The Periodic Table | SciByte 10

The Periodic Table | SciByte 10

This week on SciByte …
We take a look at the periodic table, how to read it, what some of those crazy little numbers mean, and why it’s laid out the way it is. We also take a look at where an element is on the table affects how an element interacts with other elements.
All that and more, on SciByte!

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Show Notes:

Origins to the concept of the atom
  • We know that all atoms are composed of a given set of subatomic particles: protons, neutrons, and electrons. These particles have definite arrangements for any given element.
  • The earliest known concept of the atom came from the Greek philosopher/scientists Leucippus and his student Democritus nearly 2500 years ago!!! (~460 – 370 BCE)
    • Democritus and Leucippus : Thought of the world as being composed of very tiny “uncuttable” particles, which they called “atomoz” or atoms, he also explained differences in materials as caused by differences in the sizes of the particles and the amount of empty space between them.
  • Aristotle argued persuasively against the concept of atoms and thought the earth was composed of matter, which he believed was made up of four elements: earth, air, fire, and water. He said that different types of matter as arising from the proportion, form, and qualities of the four basic elements that each type of matter contained.
  • The concept of atoms wasn’t widely accepted again until the early 1800’s
    • John Dalton used the concept of the atom to explain why elements always react in ratios of small whole numbers and why certain gases dissolve better in water than others to provide the earliest empirical evidence of Atoms ~1808-1810
  • Dalton estimated the atomic weights according to the mass ratios in which they combined using Hydrogen as the basic unit
    • He did not take into effect that some elements atoms exist in molecules (like pure oxygen exists as O2)
    • He also falsely believed that the simplest compound between any two elements is always one atom of each (so he thought water was HO, not H2O).
  • The flaws in Daltons theories were corrected in 1811 by Amedeo Avogadro who had proposed that equal volumes of any two gases, at equal temperature and pressure, contain equal numbers of molecules
    • Avogadro was able to offer more accurate estimates of the atomic mass of oxygen and various other elements, and firmly established the distinction between molecules and atoms
What are Protons again?
  • Protons are positively charged particles, weighing 1 atomic mass unit and located in the nucleus.
    • 1 atomic mass unit ~= 0.00000000000000000000000167 grams)
    • Symbol(s) : p, p+, N+
What about Neutrons?
  • Neutrons are neutrally charged particles, weighing approximately 1 atomic mass unit and located in the nucleus.
    • 1 atomic mass unit ~= 0.00000000000000000000000167 grams)
    • Symbol(s) : n, n0, N0
Don’t forget about Electrons!
  • Electrons are negatively charged particles weighing zero atomic mass units and located in the various orbitals of the energy levels outside the atomic nucleus.
  • It would take about 1,830 electrons to equal the mass of one proton.
    • 1 electron mass ~= 0.000000000000000000000000000911 grams)
    • Symbol(s) : e, β
The origins of the ‘Table’
  • In about 1868 a Russian scientist, Dmitri Mendeleev, arranged the 60 elements known at that time in order of each element’s increasing mass.
  • Mendeleev was able to see a periodicity in the other characteristics of the elements. PIC : Mendeleev’s original table
Wait … periodicity, like in Periodic Table?
  • Periodicity the tendency to show a regular repeating pattern, being regularly recurrent or having periods.
  • While laying the elements that he knew out Mendeleev left blank spaces, believing that there should be undiscovered elements, and actually predicted some of the properties of those elements based on their position in his table.
  • However lack of spaces for undiscovered elements and the placing of two elements in one box were criticized and his ideas were not accepted.
A new way to order the elements
  • In 1913, Henry Moseley was studying atomic structure and helped determine atomic numbers for chemical elements.
  • Ordering the elements by atomic number, the number of protons, instead of by weight, allowed the few problems Mendeleev’s table to disappear.
Periodic Table : Symbols
  • It may be either one capital letter or a combination of one capital and one lowercase letter.
