Periodic Trends

Properties of the elements exhibit trends in the periodic table. These properties include: atomic radius (or size), ionization energy, electron affinity, electronegativity, metallic and nonmetallic property.

ATOMIC SIZE

Atomic size refers to how big or how small an atom is. The size of an atom is determined by the distance of the valence electron (the outermost electron) from the nucleus of the atom. The shorter this distance is, the smaller is the atomic size. This distance is further dependent on other factors such as

a. the number of energy levels in an atom (the more is the number of energy levels, the bigger is the atom);

b. shielding effect (refers to the number of inner electrons that "shields" the attraction between the nucleus and the valence electrons-- the higher the shielding effect, the less is the attraction, therefore the bigger will be the size of the atom);

c. effective nuclear charge (the number of protons found inside the nucleus of the atom). The higher is the nuclear charge, the greater is the ability of the nucleus to pull electrons towards itself thus, decreasing the size of the atom.

The following "periodic table" shows a graphical representation of the sizes of the atoms down a group and across a period.

The absence of representations for those of the metals indicates that there is difficulty in establishing a clear trend for atomic size of these transition elements. Atomic size of the noble gases, Group 8A, are estimates from those of nearby elements.

IONIZATION ENERGY

When an atom loses an electron, it becomes a positive ion (cation). The removal of an electron from an atom requires an absorption of ionization energy. Ionization energy is the energy required to "knock-off" the most loosely held electron (the valence electron) from an atom.

The more tightly held is the electron to the nucleus (atomic size is small), the higher is the energy required to remove it. Thus a smaller atom will require higher ionization energy than a bigger one.

The following graph shows the ionization energies of the first 20 elements.

A more comprehensive survey of the ionization energies of the main group elements is shown below.

ELECTRON AFFINITY

Electron affinity is the energy released when an electron is added to a neutral atom to form a negative ion. If the electron affinity is low, the electron is weakly bound; if the electron affinity is high, the electron is strongly bonded. Generally, electron affinity increases from left to right across the period because of increase in nuclear charge and decrease in atomic size. This causes the incoming electron to experience a greater pull of the nucleus thus giving a higher electron affinity.

Electron affinity decreases down a group because of increasing number of energy levels (and atomic size). This causes the incoming electron not to experience much attraction to the nucleus thus giving a lower electron affinity.

The electron affinities of completely filled atoms (Group 8A) is almost zero. An atom does not accept an electron in its outermost energy level if it already has a stable configuration.

ELECTRONEGATIVITY

When two elements are joined in a chemical bond, the element that attracts the shared electrons more strongly is more electronegative (usually nonmetallic elements). Elements with low electronegativities (the metallic elements) are said to be electropositive. It is important to note that electronegativities are properties of atoms that are chemically bound to each other; there is no way of measuring the electronegativity of an isolated atom.

Electronegativity values increases across a period in the periodic table. This is because elements are becoming more nonmetallic (atomic size is decreasing) from left to right. Electronegativity decreases down a group because of increasing atomic size. Elements found below are more metallic thus they are more electropositive than those found above.

METALLIC and NONMETALLIC PROPERTIES

Metals are very good electron donors. In terms of atomic size, the bigger an atom is, the more metallic it is. Thus, across a period in the periodic table metallic property decreases. Down a group, where atomic size increases, metallic property also increases.

Nonmetals, on the other hand, are very good electron acceptor. The smaller is the size of the atom the easier for it to attract (or receive) an electron from an external source. Therefore across a period in the periodic table, nonmetallic property increases. Down a group, nonmetallic property decreases.

24-Item Test on Atomic Structure and the Periodic Table

This test covers the topics on atoms and the periodic table. Before starting the test, you will be asked to enter your name (please include your course, year and section, i.e. LEMUEL OWEN H. CABANA BSN1A). A quiz with no name will be scored zero. You will also need a periodic table and a calculator in answering most of the questions in the test.

Kindly print the score report ("certificate") found at the end of the quiz. Submit such to me on our next lecture class meeting.

