Section 6-1 Notes. Organizing the Elements

Transcription

1 Section 6-1 Notes Organizing the Elements

2 Organizing the Elements As new elements were discovered chemists needed to find a logical way to organize them Properties of elements were used to sort them in to groups In 1829, J.W. Dobereiner published a classification system in which the elements were grouped into triads A group of 3 elements with similar properties Ex. Chlorine, Bromine, and Iodine One element in each triad tended to have properties with values that fell midway between those of the other two elements Problem - some of the elements could not be grouped into triads

3 Mendeleev s Periodic Table 1869, Dmitri Mendeleev published a table of elements The organization he chose was a periodic table, based on a set of repeating properties of the 60 known elements. Mendeleev arranged the elements in order of increasing atomic mass see fig. 6.3 He added? because: 1 st because he knew that Bromine needed to be with Chlorine & Iodine. 2 nd because he predicted that elements would be discovered that fit in those spaces

4 Periodic Law Mendeleev arranged his element based on increasing atomic mass, but he placed tellurium(127.6amu) before iodine (126.9amu) in his table. He did this because it placed iodine in the same group as chlorine and bromine. He knew that these elements had similar properties 1913, Henry Moseley, British physicist, determined an atomic number for each known element By using the atomic number, it makes sense that Tellurium should come before Iodine Therefore, in the modern periodic table, elements are arranged in order of increasing atomic number not mass

5 Periodic Law There are seven rows (periods) in the table Each period of the table corresponds to a principal energy level The elements within a column (group) have similar properties These properties repeat in each period, a pattern known as the periodic law When elements are arranged in order of increasing atomic number, there is a periodic repetition of their physical and chemical properties

6 Metals, Nonmetals and Metalloids The elements have been grouped into 3 broad classes based on their characteristics Metals Nonmetals Metalloids

7 Properties of Metals Good conductors of heat and electricity High luster or sheen Solid at room temperature Exception - Mercury Ductile - can be drawn into wires Malleable - can be hammered into thin sheets

8 Properties of Non Metals Most are gases at room temperature Sulfur and Phosphorus are solids Bromine is a dark-red liquid Poor conductors of heat and electricity Carbon is an exception Solid nonmetals tend to be brittle

9 Metalloids Metalloids generally have properties similar to those of metals and nonmetals Depending on the conditions a metalloid can act as a metal or a nonmetal Example Pure silicon is a poor conductor of electric current, similar to nonmetals But, when mixed with boron it is a good conductor of electric current, similar to metals

10 Section 6-2 Notes Classifying the Elements

11 Squares in the Periodic Table The periodic table displays the symbols and names of the elements, along with information about the structure of their atoms. Each square include the symbol, atomic mass, and atomic number of the element There is a vertical column that which lists the number of electrons in each energy level Also some tables provide a color coordinated chart to distinguish some specific groups of elements

12 Groups of Elements Alkali metals- group 1A elements Arabic for the ashes, Na & K are common in wood ashes Alkaline earth metals- group 2A elements Halogens- group 7A elements Hals- Greek for salt genesis- Latin for to be born These elements can be produced from their salts Chalcogens- group 6A elements Noble Gases group 8A elements

13 Electron Configurations in Groups Elements can be sorted into: Noble gases Representative elements Transition metals Inner transition metals Based on their electron configurations. The noble gases all have a full outer shell so they rarely take part in a reaction

14 The Representative Elements Groups 1A-7A are the representative elements They are referred to as representative elements because they display a wide range of physical and chemical properties Some are metals, metalloids, and nonmetals Most are solids, some are gases and one is a liquid at room temperature The s and p sublevels are the highest occupied energy levels that are being filled

15 Transition Elements The B groups on the periodic table are known as transition elements These elements are all metals These elements are characterized by the presence of electrons in the d sublevel Inner transition metals are characterized by the presence of electrons in the f sublevel

16 Section 6-3 Notes Periodic Trends

17 Trends in Atomic Size To determine atomic size we use two atoms of the same element joined together Since the atoms in each molecule are identical, the distance between the nuclei of the atoms can be used to estimate the size of the atoms The atomic radius is ½ the distance between the two nuclei In general, atomic size increases from top to bottom within a group Atomic size decreases from left to right across a period

18 Group Trends in Atomic Size What happens to the # of protons we move through a group (column)? The increase in positive charge draws the electrons closer to the nucleus The increase in # of occupied orbitals shields electrons in the highest energy level from the attraction of protons Called the shielding effect The shielding effect is greater then the effect of the increase in nuclear charge As a result, the atomic size increases

19 Ions An ion is an atom or group of atoms that has a positive or negative charge Positive and negative ions form when electrons are transferred between atoms An ion with a positive charge is called a cation Na +, Ca 2+ An ion with a negative charge is called an anion Cl -, O 2-

20 Trends in Ionization Energy The energy required to remove an electron from an atom is called ionization energy Always measured when the element is in its gaseous state First ionization energy tends to decrease from top to bottom Recall the shielding effect Tends to increase from left to right across a period

21 Group Trends in Ionization Energy Recall that the atomic size increases as the atomic number increases within a group As the size increases, nuclear charge has a smaller effect on the electrons in the highest energy level. Therefore, it takes less energy to remove an electron, causing the first ionization energy to be lower

22 Periodic Trends in Ionization Energy Energy generally increases as you move left to right due to an increase in nuclear charge and the shielding effect remaining the same for the period Since there is an increase in the attraction of the nucleus for an electron, it takes more energy to remove an electron from an atom

23 Trends in Ionic Size Cations are always smaller than the atoms from which they form. Anions are always larger then the atoms from which they form When a sodium atom loses an electron, the attraction between the remaining electrons and the nucleus is increased As a result, the electrons are drawn closer, making the size smaller This is opposite for nonmetals that gain electrons because the attraction is decreased More negative charge then positive charge

24 Trends in Electronegativity Electronegativity is the ability of an atom of an element to attract electrons when the atom is in a compound Scientists use factors such as ionization energy to calculate values for electronegativity In general, electronegativity values decrease from top to bottom within a group The values tend to increase from left to right across a period Cesium is the least electronegative element Fluorine is the most electronegative element The electronegativity of transition metals is irregular