  • The letter(s) usually have something to do with some form of the name of the element
  • Some are very easy to figure out, like H for hydrogen or O for oxygen. Others refer to older names for the element in different languages like Greek or Latin.
    • Gold : Aurum : Au
    • Sodium : Natrium : Na
    • Iron : Ferrum : Fe
    • Copper : Cuprum : Cu
    • Silver : Argent : Ag
    • Tin : Stannum : Sn
    • Antimony : Stibnum : Sb
    • Tungsten : Wolfram : W
    • Mercury : Hydrargyrum (liquid silver) : Hg
    • Lead : Plumbum : Pb
Periodic Table : Atomic Number
  • Atomic number : equal to the number of protons in the nuclei of its atoms.
  • The integer, whole number, in some part of the box. This is the atomic number of the element.
    • Recall that atomic number represents the number of protons found in the nucleus
  • The atomic number or number of protons in the nucleus that determines what element you are working with.
    • Adding a proton to a Carbon atom gets you Nitrogen
    • Taking a proton from a Calcium atom gets you Potassium
Periodic Table : Atomic Weight
  • Since the heaviest naturally occurring element has only 92 electrons in its normal state, we do not count the mass of the electrons in calculating the weight of the atom. [PIC : Electron / Proton / Neutron Weights Comparison]
    • It would take about 1,830 electrons to equal the mass of one proton.
  • It is also the average of all the masses of all the isotopes of that particular atom, calculated according to the actual abundance of the isotope.
What are Isotopes?
  • An isotope is one particular form an atom of an element might take
  • The mass of an atom is determined by adding the number of protons to the number of neutrons in the nucleus
  • Since the atoms of an element must have the same number of protons, the only thing that can vary to cause this change is the number of neutrons.
An Isotope Example : Averaging Carbon Isotope weights
  • There are three basic isotopes (forms) of carbon.
  • The most common form has six protons, six electrons, and six neutrons. It is known as carbon-12, to designate the mass of the six protons plus the six neutrons.
    • Carbon-12 makes up 98.89% of carbon
  • There is an extremely rare isotope of carbon known as carbon-13. This form has six protons, six electrons, and seven neutrons. Therefore, its atomic mass number would be 13.
    • Carbon-13 makes up 1.11% of carbon
  • The least common, radioactive form of carbon is carbon-14. Carbon-14 contains six protons, six electrons, and eight neutrons per atom.
    • Carbon-14 makes up 0.01% of carbon
  • Therefore to calculate the average isotopic weight of Carbon we multiply the weight of each one by the percentage it makes up and add them all together
    • Carbon-12 : 12 * 0.9889 = 11.8668
    • Carbon-13 : 13 * 0.0111 = 0.1443
    • Carbon-14 : 14 * 0.0001 = 0.0014
    • Carbon-12 + Carbon-13 + Carbon-14 11.8668 + 0.1443 + 0.0014 = 12.0107
Carbon-14 … like in Carbon Dating?
  • Yes, Carbon-14 is an unstable radioactive isotope
  • As an unstable element, when a living object is removed from the living ecosystem a neutron can ‘decay’ into a proton
    • Like when a tree dies or gets cuts down
  • When one of the neutrons ‘decays’ into a proton the atom becomes Nitrogen, and this happens at a specific rate
  • Since we know the percentage that Carbon-14 makes up in the living world, and we know the rate at which is decays into Nitrogen we can use math to calculate how long a given object has been out of the living ecosystem
Periodic table : rows
  • The table is organized into horizontal rows called periods that read from left to right, just like a book.
  • When there is a space in the middle, just jump across it as if you were reading around a picture inserted in the text on a page.
  • When you read a page in a book from left to right, you have to drop down a line to continue. The same is true for the periodic table.
