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About 70 elements had been discovered by the mid-1800’s, but no one had found a way to relate the elements in a systematic, logical way.

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He develop the 1st

periodic table of the elements.Arranged elements in order of increasing atomic mass and created columns with elements having similar properties.

Mendeleev left blank spaces in the table because there were no known elements with the appropriate properties and masses.

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Mendeleev and others were able to predict the physical and chemical properties of the missing elements. Eventually these elements were discovered and were found to have properties similar to those predicted. There were many exceptions in his table, however.

In 1913, arranged elements in order of increasing atomic number thus reversing the order of the elements and correcting the drawbacks found in Mendeleev’s table.

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Neat-o Animation

Periodic law states the physical and chemical properties of the elements are periodic functions of their atomic number. In other words, when the elements are arranged in order of atomic number, elements with similar properties appear at regular intervals.

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A Group or Family is a column on the periodic table. Elements in the same column have similar chemical properties.

2 conventions for numbering:1-18

A/B elements

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C. Johannesson

Main Group ElementsTransition Metals Inner Transition

Metals

Group A elements all have electrons in the outer s, or s and p orbitals. These are known as representative elements. The group number indicates the number of valence, or outer shell, electrons except with helium which has 2.Examples: IIA - Ca (20) 1s22s22p63s23p64s2

VIA – S (16) 1s22s22p63s23p4

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Group 18 (VIIIA) elements are the noble gases with 8 valence electrons, except helium which has 2. Noble gases are inert (nonreactive) in nature. They do not form ions.

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Full s & p orbitals in the highest principal energy levelElectron configuration very stable, making them inertWhen other atoms of other elements gain or lose electrons in reactions, they achieve electron configuration of noble gases 16

Group B elements or transition elements (d block) have electrons in their outer d orbitals. The have varying number of valence electrons.

Example: Zn (30) 1s22s22p63s23p64s23d10

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Lanthanides and Actinides elements (f-block) have electrons in their outer f orbitals. These elements have varying numbers of valence electrons.

Example: Nd (60)

1s22s22p63s23p64s23d104p65s24d105p66s25d14f3

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HydrogenNo group numberOnly element in familyMost common element in the

universeVery reactiveCompounds of H very common-

H2OFound in proteins, carbs, and fats

with C and O

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C. Johannesson

When elements are arranged in order of

increasing atomic #, elements with similar

properties appear at regular intervals.

0

50

100

150

200

250

0 5 10 15 20Atomic Number

Ato

mic

Ra

diu

s (

pm

)

Trends on table occur vertically and horizontally

Group 1Li increasing reactivity with H2O

Na

K

Knowing the trends enables you to predict chemical behavior.

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You know from the quantum mechanical model that an atom does not have a sharply defined boundary that sets the limit of its size.

Therefore, the radius of an atom cannot be measured directly. There are, however, several ways to estimate the relative size of atoms.

The atomic radius is one-half of the distance from center to center of 2 like atoms.

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Group Trends

Atomic size generally increases as you move down a group of the periodic table.

Why?Li 2s1 adding principal energy levelsNa 3s1

K 4s1

Atoms getting larger with more energy levelsElectrons getting further away from + charged nucleus

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Periodic Trends

Atomic size generally decreases as you move from left to right across a period.

Why?

As you go across a period, the principal energy level remains the same. Each element has one more proton and one more electron than the preceding element.

The electrons are added to the same energy level, causing the increasing positive charge to pull them in closer.

Realize at some point this effect is less pronounced more electrons, more reaction between them repulsion and that force is greater than positive attraction of nucleus

shielding effect- the reduction of the attractive forces between a nucleus and its outer electrons due to the blocking effect of inner electrons

• The shielding of the nucleus by electrons also increases with the additional occupied orbitals between the outermost orbital and the nucleus.

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When an atom gains or loses an electron, it becomes an ion.

The energy required to overcome the attraction of the nuclear charge and remove an electron from a atom is called the ionization energy.

Removing one electron results in the formation of a positive ion with a 1+ charge.

Na(g) Na+(g) + e-

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The energy required to remove this first outermost electron is called the first ionization energy.

To remove the outermost electron from the 1+ ion requires an amount of energy called the second ionization energy, and so forth.

Nifty swell animation

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Group Trends

Ionization energy generally decreases as you move down a group of the periodic table.

This is because the size of the atoms increases as you descend, so the outermost electron is farther from the nucleus.

The outermost electron should be more easily removed, and the element should have a lower ionization energy.

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Periodic Trends

For the representative elements, ionization energy generally increases as you move from left to right across a period.

The atomic number and therefore positive charge increases and the shielding effect is constant as you move across. A greater attraction of the nucleus for the electron leads to the increase in ionization energy.

Also, electron configuration/ noble gas configuration harder to remove electron to get to a more stable electron configuration- easier to gain an electron

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The electronegativity of an element is the tendency for the atoms of the element to attract electrons when they are chemically combined with atoms of another element.

Electronegativity generally decreases as you move down a group.

As you go across a period from left to right, the electronegativity of the representative elements increases.

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The metallic elements at the far left of the periodic table have low electronegativities. By contrast, the nonmetallic elements at the far right (excluding the noble gases), have high electronegativities.

The electronegativity of cesium, a metal, the least electronegative element, is 0.7; the electronegativity of fluorine, a nonmetal, the most electronegative element, is 4.0.

Because fluorine has such a strong tendency to attract electrons, when it is chemically combined to any other element it either attracts the shared electrons or forms a negative ion.

In contrast, cesium has the least tendency to attract electrons.

Groovy animation

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