Good luck!

link: http://www.proprofs.com/quiz-school/story.php?title=atomic-structure--periodic-table

Chemistry: A Volatile History (a BBC Four Documentary)

As a Chemistry teacher, watching this narrative feels amazingly like watching a big-budget Hollywood film :) This superbly pieced-together documentary and Chemistry lesson in one show beautifully explains the progression of the human understanding of the elements. (If only my chemistry lessons were this good at school, all my students would have become scientists! LOL)

This three-part documentary, produced by BBC Four, features Jim Al-Khalili, a professor of Theoretical Physics, tracing the extraordinary story of how the elements were discovered and mapped. In the process, he uncovers tales of success and heartaches in the story of well-known chemists' battle to control the elements which, in many ways, have helped build our modern world.

Episode 1: Discovering the Elements

Episode 2: The Order of the Elements

Episode 3: The Power of the Elements

Episode 2 is particularly interesting for me since I am presently discussing the Periodic Table of the Elements in my classes.

Many thanks to AtheistMedia.com for uploading the complete set of the documentary on youtube.

Episode 2: The Order of the Elements Part 1

Episode 2: The Order of the Elements Part 2

Episode 2: The Order of the Elements Part 3

Episode 2: The Order of the Elements Part 4

Episode 2: The Order of the Elements Part 5

Episode 2: The Order of the Elements Part 6

Rutherford's Gold-Foil Experiment: The Nuclear Model of the Atom

In 1909, Hans Geiger and Ernest Marsden, two researchers in Ernest Rutherford's laboratory at the University of Manchester, fired a beam of alpha particles at a thin gold metal foil. An alphaparticle, identified and named a decade earlier by Rutherford, is one of the types of radiation given off by radioactive elements such as uranium. Because these particles are fast-moving and positively charged (they're now known to be high-speed Helium nuclei), Rutherford reasoned they will serve as a good probe of the atomic structure of matter.

The experiment was done to validate the prevailing atomic model then, the plum-pudding model (some refer to it as the raisin-bread model) championed by J.J. Thomson. According to Rutherford, if Thomson's atomic model is correct, each high-velocity alpha particle will just pass straight through the thin gold foil suffering, at most, minor deflections because of the weak influence of the spread-out ball of positive charge and the effect of the electrons being negligible.

What Geiger and Marsden actually observed was stunning. Most of the alpha particles did indeed traveled straight through the foil with little or no deviation. But a small fraction (about 1 in 10,000) rebounded, ending up on the same side of the foil as the incoming beam. A few were returned almost along the same tracks as they went in. Rutherford upon hearing of these rebounds, described it as the most incredible event of his life. It was, he said, "as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you." Such huge deflections could mean only one thing: some of the alpha particles had run into massive concentrations of positive charge and, since like charges repel, had been hurled straight back by them. Thomson's plum-pudding model of the atom failed to explain these observations.

After months of studying, in 1911, Rutherford, presented a new model of the atom in which all of the positive charge is crammed inside a tiny, massive nucleus about ten thousand times smaller than the atom as a whole. That's equivalent to a marble in the middle of a football stadium. The much lighter electrons, he assumed, are positioned outside the nucleus. Everyone was amazed at the implication of Rutherford’s proposal---the atoms of which planets, people, objects, and everything else are made consisted almost entirely of empty space.

Rutherford's nuclear model of the atom was considered as one giant step forward in understanding nature at the microscopic level. But even as it closed the issue on the alpha particle experiment, it threw open another one. Since the nucleus and its retinue of electrons are oppositely charged, and therefore attract one another, there didn't seem anything to stop the electrons from being pulled immediately into the nucleus. Rutherford countered by saying that the atom was like a miniature solar system: the electrons circled the nucleus in wide orbits just as planets orbit the sun. This is the picture of atoms that most of us still carry around in our heads. It's an appealing, easy-to-grasp image – one that's inspired many a logo of the atomic age. Yet theorists were well aware of its shortcomings right from the start.

The fatal flaw in Rutherford's model is that it contains charges that are accelerating. The charges are on the electrons and the acceleration is due to the electrons always changing direction as they move around their orbits. (Things accelerate when they change speed and/or direction.) Since Maxwell's time, scientists had known that accelerating charges radiate energy. What was to stop the orbiting electrons in Rutherford's atom quickly (in fact, in about one hundred-millionth of a second) losing all their energy and spiraling into the nucleus?

