Химиялық периодтылық туралы ашық сабақ, 8-сынып

THE HISTORY OF THE PERIODIC TABLE

1800ish—Johann Dobereiner, triads; 1864 John Newlands octaves; 1870--Dmitrii Mendeleev & Julius Lothar Meyer--by mass; 1913 Mosley--by number of protons.

Group IA(1A or 1)--alkali metals

Group IIA(2A or 2)--alkaline earth metals

Group VIA(6A or 16)--Chalcogens

Group VIIA(7A or 17)—Halogens

Group VIIIA (8A or 18)—Noble gas

SOME PROPERTIES OF COMMON GROUPS:

Alkali metals—the most reactive metal family; must be stored under oil; react with water violently!

Alkaline-earth metals-- except for Be(OH)2, the metal hydroxides formed by this group provide basic solutions in water ; pastes of these used in batteries

Chalcogen family – many found combined with metal ores

Halogen family – known as the “salt-formers” ; used in modern lighting

Noble Gas family – known for their disinterest in other elements; once thought to never react; neon used to make bright signs

Transition metals—fill the d orbitals.

Anomalies occur at Chromium and Copper to minimize electron/electron repulsions. If you learned that there is special stability associated with a half-filled sub-level, IT’S A LIE!! No such stability exists! It’s all about lowering energy by minimizing electron/electron repulsions.

Rare Earth metals—fill the d sublevels. Lanthanides and Actinides. These sometimes put an electron in d [just one or two electrons] before filling f. This is that dsf overlay referred to earlier—the energies of the sublevels are very similar.

PERIODIC TRENDS:

A trend is NOT an EXPLANATION!

This an important section—there is almost always an essay involving this topic on the APexam. There are several arguments you will evoke to EXPLAIN a periodic trend. Remember opposites attract and likes repel. The trick is learning which argument to use when explaining a certain trend!

Effective nuclear charge, Zeff—essentially equal to the group number. Think of the IA’s having a Zeff of one while the VII A’s have a Zeff of 7! The idea is that the higher the Zeff the more attractive force there is emanating from the nucleus drawing electrons in or holding them in place. Relate this to ENERGY whenever possible.

Distance—attractive forces dissipate with increased distance. Relate this to ENERGY whenever possible.

Shielding—electrons in the “core” effectively shield the nucleus’ attractive force for the valence electrons. Use this ONLY when going up and down the table, NOT across. There is ineffective shielding within a sublevel or energy level. Relate this to ENERGY whenever possible. {dodge ball, such a barbaric ritual—since you’re the smart kids, you figured out in elementary school to stay behind the bigger kids to keep from getting hit! The electrons in the first or second energy level, shield the outer valence electrons from the Mother Nucleus’ attractive force. }

Minimize electron/electron repulsions—this puts the atom at a lower energy state and makes it more stable. Relate this to ENERGY whenever possible.

Here we go!

ATOMIC RADIUS--No sharp boundary beyond which the electron never strays!!

Use diatomics and determine radius then react with others and determine the radius of others

radii decreases (↓) moving across a period AND increases (↑) moving down a row (family)

The effective nuclear charge (Zeff)—the more protons for the same number of energy levels—increases as we move from left to right across the periodic table.

This shrinks the electron cloud until…

…the point at which electron repulsions overcome the nuclear attraction and stop the contraction of electron shells.

the principal level, n, determines size of an atom—add another level and the atoms get MUCH larger radii

As we move down a family, the attractive force of the nucleus dissipates with increased distance.

Shielding is only a valid argument when comparing elements from period to period since shielding is incomplete within a period—use this argument with extreme caution! It should NOT be your favorite!

IONIZATION ENERGY--energy required to remove an electron from the atom IN THE GAS PHASE. Costs Energy.

Removing each subsequent electron requires more energy

Second IE, Third IE, etc.

Some more than others!! A HUGE energy price is paid if the subsequent removal of electrons is from another sublevel or, Heaven forbid, another principal E level (core).

↓ down a family—increased distance from the nucleus and increased shielding by full principal E levels means it requires less E to remove an electron

↑across a period—due to increasing Zeff. Let’s talk EXCEPTIONS!!

an anomaly occurs at “messing up a half-filled or filled sublevel” There’s nothing magical about this and electrons are not “happier” as a result. The simple truth is that when electron pairing first occurs within an orbital, there is an increase in electron/electron repulsions which makes…