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The Hitchhiker's Guide to the Periodic Table

Take a nice long look at the periodic table, Mendeleev's favorite creation. Seriously. Check out the

colors, the rows, the columns, and the symbols. Have you ever wondered why the table is the way

that it is? What was good ol' Dmitri thinking when he put certain elements in one row and other

elements in a different row? At first it might seem like a random mess of numbers and letters. There

is, indeed, a method to the madness. In fact, we might say it's elementary, my dear Watson.

It's human nature to organize things. Librarians organize books. Cooks organize their kitchens. Who

hasn't spent time organizing their Skittles to accurately reflect the rainbow? Chemists are no

different than the rest of us. Okay, maybe they're a little different. 

The periodic table is the tool chemists have concocted to organize all of the elements, which are

substances (like carbon or hydrogen) that cannot be decomposed into simpler substances. You may

have noticed the periodic table looks like a big rectangular-ish grid. Each element has its own cheat-

sheet of chemical information found in a specific place within the grid.

Don't be worried if the periodic table you're used to doesn't look exactly like the one above. Each

periodic table is unique. Some contain more information, some less. If the bells and whistles of

a fancy table don't appeal to you, stick to a more basic table like the one here. 

Let's take a closer look within an individual periodic table box. Pick your favorite element…Cobalt,

you say? It wouldn't have been our first choice, but if you insist. (Just kidding. We love all the

elements equally.)

One piece of information found in every periodic table is the atomic number (located in the upper

left-hand corner in the example above). This value, unique to each element, indicates the number of

protons present in the nucleus of an atom. For cobalt, the atomic number is 27, because all cobalt

atoms have 27 protons. Clever, is it not?

All periodic tables also contain the chemical symbols for each element. These symbols are simple

two-letter abbreviations of the elements' names. For many elements, like Cobalt, the symbol is just

the first two letters of the name…like Co. For other elements the symbols are not as

obvious. Mercury's chemical symbol, for example, is Hg. In case you are curious Hg is derived from

the Latin word "hydrargyrum" meaning "liquid silver." Very fitting if you ask us.

One final piece of information found in the elemental box of most periodic tables is the atomic

weight. As the name suggests, this is the mass of a single atom of the element. This information is

very useful when solving all kinds of chemistry problems on exams and quizzes.

The elements are placed in specific locations in the periodic table grid according to the way they

look and act according to a concept called periodicity. (More on that later.) Within the grid there

are rows and columns that help organize elements with similar properties together. So there was a

method to Mendeleev's madness.

Horizontal rows of the periodic table are called periods.

Horizontal rows of the periodic table are called periods. Even though some boxes appear to be

missing, all of the rows go left to right skipping over the blank areas. Every element in the same

period has the same number of atomic orbitals. These orbitals (s, p, d, and f) are the area around

an atom where its electrons are most likely to be found.

Confused? Let's take a closer look. The elements of the first row of the periodic table (colored in red,

above) have a 1s orbital available for their electrons to sit in—all comfy and cozy. The elements of

the second row of the periodic table, which is cleverly called the second period, have a 1s and three

1p orbitals available for their valence electrons. These are the electrons located in the last shell or

energy level of an atom. The fifth period elements have a 5s, three 5p, and five 5d orbitals available. 

Vertical columns in the periodic table are called groups (or families).

The vertical columns in the periodic table are called groups (or families, depending on who you

ask). The left-most column is called group one. The next group is called group two. Any guess what

the third column is called? Hint: It starts with group and ends with three. 

Each element in a particular group has the same number of valence electrons in their outer orbital.

For example, lithium (Li) and sodium (Na) are both members of the group one club. Lithium has

a valence electron configuration of 2s1, while sodium has a configuration of 3s1.This similarity is

significant because valence electrons are the ones that form chemical bonds with other elements. In

other words, elements of the same group tend to exhibit similar reactivity and tastes in music.

The periodic table is also split into four blocks: s = red, p = green, d = yellow, and f = blue.

To further complicate things and make your studies of the periodic table even more complicated,

the periodic table is also broken into four blocks. Check out the table above. Seriously, check it out.

We'll wait.

