Posts Tagged ‘Chemistry’
This post is a reading aid for Chapter 5 : Periodic Table from ‘Modern Chemistry’ by HRW for 10th grade students of California. Could be useful for parents and teachers as well.
This chapter is relatively easy compared to the previous one. HRW team has done a good job of organizing the material around three logical and easy to read sections :
- Section 1: History behind Periodic Table
- Section 2: Periods, Groups and Blocks of Periodic Table
- Section 3: Atomic property trends across a row/ period and across column/group
Section 3 is bit dry and could be difficult for students. This reading aid will try to distill that section to the extent possible.
Knowing electron configurations from Chapter 4 is very important to sail through section 2 and 3 of this chapter. So keep going back to Chapter 4 as you read chapter 5. Alternatively, review Teaching Atomic Model for Facebook generation if you have difficulty reading the original Chapter 4 from HRW.
First, let us look at some facts related to periodic table that may come handy for your school test. Do notice text in bold.
Section 1: Facts and History of Periodic Table
- In 1860, Cannizzaro, an Italian scientist, proposed method for determining atomic mass of element
- In 1869, Mendeleeve, a Russian teacher and scientist, published world’s first periodic table in a school text book. There were about 60 known elements in the table with some spots left for elements Mendeleev predicted would be discovered!
- In 1911, Mosley, an English scientist, was able to correlate periodicity (elements with similar characteristics re-appearing in periodic table) with atomic number which led to formal definition of periodic law:
physical and chemical properties of elements are periodic functions of their atomic number
- Scandium (Sc), Gallium(Ga), Germanium (Ge) are three elements that were unnamed in Mendeleev’s first periodic table and discovered later
Before we dive further into contents of the chapter, please take time to reflect on these two questions
1. How many metals, liquids and gas elements do you know? Visualize your kitchen, garage, bath room, your family jewelry, or what you eat, drink , and breath daily.
think for at least 5 minutes and note down names of elements that flash as you think
2. What you understand by periodicity? don’t worry about periodic table. Just think about the word ‘periodic’ and see what crosses your mind.
think for 2 minutes
If you come up with about 20 elements then you are not too far off from Mendeleev! He knew about 60 elements in 1860.
What is Periodicity ?
To understand periodicity, let us look at some real world examples:
- A BART train leaves from Fremont every 30 mins. So periodicity is 30 minutes
- Each day, at 4am, there is a goods train that passes through Fremont. So periodicity for the goods train is 24 hours.
- Sun rises every morning. Periodicity for Sun rise is 24 hours
- Ocean wave has ups and downs as it travels. Its periodicity is determined by the length of the wave as measured from zero (flat) to high (peak) and back to zero. The length could be specified in meters/feet. (BTW, see Note 1 below for some advanced remarks on waves).
As you can see, all of the above have different periodicity (recurrence). The repeat interval is specified using different units such as hours and meters. Of course, the same train comes back, the same sun rises, and the same ocean wave repeats its ups and downs.
While organizing elements based on their physical/chemical properties, Mendeleev observed elements with similar properties re-appearing in his table. We can call related element as siblings. In periodic table there are several elements with as many as 7 siblings.
Though Mendeleev and others recognized elements with similar properties repeating at some periodic intervals in the table it was not clear to them as to:
- Why elements of same properties are re-appearing periodically
- How to correlate periodicity with properties of elements
- Why there are at most 7 siblings ? We may never know. But there is a myth around the mystery number 7. See note 2 if you have time.
- Infants in their 1st week need nursing every 3 hours. Periodicity =3 hours
- Infants in their 2nd week need nursing every 4 hours. Periodicity = 4 hours
- Infants in their 2nd month perhaps need nursing every 6 hours. Periodicity =6 hours
- Infants in their 3rd month need nursing every 8 hours. Periodicity=8 hours
As you can see different age group infants have different periodicity with respect to nursing. The time gap for nursing increases for older infants. Likewise, sibling elements (i.e ones with similar properties) repeat after some intervals. As the elements get bigger successive siblings may repeat after longer gap.
