Biology 9-1

Biology, Grade 9

Chapter 2: Atoms and Molecules

Day 2 of 5  —  2.1.2 The Periodic Table, Atomic Mass, and Isotopes

Text pages 36 to 37  |  40 Minutes  |  Room 212  |  Mr. Gibbs

At a Glance

• 0:00 to 0:05  Bell work, silent

• 0:05 to 0:09  Bell work reviewed

• 0:09 to 0:17  Reading the periodic table

• 0:17 to 0:23  Activity, the average age of this class

• 0:23 to 0:32  Isotopes and atomic mass

• 0:32 to 0:39  Mass number, and counting neutrons

• 0:39 to 0:40  Close

Materials: whiteboard, textbook open to Figures 2.2 and 2.3, a periodic table each student can see. Calculators are fine.

Bell Work

5 minutes silent. On the board before the bell.

1.  Name the three subatomic particles. Give the charge of each.

2.  Which two are in the nucleus?

3.  How many electrons fit in the first shell? The second?

4.  Which electrons are the only ones involved in chemical bonding, and what is the shell they sit in called?

1. Bell Work Reviewed

4 minutes. Fast.

• 1: protons, positive. Neutrons, no charge. Electrons, negative.

• 2: protons and neutrons.

• 3: two in the first, eight in the second.

• 4: valence electrons, in the valence shell. Make them use both terms.

2. Reading the Periodic Table

8 minutes.

• Start with what makes elements different, in the book’s words: all atoms are composed of protons, neutrons, and electrons, but differ in the numbers of these particles that they have. While the numbers of all three vary from element to element, the IDENTITY of an element is determined by the NUMBER OF PROTONS in an atom’s nucleus.

• The scale of it: at present there are 118 known elements, arranged in the Periodic Table of the Elements, shown in Figure 2.2.

• The ATOMIC NUMBER: the number of protons is indicated by the atomic number, and elements are arranged left to right according to increasing atomic number.

• Use the book’s example, and it is a good one for biology: carbon, one of the author’s favorite elements, one that is necessary for life. Chemical symbol C. Carbon atoms all have six protons in the nucleus, so carbon is element number 6.

• Then the sentence that makes atomic number mean something: if a carbon atom were to GAIN another proton, which happens in certain nuclear reactions, the atom would no longer be carbon, but nitrogen, N, element number 7.

SHOW OR DEMONSTRATE:  Figure 2.3 on page 36. Walk the four labels in the carbon box: atomic number 6, chemical symbol C, element name Carbon, atomic mass 12.011. Have every student find carbon on their own periodic table and point to each of the four.

BOOK LANGUAGE:  atomic number, chemical symbol, element name, atomic mass. These are the four things in every box, and Figure 2.3 labels them. Use the book’s labels rather than any other naming.

• Then introduce the odd one: the other value present in the periodic table entry is the element’s ATOMIC MASS. Carbon’s mass is 12.011 UNIFIED ATOMIC MASS UNITS, abbreviated u.

• And ask the question the number raises: why is it 12.011 and not just 12? Take a guess or two, then say the answer is coming in about five minutes.

3. Activity, The Average Age of This Class

6 minutes. This is the book’s own analogy, run on real data. Do it before you explain isotopes, not after.

• Set it up the way the book does: say there are 20 students in a class. All the students are in the same class, which is analogous to all the carbon atoms having the same number of protons. However, it is likely that the students’ ages vary a bit.

• Now use the real room. Take a count by age. How many are 14? How many 15? How many 16? Put the tally on the board.

• Have them calculate: add up everyone’s age and divide by the total number of students. Give it two minutes and have a student put the answer on the board.

• The result will not be a whole number, and that is the entire point. Ask whether anyone in the room is actually that age. Nobody is. It is an average, and no individual has it.

• Land it in the book’s words: atomic mass works nearly the same way. Every student is still in the same class, just as every carbon atom is still carbon.

Running this on the real class rather than the book’s hypothetical 20 students costs you nothing and makes the point land. The book’s own example gives an average age of 14.75 years, so if your number comes out similar, say so.

4. Isotopes and Atomic Mass

9 minutes.

