
Proton
Neutron

3. Physical Science: Ch 4 Notes
B. Atomic Number and Mass Number
Atomic Number – the number of protons in an atom
Atoms of different elements have different numbers of protons, therefore, different atomic numbers.
Protons cannot be divided so atomic numbers are always WHOLE numbers
Atoms of elements are neutral so they have EQUAL numbers of protons (+) and electrons (-)
Atomic Mass – the sum of the protons and neutrons in the nucleus of an atom
Isotopes – atoms of the same element that have different numbers of neutrons…and therefore different mass numbers.
Isotopes of an element still have the same atomic number because the number of protons stays the same.
Average Atomic Mass – the average mass of all isotopes (reported on the Periodic Table)

4. Physical Science: Ch 4 Notes
C. Modern Atomic Theory
Niels Bohr stated: If an atom gains or loses energy then the energy of an electron can change.
energy levels – the possible energies that electrons in an atom can have.
Energy levels are like a staircase, if an electron gains energy then it can go up to a higher energy level. If an electron loses energy then it can fall down to a lower energy level.
Light (a form of energy) is given off when an electron falls to a lower energy level. Since no two elements have the same set of energy levels, different elements emit different colors of light.