Patterns Found in the Elements

A

Elements Known by the AncientsThe ancient civilizations knew of the following elements:Au, Ag, Cu, Fe, Pb, Sn, Hg, S and C.(gold, silver, copper, iron, lead, tin, mercury, sulfur and carbon)

Elements Discovered by Alchemists in the Middle AgesAlchemists experimented to turn base materials into precious materials like gold or a potion for eternal life. As they did this they discovered these new elements: As, Sb, Bi, P, Zn (arsenic, antimony, bismuth, phosphorus, zinc).

Discovery of Electric CurrentAlessandro Volta in 1800 discovered that electric current could be made by using two different metals in a salt, acid or base solution. He also discovered that the current could be increased by hooking up several cells in a row (in series). The combination of many cells in series was called a battery.

Batteries Used to Decompose Compounds into ElementsIt was found that many pure substances (like water) could be

broken apart into its elements by using the electric current that a battery provided. William Nicholson and Anthony Carlisle in 1800 decomposed water into oxygen and hydrogen gas.

Electric Current Used to Find New ElementsHumphry Davy and other experimenters (1807) used electricity to break apart many substances into elements that mankind had not seen until then (K, Na, Ca, Mg, B, Ba) . Molten table salt (NaCl at 801o C)) was split apart into a silvery, explosive metal (sodium) and an irritating yellowish gas (chlorine).

Elements Discovered From 1765 to 1845Electrical current enabled scientists to discover 42 new elements from 1765 to 1845.

How to Classify the Elements?Scientists began looking for patterns among the elements. Since the atomic weights of these elements was known by 1845, chemists began to list the elements in order by weight and then look for similarities or repetitions of characteristics.

H He Li Be B C N O F Ne Na Mg

The Octave of Elements

In 1865, John Newlands published his “Law of Octaves” which stated that every 8th element repeats the properties of the element that was 8 before it, like a piano repeats a similar tone every 8th key (an octave). It soon was found out that this pattern just works for elements 1-20 but not beyond this.

H He Li Be B C N O F Ne Na Mg

Dmitri MendeleevIn 1869, Dmitri Mendeleev published a chart of the elements in which he noted recurring properties among the elements. He formulated The Periodic Law which stated that if elements are arranged in order by their weights (today by at. number), then similar properties will recur periodically.

The Periodic

Law:

Repeating Properties Among the Elements

Mendeleev’s Chart Showing PatternsAs Mendeleev listed his elements in order by weight, he placed them alongside other elements with similar chemical and physical properties to show that they belonged together in some way. He referred to elements that had similar properties as a “family” of elements.

Gaps in Mendeleev’s ChartIn some places in his chart, Mendeleev found that the next heaviest element had properties that fit with a group of elements beyond the next family. When this happened, Mendeleev left a space, placing this element with the family it belonged with. Mendeleev predicted that an element would be discovered that would have a weight and properties that would place it in the space he had left.

Mendeleev’s Method of Predicting New ElementsMendeleev predicted that elements to be discovered would have chemical properties like the family they would be in. He predicted the atomic masses, densities and other physical properties of the elements to be discovered by averaging these from the elements on either side of the blank space and from the elements above and below the blank space. For example, Mendeleev had a blank space after Ga because As had properties like P. To predict the atomic weight of the element ?, Mendeleev averaged the weight top and bottom of ? with the weights to the left and right of ? to get the weight of 72.86 which was very close to 72.59, the weight of ? or Ge.

?

Si28.0855

Sn118.71

As74.92

Ga69.72

P30.973

The Success of Mendeleev’s PredictionsMendeleev’s predictions of several elements to yet be discovered were quickly confirmed by other chemists and his predictions about their properties proved very accurate. This made chemists realize that the idea of chemical families and Mendeleev’s periodic chart demonstrated some underlying structure among atoms still to be discovered.

The Reason for Chemical Families RevealedIn 1913, Niels Bohr presented his model of the atom which helped explain why elements repeat their properties as they are ordered from lightest to heaviest. Bohr’s model showed that elements in a chemical family all have the same number of outermost electrons.

