Chapter 4Chapter 4The Periodic tableThe Periodic table

Mendeleev’s TableMendeleev’s Table

Mendeleev’s table grouped elements with Mendeleev’s table grouped elements with similar properties into vertical columns similar properties into vertical columns called “groups” or “families” example the called “groups” or “families” example the halogen group.halogen group.

Mendeleev found only 63 elements known at Mendeleev found only 63 elements known at that time. There are 2 interesting things to note.that time. There are 2 interesting things to note.

The elements do not always fit neatly in the The elements do not always fit neatly in the order of increasing atomic mass.order of increasing atomic mass.

There were gaps in his table where elements There were gaps in his table where elements with a particular atomic mass should occur. He with a particular atomic mass should occur. He was able to predict the properties of these was able to predict the properties of these undiscovered elements which he called undiscovered elements which he called ekaaluminium , ekaboron and ekasilicon.ekaaluminium , ekaboron and ekasilicon.

Eka meaning “one beyond” in Sanskrit.Eka meaning “one beyond” in Sanskrit.

Mosley an England scientist worked with x-ray spectra Mosley an England scientist worked with x-ray spectra to study atomic structure which helped him determine to study atomic structure which helped him determine atomic numbers for chemical elements. He discovered atomic numbers for chemical elements. He discovered isotopes, explaining how atomic mass did not order the isotopes, explaining how atomic mass did not order the elements appropriately. Moseley was the catalyst for the elements appropriately. Moseley was the catalyst for the periodic law, a rule stating that the properties of periodic law, a rule stating that the properties of elements are a periodic function of their atomic numbers. elements are a periodic function of their atomic numbers. Using the atomic numbers, He was able to more Using the atomic numbers, He was able to more accurately position the elements in the periodic table. accurately position the elements in the periodic table. Moseley's change to the periodic table allowed the few Moseley's change to the periodic table allowed the few problems with Mendeleev's periodic table to disappear. problems with Mendeleev's periodic table to disappear. The modern periodic table is now based on atomic The modern periodic table is now based on atomic numbers.numbers.

Moseley’s theoryMoseley’s theory

..

We now follow the periodic table We now follow the periodic table recommended by IUPAC.recommended by IUPAC.

The main group elements:The main group elements: Groups 1,2, and 13 through 18 are Groups 1,2, and 13 through 18 are

referred to as the main group elements.referred to as the main group elements. Group 1 the alkali metals. These metals Group 1 the alkali metals. These metals

react with water to form alkaline solution. react with water to form alkaline solution. They are excellent conductors of They are excellent conductors of electricity.electricity.

Group 2- The alkaline earth metals. These Group 2- The alkaline earth metals. These are harder than alkali metals. They are are harder than alkali metals. They are less reactive than alkali metals. Mg from less reactive than alkali metals. Mg from this group when reacts with oxygen from this group when reacts with oxygen from the air forms magnesium oxide which the air forms magnesium oxide which slows corrosion of the magnesium metals slows corrosion of the magnesium metals underneath. Forms good alloys.underneath. Forms good alloys.

Lanthanides and actinides: the lanthanides have atomic Lanthanides and actinides: the lanthanides have atomic numbers from 58 to 71.The name originated from the numbers from 58 to 71.The name originated from the element 57. they are shiny, reactive metals that have element 57. they are shiny, reactive metals that have irregular electronic configuration.irregular electronic configuration.

Actinides are unique in that their nuclear structures are Actinides are unique in that their nuclear structures are of more importance than their electron configurations. of more importance than their electron configurations. They have an unstable arrangement of protons and They have an unstable arrangement of protons and neutrons in the nucleus so they are called radioactive. neutrons in the nucleus so they are called radioactive. Example Uranium. Nuclear disintegration of the uranium Example Uranium. Nuclear disintegration of the uranium nucleus releases sufficient energy to run power nucleus releases sufficient energy to run power parts,submarines also aircrafts.parts,submarines also aircrafts.

Group 17-Halogens Derived from the Group 17-Halogens Derived from the Greek word meaning “salt former”. Most Greek word meaning “salt former”. Most reactive group of nonmetals.reactive group of nonmetals.

Group 18- Noble gases. Formerly called Group 18- Noble gases. Formerly called inert gases because they were thought to inert gases because they were thought to be completely unreactive. Except for be completely unreactive. Except for helium these atoms are characterized by helium these atoms are characterized by an octet of electrons, nsan octet of electrons, ns2 2 and np and np66 in the in the outermost energy level.outermost energy level.