• Now answer the 12.011 question: the atomic mass tells us how heavy the element is ON AVERAGE. This value is an average because even though all carbon atoms have six protons, the number of NEUTRONS they have varies, and thus the mass of carbon atoms varies as well. This is the case for every element.

• Define the term: the varieties of atoms of the same element are called ISOTOPES. The isotopes of carbon are all identical, EXCEPT with respect to the number of neutrons in the nucleus.

BOOK LANGUAGE:  isotopes. Note carefully what the book says isotopes differ in. Only neutrons. Same protons, same electrons in the neutral atom, different neutrons.

The carbon isotopes, from the book

• The most common isotope of carbon, by far, is CARBON-12, with an atomic mass of 12 u. That mass comes from six protons plus six neutrons.

• Point out why the periodic table reads 12.011: you can see that 12 is very close to 12.011, the average mass of the element, because carbon-12 is so much more common than the others.

• A much smaller number of carbon atoms exist as CARBON-13, six protons and seven neutrons, or CARBON-14, six protons and eight neutrons.

• And the rule that holds all of it together: atoms of every isotope of carbon have six protons. They MUST have six protons in order to BE carbon. Only the number of neutrons varies.

SHOW OR DEMONSTRATE:  Draw three nuclei side by side on the board: six protons and six neutrons, six and seven, six and eight. Label them C-12, C-13, C-14. Keep the proton count visually identical in all three so the eye sees what stays the same.

5. Mass Number, and Counting Neutrons

7 minutes. Work both examples on the board.

• Introduce the term: like carbon, most elements have a particular isotope that is far more abundant in nature than the others. As a result, the atomic mass is in many cases very close to the whole number representing the mass of the most abundant isotope. This whole number is called the MASS NUMBER.

• Define it precisely: the MASS NUMBER is the total of all the protons and neutrons in the most common isotope of an element.

BOOK LANGUAGE:  mass number versus atomic mass. Students confuse these constantly. Atomic mass is a decimal average from the periodic table. Mass number is a whole number for a specific isotope. Put both on the board at once and label them.

Worked example: sodium, from page 37

• Sodium (Na) is a relatively soft, shiny metal, pictured in Figure 2.4.

• Its atomic number is 11, so atoms of sodium have 11 protons.

• Its atomic mass is 22.9898 u, so its mass number is 23.

• The formula, written exactly as the book writes it: # of neutrons = mass number − atomic number, so # of neutrons = 23 − 11 = 12 neutrons in the isotope Na-23.

Then Example 2.2 from page 39, chlorine

• Set up the wrinkle the book sets up: when you look at chlorine’s entry in the periodic table, its atomic mass is 35.45 u. This value is nowhere close to any whole number, because it is the weighted average of chlorine’s naturally occurring isotopes. In any given sample of chlorine found on earth, 76 percent are Cl-35, 24 percent are Cl-37, and a trace amount is Cl-36.

• And why that changes the method: since the question specifically asks for the number of neutrons in the Cl-35 and Cl-37 isotopes, we use 35 and 37 as the mass numbers, rather than working from the atomic mass found in the periodic table.

• For chlorine-35: # of neutrons = 35 − 17 = 18.

• For chlorine-37: # of neutrons = 37 − 17 = 20.

• Have them do one cold: give them oxygen-16 and have them find the number of neutrons. Answer is 8. Then nitrogen-14. Answer is 7.

The two ways to write an isotope

• From the book’s side note: one way is to place the atomic mass after the name of the element, as in carbon-14 or C-14. Another way is to place the mass number as a superscript in the top left corner, such as ¹⁴C. In that method it is also appropriate to place the atomic number in the bottom left corner. However, since carbon always has six protons, the atomic number is implied and often omitted.

6. Close

1 minute.

• Leave them with the hook for tomorrow: the book says the existence of isotopes turns out to be incredibly useful in science. Tomorrow we find out what one particular isotope of carbon lets scientists do, and it involves the Dead Sea Scrolls.

• Assign the reading. Dismiss by procedure.

Objectives

• Students will identify the atomic number, chemical symbol, element name, and atomic mass in a periodic table entry.

• Students will explain that the identity of an element is determined by its number of protons.

• Students will define isotope and explain why atomic mass is not a whole number.

• Students will distinguish mass number from atomic mass.

• Students will calculate the number of neutrons in an isotope using mass number minus atomic number.