The first two columns (shown in red) comprise the s-block. The next 10 columns (shown in yellow)

comprise the d-block. We'll let you use your super powers of deduction to determine the location of

the p-block and the f-block. The highest-energy electrons of each element in a block belong to the

same atomic orbital type. In other words, elements in the s-block have their highest energy

electrons in an s orbital, while elements in the d-block have their highest energy electrons in a d-

orbital.

Main-group elements and transition elements of the periodic table.

Did you ever think one table could be split into so many classifications? Well, we're not done yet.

Our favorite table can also be broadly divided into main-group elements and transition

elements (or transition metals). The main-group elements are shown in red in the table above,

and their properties are easily predictable based on their position in the periodic table. The

transition elements are shown in yellow, and their properties are not as easily predictable. 

The elements of the periodic table can also be classified into metals, nonmetals, and metalloids.

We'll get into the nitty-gritty details of each column in the next few sections, but let's ease our way

into this adventure for now.

Periodic table color-blocked into metals, nonmetals, and metalloids.

Metals occupy the left side of the periodic table. Check out the boxes shaded in those warm yellow

and orange shades in the table above. Don't forget the two long rows at the bottom, either. Boom.

Metals. They are good conductors of heat and electricity, which is a fancy way of saying heat or

electrons can easily flow through the chunk of metal. Another term usually thrown around when

talking about metals is malleability, which means metals can be pounded into flat sheets or

different shapes. Good examples of metals that we're all familiar with are iron (Fe), silver (Ag), and

sodium (Na).

On the right side of the periodic table, colored in that awesome purples, pinks, and blues are the

elements cleverly named the nonmetals. These elements have properties that are more varied than

their metallic cousins. Some are solids at room temperature like carbon (C), while others are gases

like helium (He) or oxygen (O). Nonmetals tend to be poor conductors of heat and electricity.

The elements in that funky zigzag line shaded in green in the periodic table above are called

metalloids. These elements are neither metals nor nonmetals, but they do share some properties

with both groups. For example, metalloids can conduct electricity like metals. Silicon (Si) is a super

example of a metalloid element.

Silicon (Si) is a metalloid.

While we won't go into specific details about the f-block in this module it is important for you to

know that there are two types of compounds in this series. The first row of the f-block is called the

lanthanides. The second row of the f-block is called the actinides.

Brain Snack

The only letter that does not appear anywhere on the periodic table is the letter J

Practice Problems:

1. The recurring pattern in the properties of the elements when they are arranged in order of

increasing atomic number is called _________.

A. the periodic law

B. Dimitri's law

C. Mendeleev's law

D. the elemental law

E. none of the above

2. In the first periodic table, elements were arranged by increasing ______.

A. size

B. atomic number

C. molecular weight

D. relative mass

E. none of the above

3. In the modern periodic table elements are arranged from left to right by _______.

A. increasing atomic number

B. decreasing atomic number

C. increasing molecular weight

D. decreasing molecular weight 

E. none of the above

4. The gaps or blank spaces in the first periodic table allowed Mendeleev to _____.

A. take a nap

B. trick question—there were no gaps

C. predict the abundance of undiscovered elements

D. predict the properties of undiscovered elements

E. none of the above

5. There are ____ known elements today.

A. 42

B. 63

C. 100

D. 108

E. 118

The Periodic Table is Oh So TrendyAt this point, we've examined various elemental occupants of the periodic table. Trust us,

understanding general trends within families will come in handy one day, either on a test or when

playing Jeopardy!. But that's not all the periodic table has to offer. The truth is, if you look at the

table as a whole, some even more powerful trends start to emerge that can help us compare

elemental properties and even predict reactivity. It's like having a flat, gigantic crystal ball.

Atomic Radius

By definition, the atomic radius is one-half the distance between nuclei of two atoms. In other

words, atomic radii are used to measure atomic size. In general, atomic size decreases as we move

left to right across the periodic table. This can be a little confusing, but we have you covered—read

on. 

As we move left to right across a period we are also observing an increase in atomic charge (the

number of protons is increasing). This greater nuclear attraction pulls the electrons in more closely,

and the atomic radii actually decrease. Think of it like a big nuclear hug. The more protons, the

more love, and the electrons are pulled in closer to the nucleus.