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Characterestics Of metalsCharacterestics Of metals

Metals are good conductors of heat and Metals are good conductors of heat and electricity. Metals in the form of crystals .electricity. Metals in the form of crystals .

A crystal is a substance in which atoms or A crystal is a substance in which atoms or molecules are arranged in an orderly molecules are arranged in an orderly geometric fashion.geometric fashion.

Crystal and conduction bandCrystal and conduction band

The outer electrons in the atoms of each The outer electrons in the atoms of each element form the bonds that bind the element form the bonds that bind the atoms together in a crystal. In a crystal, atoms together in a crystal. In a crystal, each atom is bonded to all neighboring each atom is bonded to all neighboring atoms. These electrons exist in orbitals atoms. These electrons exist in orbitals just as electrons do in atoms. There are just as electrons do in atoms. There are vacant orbitals in the bond just as in vacant orbitals in the bond just as in atoms. Electrons are free to move in three atoms. Electrons are free to move in three dimensions through the crystal, forming dimensions through the crystal, forming the conduction band.the conduction band.

Conduction band is a band within which Conduction band is a band within which electrons must move to allow electrical electrons must move to allow electrical conduction. In metals the orbitals of the conduction. In metals the orbitals of the conduction band actually overlap conduction band actually overlap

and are continous with and are continous with

the bond orbitals so even the bond orbitals so even

at room temp. thereat room temp. there

are many electrons in are many electrons in

the conduction band.the conduction band.

The metal atoms are vibrating back and The metal atoms are vibrating back and forth with thermal motion, interfering with forth with thermal motion, interfering with the motion of electrons. The vibration the motion of electrons. The vibration increases as the temperature increases so increases as the temperature increases so the conductivity decreases.the conductivity decreases.

                               

     

                               

     

In semiconductors the energy of the higher In semiconductors the energy of the higher orbitals is not very high compared with the orbitals is not very high compared with the energy of the filled bonding orbitals.less energy of the filled bonding orbitals.less energy is required to excite electrons to energy is required to excite electrons to these orbitals, so even at room these orbitals, so even at room temperature they carry a certain amount of temperature they carry a certain amount of current. At higher temperature there are current. At higher temperature there are more electrons excited in the conducting more electrons excited in the conducting band, so conductivity of semiconductors band, so conductivity of semiconductors increases as the temperature increases.increases as the temperature increases.

In typical non metals these orbitals have In typical non metals these orbitals have very high energy so it is practically very high energy so it is practically impossible to excite electrons to the impossible to excite electrons to the orbitals of the conduction band example orbitals of the conduction band example diamond is a nonconductor.diamond is a nonconductor.

Trends in periodic tableTrends in periodic table

Trend is a periodic change in a particular Trend is a periodic change in a particular direction.direction.

Periodic trends in atomic radii: The size of Periodic trends in atomic radii: The size of an atom is determined by the volume an atom is determined by the volume occupied by the electrons surrounding the occupied by the electrons surrounding the nucleus. Radii are usually determined for nucleus. Radii are usually determined for atoms that are chemically bonded or close atoms that are chemically bonded or close together in the solid state, as in Bromine together in the solid state, as in Bromine and Iodine.and Iodine.

Bond radii: Half the distance between the Bond radii: Half the distance between the nuclei of the atoms in each molecule is the nuclei of the atoms in each molecule is the bond radius.bond radius.

The distance between the The distance between the

nuclei in adjacent nonnuclei in adjacent non

bonded molecules, which bonded molecules, which

is twice the distance, calledis twice the distance, called

the van der Waals radius. the van der Waals radius.

Van der waal’s radii are not very precise Van der waal’s radii are not very precise because of the fuzziness of the atoms. because of the fuzziness of the atoms. Bond radius is usually considered to dfind Bond radius is usually considered to dfind the size of the atom.the size of the atom.

Atomic radius increases as you move Atomic radius increases as you move down a group. This is caused by the down a group. This is caused by the addition of another principal energy level addition of another principal energy level from one period to the next.from one period to the next.

Electron shielding: The electrons in the Electron shielding: The electrons in the inner energy levels are between the inner energy levels are between the nucleus and the outermost electrons and nucleus and the outermost electrons and therefore shield the outer electrons from therefore shield the outer electrons from the full charge of the nucleusthe full charge of the nucleus

Atomic radius decreases as you move across a period.Atomic radius decreases as you move across a period.

As you move from left to right across a period, each As you move from left to right across a period, each atom has one more proton and one more electron than atom has one more proton and one more electron than the atom before it has. the atom before it has.

