Chemistry 0010

Atomic Structure

Topics you should learn:
1. Structure of atoms

2. Isotopes
   • Step-by-step calculation of the atomic mass of carbon

3. Arrangement of Electrons in Principal energy levels
   • Bohr Diagrams

4. Electron dot symbols for elements
   
   • Rules for writing electron-dot symbols

5. Electronic configurations of an atom
   
   • Order of filling atomic orbitals

Structure of atoms

Let's look at what is inside an atom.

There are three fundamental particles that make up all atoms:

1. protons
2. neutrons
3. electrons

The main things we need to know about these three particles are their charge and mass.

Points to note:

• Notice that the mass of the electron is much less, about 1/1835 that of the proton. In other words, it takes about 1835 electrons to equal the mass of one proton.
• Protons and electrons carry electric charges (positive and negative charges respectively).

  like charges REPEL	unlike charges ATTRACT

• Neutrons have no charge. There is no electrostatic force of attraction associated with neutrons. The mass of the neutron is very close to that of the proton.

The general symbol for an atom of an element has the general format.

• A = mass number
• E = symbol of the element
• Z = atomic number

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.

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

Points to note:

• The only change in the number of fundamental particles between the atom and its ion is in the number of electrons.
• Ca²⁺ and Cl^- have the same number of electrons. They are isoelectronic .

National Institute of Standards and Technology Physics Laboratory (NIST) 1986 CODATA Recommended Values of the Fundamental Physics Constants Proton mass Electron mass Neutron mass

Read: 4-2 - Dalton's Atomic Theory (page 80) 4-3 - Subatomic Particles:Electrons, Protons, and Neutrons (page 82) 4-4 - General Arrangement of Electrons, Protons, and Neutron. Atomic Number (page 83)

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Isotopes

The periodic table does not give atomic masses of the elements by whole numbers (only mass numbers are whole numbers). The periodic table shows that:

• the atomic mass of carbon = 12.011 amu
• the atomic mass of magnesium = 24.3050 amu
• the atomic mass of copper = 63.546 amu ..... etc ...

Let's calculate the atomic mass of carbon. (This is a 4 step process.)

Carbon, as we find it in our universe, does not exist as solely as ¹²C. There is another naturally occurring isotope of carbon, ¹³C. ¹³C also contributes to the atomic mass of carbon.

Are you wondering why we don't include the contribution of 14C to the atomic mass of carbon?

1. "How many naturally occurring isotopes does carbon have?"
2. "What is the abundance of each of the isotopes?"

Ordinarily, you would take a stroll to your library to dig out the information. Since you are on-line already, let's go to the World Wide Web (WWW). We will tab into the WebElements database at the University of Sheffield in England. So, depending on the time of day, Internet traffic may be heavy. Be patient, it might take a moment.

Step 3

Points to note about the isotopic data:

1. There are two naturally occurring isotopes of carbon:
   • ¹²C
   • ¹³C
   So these are the masses that will contribute to the atomic mass of carbon.
2. From the fraction of abundance column, note that there are much more ¹²C isotopes than ¹³C isotopes.
3. The sum of all the fractions of abundance of each naturally occurring isotopes (ie - add up column 3) should equal 1.0000 or 100%.
4. Look at the mass of ¹²C. This is defined to be 12.000000 amu. Now look at ¹³C, it is,as expected, heavier than ¹²C. All the masses reported here have 8 significant figures!!

Step 4

Courtesy of the University of Sheffield, we can quickly obtain the isotope pattern for compounds. Enter a chemical formula, such as CO, into the calculator and see if you understand the isotope pattern. Isotope Pattern Calculator

Read: 4-5 - Isotopes (page 85)

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Arrangement of Electrons in Principal energy levels

Electrons are found in the space outside the nucleus of the atom. These electrons are always moving around the nucleus and so possess potential and kinetic energy. But they can only possess certain values of energy, or specific energy levels. (Credit should be given to Niels Bohr for proposing this theory.) Click here if you need an illustration of this concept.

Bohr deduced that each energy level of an atom could only house a certain number of electrons at a time.The maximum number of electrons that can be in a certain energy level is given by the formula

These energy levels, n, are called principal energy levels.
n = integers 1 to 7 of the principal energy levels.
n = 1 has the lowest energy and is closest to the nucleus.

• For the first energy level (n=1), the maximum number of electron is 2(1)², or 2.

• For the second energy level (n=2), the maximum number of electron is 2(2)², or 8.

• For the third energy level (n=3), the maximum number of electron is 2(3)², or 18.

1. For a hydrogen atom, H, the one electron goes into the first energy level.
2. For a helium atom, He, the two electrons go into the first energy level.
3. For a lithium atom, Li, two of the three electrons go into the first energy level. The third electron goes into the second energy level. This electron in the outer energy level is called the valence electron.

Theory of Atoms in Molecules I do not expect you to understand everything in this site. But it might be interesting to realize that scientists are still studying the atom. Meet Nobel prize winners Richard Feynman and Julian Schwinger at this site.

Read: 4-6 - Arrangement of Electrons in Principal Energy Levels (page 87)

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Electron dot formulas for elements

Gilbert N Lewis is widely known for his use of simple symbolic representations of elements that show valence electrons as dots. You've seen what the Bohr diagrams for the first 20 elements. Sometimes it is more convenient to represent the elements by its Lewis electron-dot symbol. It is not to say one is better than the other. The Bohr diagrams show arrangement of all the electrons. The Lewis electron-dot symbols focus on the electrons in the highest principal energy level in the atom, the valence electrons.

Read: 4-7 - Electron-Dot Formulas of Elements (page 89)

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Electronic configurations of an atom

The principal energy levels we talked about earlier consists of one or more sublevels. Points to remember about the sublevels:

1. An energy level with a principal energy level , n, contains n sublevels.
2. Each sublevel has one or more atomic orbitals with a specific three dimensional shape.
3. The different types of atomic orbitals are designated by the lowercase letters. They are s, p, d, and f.

Order of filling atomic orbitals

The order of filling electrons in the sublevels of the atoms is illustrated below.

• Follow the arrows starting with the orbital with the lowest energy, the 1 s orbital.
• The maximum number of electrons for each type of orbital is summarized below.

  Type of Atomic Orbital	Maximum number of electrons allowed in the Atomic Orbital
  s	2 (There is 1 s-type orbital.)
  p	6 (There are 3 p-type orbitals.)
  d	10 (There are 5 d-type orbitals.)
  f	14 (There are 7 f-type orbitals.)

  The electrons fill to occupy the same type of orbitals singly before they start pairing up.

• Notice that the 4 s orbital is filled before the 3 d orbitals. Likewise, the 5s orbital is filled before the 4 d orbitals etc ...

When the electrons are arranged in their lowest energy state, the atom is in the

Read: 4-8 - Arrangement of the Electrons in Sublevels (page 91)

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Rosamaría Fong, Instructor of Chemistry,
British Columbia Institute of Technology, Burnaby, B.C. Canada.

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All contents copyrighted © 1996
Rosamaría Fong, Instructor of Chemistry
All Rights reserved

: Last updated on June 15, 1996