Going down a group, atomic radii increase. This time, we're comparing electrons in different shells.

Electrons in a 1s orbital are closer to the nucleus than electrons in a 2s orbital. In essence, the

electrons in the outer shells don't "feel" the pull of the nucleus as strongly as those that are closer to

the nucleus. Think about all the funky magnets you keep on your refrigerator. You use one of the

magnets to hold up your prized study hall doodle. No problem—the magnet holds the single piece

of paper with ease. But then you start adding more and more pieces of paper. (You've created some

mad cool doodles this week.) Eventually, as you add more and more layers, the magnet loses its

hold and falls off. This is similar to the idea of electron shielding. As we add more and more orbitals,

the outer elections feel less of a pull from the nucleus and are able to get further away—hence the

larger atomic radii as we go down the group.

Atomic radii trends in the periodic table.

Electronegativity

Electronegativity is the property describing an atom's ability to attract an electron. Electrons are

drawn to elements with high electronegativites like Taylor Swift is drawn to boyfriends. Remember,

elements are just dying to have a stable noble gas configuration. The elements that only need one or

two extra electrons to get to that state are the most electronegative—they want electrons and

they'll do anything to get them.

As we move to the right across a period of elements, electronegativity increases. Atoms can either

gain electrons or lose electrons. When the valence shell of an atom is less than half full, it's easier to

lose an electron. (Those elements want to downsize, so they are practically giving electrons away.)

When the valence shell of an atom is more than half full, it's easier to gain an electron; those

elements will try to get a full shell by adding on.

As we move down a group, electronegativity decreases. As we navigate down a group the atoms get

bigger and bigger with more and more electrons. This means the outermost electrons get further

and further away from the positively charged nucleus. The consequence? Electronegativity

decreases. Again, think of the magnet being shielded by all those doodle-pages.

Electronegativity trends in the periodic table.

It's really important to remember these are just trends not rules. There are always exceptions. For

example, the noble gases have negligible electronegativity values despite being on the very right

hand side of the periodic table and the transition metals have only very small differences in their

electronegativity values. 

Knowing what we know, what is the most electronegative element? 

Both arrows point to fluorine.

Ionization Energy

is the amount of energy needed to remove an electron from an atom. Elements on the left hand side

of the periodic table like alkali and alkaline earth metals have low ionization energies because

losing an electron (or two) would make them achieve the noble gas configuration. Sometimes it's

just easier to downsize to a lower shell, especially when you only have to lose a couple electrons to

do it. Elements on the right hand side of the table have higher ionization energies because

want more electrons, not less. Think of the elements on the right as being like Gollum and the

golden ring represents electrons. Their electrons are precious, and it would take a lot of energy to

take them away. Thus, ionization energies increase from left to right across the table—the left side

is giving, the right side is Gollum.

We've already figured out that as we go down a group the atoms get larger and the outermost

electrons move further away from the positively charged nucleus. Which are stronger, long-

distance relationships or short-distance relationships? As any rom-com fan will know, long-distance

relationships rarely stick. The answer is the same for electrons. Electrons that are further away

from the nucleus are not as tightly bound as electrons that are closer; therefore as we move down a

group the ionization energy gets smaller.

Ionization energy trends in the periodic table.

The trends for electronegativity and ionization energy are eerily similar. Remember that for your

next quiz.

Practice Problems1. Which of the following is not a periodic trend?

A. Atomic Radii

B. Reactivity

C. Ionization Energy

D. Electronegativity

2. Electronegativity _____ as you proceed left to right across a period and _____ as you proceed down

a group.

A. stays the same, increases

B. increases, decreases

C. increases, increases

D. decreases, increases

E. decreases, decreases

3. Ionization Energy _____ as you proceed left to right across a period and _____ as you proceed down

a group.

A. stays the same, increases

B. increases, decreases

C. increases, increases

D. decreases, increases

E. decreases, decreases

5. Atomic radii _____ as you proceed left to right across a period and _____ as you proceed down a

group.

A. stays the same, increases

B. increases, decreases

C. increases, increases

D. decreases, increases

E. decreases, decreases

Done? Please get a laptop and go tohttp://www.sciencegeek.net/Chemistry/taters/Unit2PeriodicTrends.htm

for more practice