About 100 years ago, scientists came up with a term for the smallest piece of matter: the atom.  The word atom comes from the Greek word “atomus” which means “indivisible.”

–      Since scientists didn’t have very good laboratory equipment over one hundred years ago, they were slightly mistaken about atoms.  The atom is the smallest piece of one specific elementAtoms, however, can be broken down into three smaller pieces.


–      There are three particles inside the atom: protons, neutrons, and electrons.

–      Protons:

  • Protons are positive particles inside the nucleus (middle core) of the atom.
  • They have a charge of +1.
  • Protons are given a mass of 1 amu (atomic mass unit).

–      Neutrons:

  • Neutrons are also located in the nucleus of the atom.
  • As their name suggests, they are neutral particles; they have a charge of zero.
  • Neutrons also have a weight of 1 amu.

–      Electrons:

  • Electrons are negatively charged particles.  They have a charge of -1.  As we’ll learn later in the course, they are responsible for electricity.
  • Electrons are not located in the nucleus of the atoms.  Electrons spin outside of the nucleus in energy levels called “orbits” or “shells.”
  • Electrons are extremely light.  They are 2000 times lighter than a proton or a neutron.  Since they are so small and light, we say they have a mass of zero.


–      The number of protons in an atom determines what element an atom is.  An atom with six protons will always be carbon.  An atom with two protons will always be helium.

–      The number of protons in an atom is called the atomic number.  The atomic number will always tell you how many protons AND how many electrons are in an atom.

–      Atoms ALWAYS have an overall charge of zero, therefore atoms are neutral.

–      For example, let’s look at nitrogen.  The atomic number of nitrogen is seven.  This means that nitrogen has seven protons and seven electrons.  Every proton is positive and every electron is negative, therefore:

(+7)   +  (-7)   =   0

Therefore, every atom of nitrogen is neutral (no charge).

–      Another important number we need to consider is the “atomic mass number.”  The atomic mass number tells us how heavy the atom is.

–      The atomic mass number of an atom only weighs the nucleus.  Do you remember which two particles are in the nucleus?  The protons and the neutrons, and each of these weigh 1 amu.

–      Therefore, the atomic mass number is the sum of the number of protons and the number of neutrons in an atom.

–      For example, let’s look at fluorine.  Fluorine has an atomic number of nine, and an atomic mass number of 19.  Therefore, fluorine has nine protons (the same number as the atomic number) and nine electrons (also the same as the atomic number).

–      The atomic mass number of 19 tells us there are a total of nineteen protons AND neutrons in the atom (specifically in the nucleus of the atom).  If there are nine protons, how many neutrons are there?

Number of Neutrons = Atomic Mass Number –  Atomic Number

–      So for fluorine:

Number of Neutrons =   19 –  9 =   10 neutrons

–      Let’s try a harder one:  What is the atomic number of Cesium55.  What is the atomic mass number of cesium?  140.  Now let’s see how many of each subatomic particle cesium has.

–      Cesium has 55 protons and 55 electrons (since the atomic number tells us how many protons and electrons are in an atom).

–      Let’s use the formula to find out how many neutrons cesium has:

Number of Neutrons = Atomic Mass NumberAtomic Number

=   140 –   55

=   85 neutrons

–      If an atom has ten protons and 19 neutrons, what is the atom’s atomic number?

–      Remember that the atomic mass number is the number of protons and neutrons added together.  Therefore the atomic mass number for this example is:

Number of Neutrons = Atomic Mass Number – Atomic Number

19 =  Atomic Mass Number – 10

19 +   10 =  Atomic Mass Number

29 =  Atomic Mass Number

–      When the atomic mass number of an atom is not a whole number (for example: the atomic mass number of magnesium is 24.3), round the number to the nearest whole number (24.3 rounds to 24).

–      When we write the notation for an atom, we use the following format:

  • Where A is the symbol of the element,
  • Z is the atomic mass number,
  • And X is the atomic number.

–      This format is used internationally and is called the “standard atomic notation.”


–      In the early 1900’s, two European scientists, Neils Bohr and Ernst Rutherford, made major discoveries about the atom.

–      They came up with the following ideas through careful experimentation:

  • Electrons move in orbits around the atom’s nucleus just like planets move in orbits around the Sun.
  • The electron orbits are at specific distances away from the nucleus.
  • Electrons can move from one orbit to another, but they cannot stay in the spaces between orbits.
  • The first orbit can hold a maximum of 2 electrons.  The second and third orbit can hold a maximum of 8 electrons.
  • Electrons are more stable (less likely to react with other chemicals) when they are closer to the nucleus.

–      Using this information, a system was devised to draw diagrams of atoms to show where electrons are located.

–      A circle is drawn to represent the nucleus of an atom.  Within this circle we write how many protons and neutrons are in the atom.

–      The next thing is to draw electron orbits around the nucleus.  The number of orbits depends on how many electrons an atom has.

–      Circles drawn on the orbits represent electrons.

–      Let’s look at an example:  Draw the Bohr-Rutherford diagram for lithium.

–      First, ask yourself how many of each subatomic particle is in a lithium atom.

  • Protons = 3 (the atomic number)
  • Electrons = 3 (the atomic number)
  • Neutrons = 4 (atomic mass number – atomic number)

–      Now let’s draw the nucleus:

–      Then add the first electron orbit:

–      What is the maximum number of

Electrons allowed in the first orbit?  2.

Draw the electrons:

–      Does lithium have more than 2 electrons?

Yes.  Draw another electron orbit:

–      How many more electrons do we place in this orbit to show that lithium has three electrons altogether? 1.  Draw the last electron:

–      Let’s try another one.  Draw the Bohr-Rutherford diagram for neon.

  • Protons = 10
  • Electrons = 10
  • Neutrons = 2010

= 10

–      Outside of the first orbit, electrons are usually drawn in pairs.

–      Don’t’ forget the maximum number of electrons allowed in each orbit:

  • First orbit > maximum 2 electrons
  • Second orbit > maximum 8 electrons
  • Third orbit > maximum 8 electrons

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