The Quantum Atomic Model Also Explains PeriodicityElements in the same chemical family have the same electron configuration, just at increasing energy levels.

The Periodic Chart Reflects Quantum Atom StructureThe ordering of the elements follows the Aufbau Principle of the Quantum Model of the Atom.

The Modern Periodic ChartThe modern Periodic Chart of the Elements arranges elements into vertical columns and horizontal rows. Vertical columns are called chemical families and elements in vertical columns have similar chemical and physical properties.

The Alkali Metal FamilyThe alkali metal elements are silvery, have a low density (float on water), react very strongly with water to form hydrogen gas and very strong bases (alkalis).

The Alkaline Earth FamilyThe alkaline earth metal family is less reactive than the alkali metals but alkaline earth metals burn brightly in air, some very colourful (strontium is used in red fireworks). Alkaline earth elements also react with water and are very common elements in earth rocks (calcium and magnesium especially).

The Halogen FamilyThe halogens are very reactive and corrosive, especially with metals (Na + Cl NaCl {table salt}). At room temperature, fluorine and chlorine are gases while bromine is a liquid and iodine is a solid. Chlorine, bromine and iodine are very good anti-bacterial agents.

The Noble Gas FamilyThe noble gases are non-reactive and stable by themselves. They are all colourless and odourless gases. They glow with distinctive colours (Ne is reddish) when electricity is passed through them. Their ion charges of zero indicate that they do not form charged ions.

A Major Pattern Within The ElementsThe elements can be classified as either metals or nonmetals. Metals are shiny, malleable, ductile, good conductors of heat/electricity, and form + ions. Nonmetals are dull, brittle/crumbly, poor conductors, and form – ions. Metalloids or semi-metals have characteristics inbetween metals and nonmetals.

Some Periodic Properties of AtomsAtomic and Ionic size, Ionization energies and Electron Affinities are properties which can be predicted using the Quantum Model of the atom.

Atomic and Ionic SizeAn important atomic property is its size relative to other atoms. An atom’s size changes when it becomes an ion.

Ionization EnergyIonization energy is the amount of energy needed to remove the outermost electron from an atom of a gaseous element.

Electron AffinityElectron Affinity is the amount of energy released when an electron is added to a neutral atom in the gaseous state.

Factors Used to Predict Atomic Properties1. The pel (principal energy level) of the outermost electrons. This factor relates to the distance that the outermost electrons are from the nucleus. It is the primary factor for determining many atomic properties. Check the highest pel first!

Factors Used to Predict Atomic Properties2. The size of the nuclear charge is the second most important factor in determining atomic properties. A greater nuclear charge (+) pulls electrons (-) closer because of the attraction of opposite charges. Electrons are held more tightly (attracted more strongly) by the nucleus when they are closer to the nucleus.

Factors Used to Predict Atomic Properties2. The size of the nuclear charge is the second most important factor in determining atomic properties. A greater nuclear charge (+) pulls electrons (-) closer because of the attraction of opposite charges. Electrons are held more tightly (attracted more strongly) by the nucleus when they are closer to the nucleus.

Factors Used to Predict Atomic Properties3. The screening effect of inner electrons between outer electrons and the nucleus is the third most important factor (after nuclear charge and pel) in determining atomic properties. The layers of inner electrons affect the attraction force that the nucleus has for its outer electrons by weakening or diluting the effective nuclear attraction as these layers increase.

Factors Used to Predict Atomic Properties4. The repulsive effect of electrons within the same orbital is the fourth factor affecting atomic properties. The repulsive effect only becomes significant in the rare occasions when, for 2 situations, the effects of pel, nuclear charge and screening are generally equivalent. Repulsive effects tend to move electrons slightly apart from each other due to their identical charge.

Using the 4 factors to Predict SizeWithin the alkali metal family, the size of atoms increases going from Li to Na to K because in this progression, a new pel is being added with each successive element. A new pel being filled means successive outer electrons are farther from the nucleus.

Using the 4 Factors to Predict SizesFor the atoms, Mg and Cl, their outer electrons are on the same pel but the nuclear charge of Cl is greater. Thus an atom of Cl will be smaller than an atom of Mg due to its stronger attraction for electrons on the same pel.