  • Elements get heavier as you go across from left to right
  • Energy levels of the atoms correspond to the rows of the table
  • You are moving to a row of elements with another energy level for the atoms’ electrons to fill. This means that the top row has only one energy level. The second row (period) adds a level to have a total of two energy levels that the electrons must fill. The third period contains three energy levels for electrons, and so on.
    • 1st Row : 2 elements == 1st shell holds 2 electrons
    • 2nd Row : 8 elements == 2nd shell holds 8 electrons
What are orbitals?
  • We’ve all seen the nice neat little circles that we picture electrons in … it’s not quite like that
  • There are actually more like spheres, or not spherically shaped regions
  • At any given time, we can’t actually tell where an Electron is
  • So an orbital is a region in which the electron can probably be found.
  • Each of these ‘levels’ of electrons regions have very specific shapes
Energy Levels
  • Energy levels are built up from the level closest to the nucleus outward.
  • The most energy levels currently found in an atom of an element at this time is seven. We have seven periods in the periodic table (the two bottom rows are actually continuations of the 6th and 7th periods.) The period number is usually found to the left of the first element box for each row.
  • Hund’s rule states that each p orbital must receive one electron before any p orbital can receive a second filling electron.
    • “ Everybody gets one before anybody gets seconds. “
  • The process of filling in the electrons from the first, lowest energy level to the second, slightly higher energy level to the third, even higher energy level is called filling the electrons in by the Aufbau Principle.
    • Aufbau principle : start at the lowest energy level and build up to the higher energy levels only after the lowest are filled.
Periodic table : Columns
  • Groups are numbered according to several different conventions, but the most commonly accepted method is the IUPAC (International Union of Pure and Applied Chemistry) system
  • There are 18 vertical columns of varying length going across the table. These columns are commonly known as groups, or families of elements.
  • All elements in a column have similar chemical properties because each column has the same number of outer electrons, and it is the outer electrons (Valance Electrons) are involved in chemical reactions
Example – Water Molecule H2O
  • Hydrogen (H) : needs 1 electrons in outer shell to be stable
    • Cell 1 of 2 of it’s row; therefore has 1 electron in an outer shell capable of holding 2
  • Oxygen (O) : needs 2 electrons in outer shell to be stable
    • Cell 6 of 8 of it’s row; therefore has 6 electrons in an outer shell capable of holding 8
  • SO if you have 2 Hydrogen and 1 Oxygen they can share valance electrons
  • Now all three elements have full (stable) outer shells
Column = Groups/Families
  • Column I A- Alkali Metals
    • The alkali metals are the most reactive of all of the metals. Adding them to water causes the hydrogen in the water to be released as a gas
  • Column II A- Alkaline Earths
    • alkaline earths become increasingly soluble with a decrease in temperature
  • Columns III B through I B – Transition elements
  • Column VII A – Halogens
    • Halogens form salts when they react with a metal. All of these elements exist as diatomic molecules in their gaseous state. This means that two atoms bond together to form a molecule of the gas.
  • Column 0 – Noble Gases
    • The noble gases are generally chemically inert,perceived lack of participation in any chemical reactions, and used in industry in arc welding, to dilute the oxygen in deep-sea divers’ gas tanks, and to fill light bulbs.
  • Obviously an atom is way too small for anyone to be able to accurately weigh one. There are no scales in the world sensitive enough to accomplish this. So scientists have come up with another method for measuring mass. They use the concept of a specific quantity, known as a mole, to compare the mass of different elements.
  • One mole of any substance is 6.023 X 1023 atoms or molecules of that substance. In this way, moles are a counting unit kind of like dozens. Just as there are 12 items in a dozen of anything, there are 6.023 x 1023 items in a mole.
  • By definition, a mole of any element is the amount of that element in the gaseous state at standard temperature and pressure occupying a volume of 22.4 liters.
  • For example, if we have a box of 12 golf balls and an identical size box of 12 ping-pong balls, we can figure out how much the mass of one golf ball is compared to a ping-pong ball. This would be its relative mass.
Online interactive Periodic table quiz/games
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