The answer came from a young Dane, Niels Bohr, who joined the team at Manchester for a six-month spell in 1912, shortly after Rutherford went public with his new vision of the atom. Bohr played a hunch. He knew about Planck's quantum. He knew there was no way to save an electron inside an atom from plummeting into the nucleus if it could give off energy continuously. And so he said simply that electrons inside atoms can’t radiate continuously. They can only radiate in lumps, and these lumps are the same as Planck's quanta. For a given type of atom, say hydrogen, there's a limited number of stable orbits that an electron can occupy. Each of these orbits corresponds to a whole multiple of the basic quantum. As long as an electron is in one of these orbits, its energy, contrary to whatever classical physics might say, stays the same. If it jumps from an outer (higher energy) orbit to an inner (lower energy) orbit, the energy difference between the two is given off as a quantum of light. Once the electron reaches the lowest energy orbit, it can't fall any further and is safe from the clutches of the nucleus.

GenChem Midterm Lab Activities

7 is here...

8 is here...

9 is here...

you know what to do... :)

Chemistry PUN

A neutron walks into a bar. "I'd like a beer" he says. The bartender promptly serves up a beer. "How much will that be?" asks the neutron. "For you?" replies the bartender, "no charge."

Links to GenChem Lab Activities 4, 5 and 6

Lab Activity 4

Lab Activity 5

Lab Activity 6

Gen Chem Prelim Experiments

Here is the link to the download page of your first 3 lab experiments. You may download the files individually by clicking on each file or download all 3 at once by clicking "download this folder."

Please make sure that, aside from the other materials you're asked to bring to the lab, you carry a hardcopy of the laboratory activity for the day. Remember, NO HARDCOPY, NO EXPERIMENT and you’ll get 60 for the day.

Other reminders:

1. Make use of short-sized bond paper when printing or photocopying the activity sheets.
2. Brief answers to questions are encouraged. Confine your answers to the spaces provided after each question.
3. Write legibly. Maintain cleanliness of your report papers. Use a pen when writing down your answers. A pencil may be used if you're asked to draw or illustrate certain set-ups.
4. Staple the pages of your report before submitting such to me.

Thank you very much...

m.cabaña

Now you can stop bugging me.... :)

Lately, whenever they see me, some of my students would pester me about the class pictures I promised to post on this site. I'm not really sure but, I suspect, they have this notion that they'll get their "well-deserved" 15 minutes of fame through me... LOL!

Kidding aside, I was lucky to have handled only 4 classes this semester. 3 Biochem lab classes and 1 Organic Chem lec-lab class. It was the first semester I was actually given a "lighter' load although the schedule assigned to me was annoyingly spread out for 6 days, mondays through saturdays... arrrgghhh!

So here are the students who were "lucky" enough (tsk! I can hear some of you protesting to my use of that word) to have me as their teacher this semester...

BSN 1A

The first time I met 1A, I felt snubbed. (Yes, that's right 1A!) They thought they'll be handled again for the second time by their favorite chem teacher, Sir Roger (not Moore). I can still "hear" their collective disappointed sighs when they found out that Sir Roger and I decided to swap classes for the second semester. I just hope that their "We'll miss you ma'am!" the last time we met was a sincere declaration that they no longer regret entering my class... hehehe!

This is one responsible group. Everyone is busy whenever an activity is performed. Everyone listens when a discussion is going on. They ask questions. They probe. They share what they know. It's the kind of class that's easy to facilitate. It's the kind of class teachers generally look for :)

BSN 1D

Hmmm... 1D... the class that really knows how to push my buttons.... 23 girls, 1 gentleman. (Yes girls of 1D. Based from what you have shown me for 2 semesters, you're still in that girls/boys category lol!)

1D may be the noisiest among my classes, but it's generally the sweetest. Thanks to Abbie for always offering me food (awww...), which I always decline since eating is not allowed in the lab... hehehe!

This class has the tendency to "forget" things they have to bring to class. There was even a time when I have to ask 4 among the 5 groups to leave the lab room for non-bringing of materials. And whenever they're asked if they have understood the pre-lab discussed, they would all say yes but the minute we switch into performing the actual activity, all questions, sensible or otherwise, would be thrown at me... grrrr!

