AIDS to chemistry: lesson 3
ATOMIC STRUCTURE
Francine Taylor-Campbell, Contributor

Saying hi to friends is an important part of the day at Seaforth High School. - Photo by Oliver Wright

YOU SHOULD BE ABLE TO:

• Show the electronic arrangement of an element given its atomic number.
  
  
• Define isotopy and give three uses of isotopes.
  
  
• Explain the basis of arrangement of elements in the periodic table.

POINTS TO NOTE

• Electrons are arranged in shells around the nucleus.
  
  
• Each shell or energy level can hold a fixed number of electrons. When a shell is filled, electrons will enter the next shell with the lowest energy.
  
  
• Isotopes are atoms of the same element with the same atomic number but different mass numbers.
  
  
• Elements are arranged in the periodic table in order of increasing atomic (proton) number.

Last week, we found out that protons and neutrons are present in the nucleus while the electrons orbit the nucleus. These electrons are not stationary but continue to move in different energy levels around the nucleus. The arrangement of electrons in the shells or energy levels is governed by certain rules or guidelines.

• Electrons start to fill up the shells of lowest energy first, that is the closest one to the nucleus.
  
  
• The first shell (K shell) can hold a maximum of two electrons.
  
  
• The next shell (L shell) can hold a maximum of eight electrons.
  
  
• The third shell (M shell) holds eight electrons for the first 20 elements but can hold up to 18 electrons.

So let's take the element Boron for example, 11 5B Boron has a mass number of 11 and an atomic number of five. This means it also has five electrons. Using the guidelines B can have two electrons in the first shell, which leaves three electrons in the second shell. The electronic configuration E. C. of B = 2:3.

Example 2 - sodium

23 Na

11 E.C. of Na = 2:8:1 Remember the second shell can hold no more than eight so the other electron must go into the third shell.

Try the following: Write the electronic arrangement of the following elements; He, C, Al and K.

There are some elements that exist as isotopes. For example, 3517 Cl and 3717 Cl.

Notice that both elements have the chlorine symbol and have the same atomic number but their mass numbers are different. Remember mass number = P + N

If #protons = 17 then for Cl - 1 the # neutrons = 35 - 17 = 18 while for Cl ñ2, #neutrons = 37-17 = 20 neutrons.

These two chlorine atoms will have the same chemical properties, as their protons/electrons will be the same. It is the electrons in atoms that react and influence the chemical properties.

Let's look at another example oxygen-16 and oxygen-18. 16 O and 18 O

Both atoms have eight electrons, but one atom has eight neutrons while the other has 10.

Atoms of some isotopes are unstable and can split to form smaller atoms. When this occurs their nucleus splits and radiation is released. These isotopes are called radioisotopes or radioactive isotopes. Radioactive particles released during splitting may be: 1. alpha (a) particle which is a helium ion (helium atom that has lost its electrons He2+).

2. Beta (b) particles which are electrons (charge of - 1).

3. Gamma (y) particles are high energy electromagnetic radiation.

Radioisotopes are used in medicine and in industry.

In the treatment of cancer, gamma radiation from cobalt-60 is used to destroy cancerous cells.

In biological research, iodine-131 and carbon-14 are used as tracers. Iodine - 131 is given to patients with defective thyroid glands, while C-14 is used in the dating of plant and animal remains.

Radioactive isotopes, like uranium-235, are used to generate electricity. When a nucleus of this atom splits, a chain reaction is set up which releases vast amounts of heat energy. If this reaction is not controlled, this can lead to an atomic bomb.

Elements are arranged in the periodic table in groups based on the number of electrons in the outer shell. So all elements with one electron in the outer shell are in group 1, and those with two electrons are in group 2. Elements with a filled shell (two or eight) electrons are in group 8 or 0. The arrangement of elements in groups influences reactivity and the types of reactions they will participate in.

Francine Taylor-Campbell is an independent contributor.