Introduction to Human Anatomy and Physiology
Chapter 2 - Chemical Basis of Life
1. How can I use an element's atomic number & atomic weight to learn more about its structure?
Some of the information you can derive from atomic weight & number number are the atom's number of protons, number of electrons, its tendency for to react with other elements, and the number of neutrons the atom has.
The atomic number of an element tells you how many protons it has and, therefore, how many electrons it has when it has not already been involved in a chemical reaction. For example, the atomic number of hydrogen is 1, which tells you it has 1 proton & 1 electron in the uncombined form. Sodium has the atomic number 11: it has 11 protons & also 11 electrons when uncombined with another atom.
Since the atomic mass equals the sum of an atom's protons plus its neutrons, you can determine how many neutrons a particular atom has by subtracting its atomic number from its atomic weight. For example, sodium has an atomic number of 11 and an atomic weight of 23.
You can use an atom's atomic number to determine how many of its electrons will be involved in forming chemical bonds with some other atom in a chemical compound. As is shown in Table 2.2 on page 33 of your textbook, the elements we most commonly work with in biological reactions have electrons arranged in specific energy shells. There is a maximum number of electrons that can be in each of these electrons shells:23 (atomic weight, which is protons + neutrons)-11 (atomic number, the number of protons)
12 neutrons
In this Introduction to Human Anatomy & Physiology course, we will not be dealing with very "heavy" elements, & so you probably will not make use of the above pattern beyond the third or fourth shell. However, you can easily determine the number of electrons in a shell by using the pattern. For example, sodium has the atomic number 11, indicating it has 11 protons & consequently 11 electrons before a chemical reaction. If you start from sodium's innermost electron shell:first electron shell: 2 electronssecond electron shell: 8 electrons
third electron shell: 8 electrons
fourth electron shell: 8 electrons
fifth electron shell: 8 electrons
and so on...
In this case, you are now able to the answer the question, "How many electrons are in sodium's outermost shell?" The answer is "One electron." This is an indication that sodium is a chemically active element, because unless an atom's outermost shell is filled to capacity, it tends to become involved in chemical reactions with other atoms whose outermost shell is also not filled to capacity. Since a third shell requires 8 electrons to be filled to capacity, sodium will tend to be a reactive element. When a specific element has its outermost shell filled to capacity, it does not tend to take part in chemical reactions. We call these elements (such as neon and helium), inert elements.put 2 electrons in the first shell (maximum number possible)put 8 electrons in the second shell (its maximum number)
2 + 8 = 10 electrons used so far, leaving 1 electron to be placed in a shell
To find more
information about atomic structure, see pages 32 to 35 in your textbook.