Section 5-2: Electron Configuration and the Periodic Table Coach Kelsoe Chemistry Pages 138-149

Section Objectives Describe the relationship between electrons in sublevels and the length of each period of the periodic table. Locate and name the four blocks of the periodic table, as well as explain the reason for these names. Discuss the relationship between group configurations and group numbers. Describe the locations in the periodic table and the general properties of the alkali metals, the alkaline-earth metals, the halogens, and the noble gases.

Periods and Blocks of the Periodic Table Remember that elements are arranged vertically in groups and horizontally in periods. Elements in the same group share chemical properties. The length of each period is determined by the number of electrons that can occupy the sublevels being filled in that period.

Periods and Blocks of the Periodic Table The first period consists of two elements because the 1s sublevel can only 2 electrons. In the second period, the 2s sublevel can hold two electrons and the 2p sublevel can hold 6, so the second period has 8 elements. The same applies to the third period. At the fourth period, the 4s sublevel can hold 2 electrons, the 3d sublevel can hold 10 electrons, and the 4p sublevel can hold 6.

Periods and Blocks of the Periodic Table The period of an element can be determined from the element’s electron configuration. For example, arsenic (As) has the Noble Gas configuration [Ar]3d104s24p3. The 4 in the 4p3 indicates that the highest occupied energy level is the fourth energy level.

Periods and Blocks of the Periodic Table Based on the electron configurations of the elements, the periodic table can be divided into four blocks, the s, p, d, and f blocks. The name of each block is determined by whether an s, p, d, or f sublevel is being filled in successive elements.

Important Notice It is important to realize that not all periodic tables have the information arranged in the same way. Atomic numbers will never have a decimal and will always be in numerical order.

The s-Block Elements: Group 1 & 2 The elements of the s block are chemically reactive metals. Group 1 elements are extremely reactive because of how easily they lose their electron. Using n for the number of the highest occupied energy level, the outer, or group, configurations of the Group 1 and 2 elements are written as ns1 and ns2 respectively.

Alkali Metals The elements of Group 1 of the periodic table (lithium, sodium, potassium, rubidium, cesium, and francium) are known as the alkali metals. In pure state, all have a silvery appearance and are soft enough to be cut with a knife.

Alkali Metals Properties of alkali metals: Silvery appearance Soft in texture Not found in nature as free elements because they combine with other elements. Combine vigorously with most nonmetals React strongly with water to produce H2 gas and alkalis.

Alkali-Earth Metals The elements of Group 2 of the periodic table (beryllium, magnesium, calcium, strontium, barium, & radium) are called the alkali-earth metals. Atoms of alkali-earth metals contain a pair of electrons in their outermost s sublevel.

Alkali-Earth Metals The Group 2 metals are harder, denser, and stronger than the alkali metals. They also have higher melting points. They are less reactive than alkali metals, but they too are too reactive to be found free in nature.

Hydrogen and Helium Hydrogen and helium are special cases in the periodic table. Hydrogen has an electron configuration of 1s1, but it does not share the same properties as the elements of Group 1. Helium is also different from the Group 18 elements. It is very stable because its outermost energy level is filled by 2 electrons.

Sample Problem Without looking at the periodic table, give the group, period, and block in which the element with the electron configuration [Xe]6s2 is located. Group 2, Period 6, Block s Without looking at the periodic table, write the electron configuration for the Group 1 element in the 3rd period. Is this element likely to be more active or less active than the previous? 1s22s22p63s1; it is likely to be more reactive

The d-Block Elements: Groups 3-12 For energy level n, there are n possible sublevels, so we don’t see the d sublevel until the 3rd energy level. Remember that the 3d sublevel is higher in energy than the 4s, so they will fill up in order of 4s3d. The d sublevel is made up of 5 orbitals that can hold a total of 10 electrons.

The d-Block Elements: Groups 3-12 The group configuration for Group 3 is (n-1)d1ns2. The group configuration for Group 12 is (n-1)d10ns2. There are a few exceptions. For elements like Palladium and Platinum, those lower energy levels do not fill up first.

The d-Block Elements: Groups 3-12 The d-block elements are metals with typical metallic properties and are often referred to as transition elements. They are good conductors of electricity, have a high luster, and are less reactive than alkali and alkali-earth metals.

Essentials for Life These elements are essential for life and make up over 96% of the human body. Carbon Hydrogen Oxygen Phosphorus Sulfur Nitrogen

Sample Problem An element has the electron configuration [Ar]3d84s2. Without looking at the periodic table, identify the period, block, and group in which this element is located. 4th Period – indicated by the highest occupied energy level d-block – indicated by the (n-1)d1-10ns0-2 notation Group 10 – indicated by the total number of electrons in the outer energy level.

The p-Block Elements: Groups 13-18 The p-block elements consist of all the elements of Groups 13-18 except Helium. Electrons add to a p sublevel only after the s sublevel in the same energy level is filled. Atoms of all p-block elements contain two electrons in the ns sublevel.

The p-Block Elements: Groups 13-18 The p-block elements and the s-block elements are called the main-group elements. For Group 13, the group configuration is ns2np1. For Group 18, the group configuration is the ns2np6.

The p-Block Elements: Groups 13-18 For atoms of p-block elements, the total number of electrons in the highest occupied level is equal to the group number minus 10. In the p-block elements, there are metals, metalloids, and nonmetals.

The Halogens The elements of Group 17 (fluorine, chlorine, bromine, iodine, and astatine) are known as the halogens. The halogens are the most reactive nonmetals. They react with most metals to form salts. For all atoms, reactivity is based on the presence of electrons in the outermost energy level.

The Halogens Characteristics of some of the halogens: Fluorine and chlorine are gases at room temperature. Bromine is a reddish liquid. Iodine is a dark purple solid. Astatine is a synthetic element prepared in only very small quantities.

Metalloids Metalloids are semiconducting elements that are mostly brittle solids with some properties of metals and some of nonmetals. The metalloids have electrical conductivity intermediate between that of metals and nonmetals.

Properties of p-Block Elements The p-block elements are generally harder and denser than the s-block elements, but softer and less dense than the d-block elements. With the exception of bismuth, these metals are sufficiently reactive to be found in nature only in the form of compounds.

Sample Problem Without looking at the periodic table, write the outer electron configuration for the Group 14 element in the second period. Then name the element. It is in the p-block because its group is higher than 12 The total number of electrons in the highest occupied s and p sublevels is equal to the group number minus 10, so 14-10=4 Being in the 2nd period, it must be 2s22p2.

The f-Block Elements: Lanthanides and Actinides The f-block elements are wedged between Groups 3 and 4 in the 6th and 7th periods. With seven 4f orbitals to be filled with two electrons each, there are a total of 14 f-block elements between lanthanum and hafnium. There are also 14 f-block elements between Actinium and Rutherfordium.