I have a feeling that, all things considered, this class likes me. I have to limit class size so I only took in those who were around the first day we met last November. Some were asking-- begging-- that they be considered for inclusion in my class list later but, i have to refuse. One student in particular-- what's his name? Adrian something?--- I have to refuse because of an entirely different reason. (Sssshhh... don't tell him but he asks way too many questions... lol!).

BSND1

In my more than 10 years of teaching, this is the first time I handled a class who dared to show me their obvious disgust for learning organic chemistry. This is the class who makes me feel embarrassed whenever I give them a test after a discussion... This is the class whose test scores makes me feel that they were actually taking the test to see who would get the lowest score... This is the class I decided to give the exact same test the next meeting just so they'll get better scores only to find out that their second result is no better than the first... This is the class who has the guts to stretch their arms up and utter ho-hum while I'm busy illustrating the structure of trinitrotoluene on the board... lol!

But then, although they don't appreciate Organic Chemistry that much, I am sure that they appreciate me. They were even asking if I will be handling them in Biochem. You know what ladies? I will be happy to teach you Biochem. Just promise that you'll give up the arms-stretching and ho-hums when I deliver the lectures :D

BSN 1E

Yeah... NO IMAGE AVAILABLE. But that doesn't mean they're literally invisible. This is the same half-class whose Gen Chem I handled too. The same class I always have a problem meeting regularly because their class schedule, wednesdays and saturdays, would often overlap with school programs and activities.

Although it's an all-ladies class, I have not felt any displeasure in handling them. They're manageable and they work efficiently. Although most of them would just rely on 1 person --- Imee, the class president --- to provide all the groups with most of the materials needed in the lab, I don't have complaints. Imee I'm sure has a different story lol!



To all my 2nd semester students--- ladies and gentlemen... and girls --- thank you for making the last five months interesting and fun. For most of you, I pray, that we will never meet again in that same lab room performing the same activities and talking about the same things...

You know what I mean ;)

Long Test - Energy Metabolism (40 Items)

To My BSMT Classes...

The questions I have posted for this test are of 2 types: True or False and Identification. The questions will randomly appear as you click the "submit my answer" tab on each page. This means the sequence the questions will follow is unique for each quiz-taker. Make sure you enter the correct spelling of the words for questions under the identification type.

Before starting the test, you will be asked to enter your name. Please 
include your course, year and section, i.e. LEMUEL OWEN H. CABANA BSN 1A.

You are given 30 minutes to finish answering all the questions. Yes, I know I promised to give you an hour but I have taken the test myself and realized that 1 hour is too long a time to allow you to go over your lecture notes ( Yes, i know that you do that, too! ;) ). Your result will appear after the 40th question is answered. You are required to print the certificate generated by the site at the end of the test. Submit such to me on the next lecture class meeting.

Please click here to go to the quiz page... Good luck!

P.S.

To help you get the hang of taking online tests, I have posted a 10-item practice test found on this page. There's no limit to the number of times you may take this 4-minute test. You also have the option to print the score report that will appear at the end. Scores you get in this test will not be recorded.

Biochem Experiments 15 to 18....

15 is here...

16 is here...

17 is here...

18 is here...

When Atoms Go To War... :))

Biochem Experiments 13 and 14

Lab Activity 13 is here...

14 is here...

Test for Reducing Sugars

All monosaccharides and disaccharides, except sucrose, are reducing sugars. This means they reduce oxidizing agent-containing reagents like Tollen's, Fehling's and Benedict's.

The following video clip shows the difference in the manner sucrose and glucose react with Fehling's reagent...

For My Organic Chemistry Class (BSND1)

Lab Activity No. 5: Test for Unsaturation: Differentiating Alkanes from Alkenes

Because an alkene contains at least 1 double bond in its structure, it is classified as unsaturated. Bromine (Br2) is used to determine whether an unknown hydrocarbon is saturated (alkane) or unsaturated (alkene or alkyne). The following video clip shows the difference in the manner cyclohexane and cyclohexene react with bromine.





Questions: (Please use 1 short-sized bond paper to copy and briefly answer the following questions.)

1. Describe the difference in the reactions of cyclohexane and cyclohexene with bromine. What does this difference imply?

2. What kind of reaction occurs when Br2 is reacted with cyclohexene?

3. Complete the following reactions. Illustrate and name the structures of the product/products formed.

Biochem Midterm Laboratory Activities

you may now download lab activities 7 to 12 here...