THE PERIODIC TABLE
Early in the 19th century, chemist began to understand that there are similarities in the chemical properties of elements, and suggest a systematic classification of elements based on their properties.
Dobereiner arranged the elements in order of increasing atomic weights. But he was not successful, because the atomic weights of elements were not determined exactly, and many of the elements were not discovered yet. (1829)
Dmitri Mendeleev, and Meyer showed that the properties of elements are periodic functions of their atomic weights. However this kind of arrangement created exceptions in a few cases. ( such as K and Ar or I and Te)
The table which elements are placed with respect to atomic number and their similarities is called periodic table.
- The horizontal rows are called periods.
- The vertical columns are called groups.
In the periodic table there are two kinds of elements. These are metals and nonmetals. Some elements in the periodic table have both metallic and nonmetallic properties. They are called metalloids. ( B, Si, Ge, As, Sb, Te, Po, At )
There are 8A groups and 10B groups.
A groups are called representative elements.
B groups are called transition metals.
The two series of elements below the periodic table are called the inner transition elements.
A few common groups are given family names:
1A: alkali metals
2A: alkaline earth metals
7A: halogens
8A: nobel gases
The Periodic Properties in Periodic Table
- Atomic Radius: from right to left and from top to bottom atomic radius (size) increases.
- Metallic and Nonmetallic Properties: metallic property is directly proportional to the atomic size of the element. From right to left and from top to bottom metallic property increases. From left to right and from bottom to top nonmetallic property increases.
- Acidity and Basicity: If metallic property increases basicity increases, too. As nonmetallic property increases acidity increases.
- Electro negativity: the tendency of an atom to attract electrons is called electro negativity. From left to right and from bottom to top electro negativity increases.
ISOTOPES
Atoms having same number of protons (atomic number),
but different number of neutron (atomic mass) are called isotopes.
24Mg 25Mg 26Mg
Average atomic weight:S fractional abundance * atomic mass
Ex: Natural magnesium is found to contain 78.6 % Mg 24, 10.1 % Mg 25, 11.3% Mg 26. Calculate the weighted average mass of Mg atom.
Atomic weight avarege: 78.6 % *24 + 10.1% *25 + 11.3* 26= 24.33
Ex: Naturally occurring boron consist of 80.4% B-11 and 19.6% B-10. Calculate the average atomic mass of boron. (10.8)
Ex: The element chlorine consist of two isotopes whose atomic masses are 35 and 37. What are the natural abundances of these isotopes if the average atomic mass of chlorine is 35.5? (75% and 25%)
ELECTRON CONFIGURATION and SUBATOMIC PARTICLES
In the nucleus:
-protons are positively charged
-neutrons are neutral
They are heavy particles, they have some atomic mass. (1amu)
Around the nucleus:
-electrons are negative charged particles
We can ignore their masses (1/1840amu)
*Electrons are found at different energy levels. They are called as shells.
*In every energy level(shell), there exist some sub-shells: s, p , d, f.
*Energies of sub-shells increase in order of Es áEp áEd áEf
*Location of electrons in every energy level can not be determined exactly.
*Probable regions in which electrons might be found in a shell are called orbital.
*Each orbital has a capacity of 2 electrons.
s sub-shell has 1 orbital ® 2 electrons
p sub-shell has 3 orbital ® 6 electrons
d sub-shell has 5 orbital ® 10 electrons
f sub-shell has 7 orbital ® 14 electrons
*Orbitals are indicated as a square ( )
s ®
p ®
d ®
f ®
*The maximum electrons in a shell can be calculated by 2n2 where n stands for the number of shell.
n=1 ® 2n2 = 2
n=2 ® 2n2 = 8
n=3 ® 2n2 = 18
*The last shell (valance shell) can hold maximum 8 electrons.
*In chemical reactions, elements want to reach 8 electrons in the last shell, by giving or taking electrons.
Energy levels: 1s 2s 2p 3s 3p 4s 3d 4p 5s
Rules for Electron Configuration
1. Electrons are placed from lowest energy levels to highest energy levels.
2. Electrons are located in an orbital first unpaired with same spin.
1H: 1s1
2He: 1s2
11Na:1s22s22p63s1
8O:1s22s2sp4
32Ge:1s22s22p63s23p64s23d104p2
15P: 1s22s22p63s23p3
17Cl: 1s22s22p63s23p5
10Ne:1s22s22p6
20Ca:1s22s22p63s23p64s2
13Al: 1s22s22p63s23p1
21Sc:1s22s22p63s23p64s23d1
26Fe:1s22s2p63s23p64s23d6
FINDING GROUP and PERIOD NUMBERS
We know that the elements are arranged in the order of increasing atomic numbers. Elements which have the same electron configuration in their valance shells are placed in the same group.
Valance electrons: the electrons in the outermost shell (valance shell) of an atom.
In order to find the group and period number of any atoms, the electron configuration is written first. Then the following points are taken into consideration.
1. The biggest shell number of any atom shows the period number of that atom.
2. If the electron configuration of an atom ends with s or p orbitals, this atom belongs to group A and the group number is determined by adding the number of electrons in the valance shell.
3. If the electron configuration of an atom ends with the d orbitals, this atom belongs to group B and the group number is determined by adding the number of electrons in the d orbital and electrons in the last s orbital. This rule does not work for 1B, 2B, 8B groups.
4. All the elements of which the electron configuration ends with the f orbital belong to group 3B.
1H: 1s1 1st period 1A
2He: 1s2 1st period 8A (noble gas)
11Na:1s22s22p63s1 3rd period 1A
8O:1s22s2sp4 2nd period 6A
32Ge:1s22s22p63s23p64s23d104p2 4th period 4A
15P: 1s22s22p63s23p3 3rd period 5A
17Cl: 1s22s22p63s23p5 3rd period 7A
10Ne:1s22s22p6 2nd period 8A
20Ca:1s22s22p63s23p64s2 4th period 2A
13Al: 1s22s22p63s23p1 3rd period 3A
21Sc:1s22s22p63s23p64s23d1 4th period 3B
26Fe:1s22s2p63s23p64s23d6 4th period 8B