Atomic Size Trends in the Periodic ChartAtoms increase in size from top to bottom due to the addition of new pels. Atoms decrease in size from left to right due to increasing nuclear charge, increasing effective nuclear charge on outer electrons.

Predicting Ion Sizes Relative to Atom SizesHow would the sizes of the following species compare : 9F, 9F-, 10Ne, 11Na, 11Na+, 12Mg, 12Mg2+ ?Write the electron configuration for each species:9F : 1s22s22p5

9F- : 1s22s22p6

10Ne : 1s22s22p6

11Na : 1s22s22p63s1

11Na+ : 1s22s22p6 11Na

12Mg

12Mg : 1s22s22p63s2

12Mg2+ : 1s22s22p6

Based on the above, neutral sodium and magnesium atoms would be the largest since they have a 3rd pel (1st factor). A magnesium atom would be smaller than a sodium atom because it has more nuclear charge (2nd factor).

Predicting Ion Sizes Relative to Atom Sizes (Cont.)The remaining species all have the same outer pel. Thus the second factor (nuclear charge) is checked to determine size.9F : 1s22s22p5

9F- : 1s22s22p6

10Ne : 1s22s22p6

11Na+ : 1s22s22p6

12Mg2+ : 1s22s22p6 9F-

9F

The smallest nuclear charge (the smallest or weakest effective attraction) is found in 9F and 9F-, making these the largest. Of these two, both with same pel and nuclear charge, 9F- has one more electron which causes it to be the larger of the two due to factor 4, the repulsion effect.

Predicting Ion Sizes Relative to Atom Sizes (Cont.)In the remaining species which have the same outer pel, the second factor, nuclear charge, is checked for differences.10Ne : 1s22s22p6

11Na+ : 1s22s22p6

12Mg2+ : 1s22s22p6

10Ne 11Na+ 12Mg2+

The smallest to largest nuclear charge, the weakest to strongest effective electron attraction, hence the largest to smallest sized, is 10Ne, 11Na+, 12Mg2+ .

Relative Sizes Compared9F : 1s22s22p5

9F- : 1s22s22p6

10Ne : 1s22s22p6

11Na : 1s22s22p63s1

11Na+ : 1s22s22p6

12 Mg : 1s22s22p63s2

12Mg2+ : 1s22s22p6 A

11Na 12 Mg 9F-

9F 10Ne 11Na+ 12Mg2+

Periodic Ionization Energy Patterns

Ionization energy (The energy needed to remove the outermost electron) increases across a period due to increasing nuclear charge which attracts electrons more strongly, making it more difficult to remove them. Having fewer pels means outer electrons are closer to the nucleus, making it more difficult to remove them.

Explanation for Ionization Energy Patterns

Within each period, the noble gases have the largest nuclear charge so they attract their outer electrons most strongly so that their ionization energies are at the top in the graph above. In contrast to this, in each period, the alkali metals have the lowest nuclear charge which means they have the least attraction for their outer electrons and are at the bottom of the graph

Ionization Energy Patterns Reflect Quantum Atom

There is an ionization energy drop from Be to B despite the fact that B has a greater nuclear charge and same pel. This is explained by noting that in the quantum atom, B’s outermost electron is in the second minor energy level (p) which makes it slightly farther from the nucleus and thus easier to remove.

Ionization Energy Patterns Reflect Quantum Atom

Oxygen has a lower ionization energy than nitrogen despite the fact that oxygen has greater nuclear charge and the same pel. This is explained by considering that oxygen’s outermost electron is the first paired p electron. Being in the same orbital as another electron, this outermost electron experiences repulsion force and thus is easier to remove.

Electron Affinity Trends

Electron Affinity is the energy an atom gives off when an electron is added to it. A large electron affinity (attraction for) means that an atom has a strong “desire” to get an electron. The nonmetals tend to have higher electron affinities because they have higher nuclear charge on a pel and getting electrons gives them electron configurations like the noble gases.

Summary of Atomic Property Trends

End of Presentation