In mathematics, a periodic function is one that produces same characteristic value for x and x+p where p is the period of the function:
func ( x ) = func (x+p)
Though not quite same as above, we can say that for each element the period after which its sibling element appears is a function of the element’s atomic number:
p = func (x) where p = period , and x= atomic number
Still abstract, isn’t it? If yes then let us build another analogy, albeit fictional, to help explain ‘periodic’ in the sense of what Mendeleev, Mosley and others scientists meant. In order to build the analogy quickly recal the mystery house from the reading aid for Chapter 4. Each mystery house follows the building /construction rules (the Aufbau’s principle for atom) we have described earlier – one bed room on 1st floor, two bed rooms on 2nd floor, three bed rooms on 3rd floor and son.
Let us say we have families of size 1 to 60 to be accommodated with no two families having same family size. So essentially 60 different families need accommodation.
Now the problem to solve: How often homes with similar room occupancy repeat? To find the answer let us see how families are accommodated:
Answer: 2, 8, 8 , 18, …
The repetition interval/gaps (or shall we say ‘period’) is not same as you can see above with s1 (s room with single occupant). We can ask similar question with respect to s2 (s room with two occupants), p1, p2, ..p6, d1, d2, …, d10, and f1, f2, …f14. We will have similar repetition patterns.
Repetition pattern is what the scientists meant by ‘periodicity’. The gap between two homes (the siblings if you will) of similar bed room occupancy at the outermost energy level. The gap depends on the size of the first house. For example:
street 1: for s1 home on this street, the next s1 house is 2 homes away on 2nd street
street 2: for s1 home on this street, the next s1 home is 8 homes away on 3rd street
street 3: for s1 home on this street, the next s1 home is 8 homes away on 4th street
street 4: for s1 home on this street, the next s1 home is 18 homes away on 5th street
In the example above, we are able to see and justify gap pattern given the bed room occupancy rules we laid out as part of our game for the mystery house. Mendeleev and others of 1870’s didn’t know the inside of elements as much as we do now in terms of atomic model and electron configuration. They only knew external characteristics such as color, reactivity etc of elements. Using such externally known properties they found that elements with similar characteristics repeat at certain intervals. It is only after 30 years , when Bohr and other scientists formulated atomic model , that scientists were able to piece together logical explanation for the periodic pattern 8, 8, 18 etc (see Note 2 for a Hindu myth).
Now that we understand the history behind periodic table , let us see how it is organized and how properties of elements trend across a row (period) and across column (group) of the table.
Section 2: the blocks in periodic table
Section 3: the atomic properties and their trends
- As you will learn later in your science education, ocean waves, after travelling some distance, flatten out. The energy that created the wave slowly drains out as the wave travels further away from the point where it was created as there is no new energy to replenish to keep the wave going. This is similar to how your cell phone battery drains after some use if you don’t charge it again.
- In Hindu mythology and folklore it is often mentioned that Bramha, the creator of life, some times makes seven identical copies each time he creates a new persona (soul) Each copy maintains similar facial , structural and other physical characteristics. The copies may be born to different mothers, in different ages, in different races, in different places. Reincarnation is also an essential part of Hindu psyche. One can be re-born as many as 7 times. Is it purely coincident that there are at most 7 siblings for elements? Or could it be the work of some high order to ensure there are at most 7? Despite its unscientific and irrational nature, the myth might give inspiration to biologists to search for periodicity in human genetics. Could there be Nobel prize prospects for finding ‘Periodic Table of Human Genes’ ?
- Periodic Table of Genetics – Possible? (ellanti.wordpress.com)
- Why is oxygen placed where it’s at on the periodic table (wiki.answers.com)
- Seven types of anxiety (could it be related to 7 elements in a column of periodic table) (livingwithanxiety.com)