Unit #1Unit #2Unit #3Unit #4Unit #5Unit #6Unit #7Unit #8Unit #9Unit #10
spacerUNIT # 3  

spacerof the Atom

spacerAtomic Number - Number of Protons
spacerNumber of Electrons
spacerMass Number - Number of Neutrons
spacer3.2 Isotopes
spacer3.3 Arrangement
spacerof Electrons

spacerThe Bohr Model
spacerBohr Diagram
spacerThe Lewis Electron-dot Symbol
spacerThe Quantum Mechanical Model of the Atom
spacerElectrons in the sublevels

spacer3.4 Electron

spacerFilling Order of the Sublevels

spacerElectron Configuration Notations
spacerElectron Configuration for Atoms of the First 20 Elements
spacerElectron Configuration and the Periodic Table
spacer3.5 Atomic Mass
spacerCalculation of Atomic Mass
spacerCalculate the Atomic Mass of Carbon

spacer1 | 2 | 3


3.1 - Structure of the Atom

3.1.3 - The number of neutrons in an atom

In most cases atoms of a given element have different mass. The general symbol to describe an atom of an element has the following format.

Sometimes Z is omitted because if the symbol of the element is known, including the atomic number becomes redundant.

In order to determine the number of neutrons present in the nucleus of an atom of an element, we must know the mass number of the element.
mass number definition

Since the number of protons = atomic number, it follows that

The number of neutrons in the nucleus of an atom is

Take a look at 35Cl and 40Ca and see if you can figure out how I arrived at:

  • # of protons,
  • # of neutrons,
  • # of electrons, and
  • mass #, the mass number.

# of protons # of neutrons # of electrons mass #

17 35-17 = 18 17 35
20 40-20 = 20 20 40

Let's try figuring out the number of fundamental particles for their ions, 35Cl-, and 40Ca2+.

# of protons # of neutrons # of electrons mass #

17 35-17 = 18 18 35
20 40-20 = 20 18 40

Points to note:

  • The only change in the number of subatomic particles between the atom and its ion is in the number of electrons. The number of protons of a neutral atom and its ion is the same.
    • 40Ca and 40Ca2+ both have 20 protons.
    • 40Ca has 20 electrons. 40Ca2+ has 18 electrons.
    • The +2 charge on 40Ca2+ indicates that 40Ca2+ has 2 less electrons than 40Ca.
  • 35Cl- and 40Ca2+ have the same number of electrons. They are isoelectronic.


Section 4.4
General Arrangement of Electrons, Protons, and Neutrons. Atomic Number ..p83




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