All additional electrons go into the same principal energy All additional electrons go into the same principal energy level—no electrons are being added to the inner levels. level—no electrons are being added to the inner levels.

Electron shielding does not play a role as you move Electron shielding does not play a role as you move across a period.across a period.

As the nuclear charge increases across a period, theAs the nuclear charge increases across a period, theeffective nuclear charge acting on the outer electronseffective nuclear charge acting on the outer electronsalso increases.also increases.

  

Ionization Energy: The energy used to remove the Ionization Energy: The energy used to remove the electron is the ionization energy of the atom.electron is the ionization energy of the atom.

Each element has more occupied energy levels than the Each element has more occupied energy levels than the one above it has. one above it has.

The outermost electrons are farthest from the nucleus The outermost electrons are farthest from the nucleus in elements near the bottom of a group.     in elements near the bottom of a group.     

As you move down a group, each successive element As you move down a group, each successive element contains more electrons in the energy levels between the contains more electrons in the energy levels between the nucleus and the outermost electrons. nucleus and the outermost electrons.

  

Ionization energy tends increases as you move from left Ionization energy tends increases as you move from left to right across a period. to right across a period.

      From one element to the next in a period, the number From one element to the next in a period, the number of protons and the number of electrons increase by one of protons and the number of electrons increase by one each. each.

    The additional proton increases the nuclear charge.The additional proton increases the nuclear charge. A higher nuclear charge more strongly attracts the outer A higher nuclear charge more strongly attracts the outer

electrons in the same energy level, but the electron-electrons in the same energy level, but the electron-shielding effect from inner-level electrons remains the shielding effect from inner-level electrons remains the same. same.

  

Electron affinity: The ability of an atom to Electron affinity: The ability of an atom to attract and hold an electron is called attract and hold an electron is called electron affinity. Although the atom is electron affinity. Although the atom is neutral the electrons in the orbitals neutral the electrons in the orbitals generally do not shield the nuclear charge generally do not shield the nuclear charge a 100%. The approaching electron may a 100%. The approaching electron may experience a net pull because the effective experience a net pull because the effective nuclear charge is greater than zero. The nuclear charge is greater than zero. The electron enters a vacant orbital.electron enters a vacant orbital.

For example, when a neutral chlorine atom For example, when a neutral chlorine atom in the gaseous form picks up an electron in the gaseous form picks up an electron to form a Cl- ion, it releases an energy of to form a Cl- ion, it releases an energy of 349 kJ/mol or 3.6 eV/atom. It is said to 349 kJ/mol or 3.6 eV/atom. It is said to have an electron affinity of -349 kJ/mol have an electron affinity of -349 kJ/mol and indicates that it and indicates that it

forms a stable negative ion forms a stable negative ion

Electron affinity becomes more negative across Electron affinity becomes more negative across a period and tends to decrease from top to a period and tends to decrease from top to bottom.bottom.

Electron affinity trends within groups of elements Electron affinity trends within groups of elements are not as regular as trends for ionization are not as regular as trends for ionization energy. Alkaline earth metals are an exception. energy. Alkaline earth metals are an exception. An electron added in the alkaline earth metals An electron added in the alkaline earth metals must go to the p orbital and is shielded to a must go to the p orbital and is shielded to a greater extent by the s electrons.greater extent by the s electrons.

Electron affinity of the main block Electron affinity of the main block elements in kj/molelements in kj/mol

Periodic trends in melting and Periodic trends in melting and boiling pointsboiling points

As the number of electrons increases As the number of electrons increases across the period the m.p and b.p. across the period the m.p and b.p. increases. This indicates stronger increases. This indicates stronger bonding.bonding.

However when the d orbitals become half filled However when the d orbitals become half filled the m.p and b.p decreases. This indicates that the m.p and b.p decreases. This indicates that added electrons become less involved in added electrons become less involved in bonding. As the p electrons are added we see a bonding. As the p electrons are added we see a similar rise and fall with the maximum near the similar rise and fall with the maximum near the stage where p orbitals are half filled. A minimum stage where p orbitals are half filled. A minimum is reached when the p orbitals are filled and the is reached when the p orbitals are filled and the atoms no longer bond. The noble gases are atoms no longer bond. The noble gases are monatomic and have no chemical bonding monatomic and have no chemical bonding forces between atoms. Hence their m.p. and b.p. forces between atoms. Hence their m.p. and b.p. are unusually low.are unusually low.

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