Electron arrangements Period

The alkali metals are extremely reactive. Thus, there is a dramatic change in chemistry as we pass from the inert gases to the next column in the periodic table. The chemistry of the alkali metals is interesting and often spectacular. Thus, these metals react with chlorine, water, and oxygen, always forming a +1 ion that is stable in contact with most substances. The chemistry of these +1 ions, on the other hand, is drab, reflecting the stabilities of the inert gas electron arrangements that they have acquired. 

The periodicity of chemical properties, which Is summarized In the periodic table. Is one of the most useful organizing principles in chemistry. Periodic patterns also provide information about electron arrangements in atoms. 

The periodic table of the elements is one of the most powerful tools for correlating chemical behavior. The table can be used at the same time to present the detailed arrangement of electrons in atoms. Indeed, these facts point toward a connection between chemical properties and electron arrangement, a point that will be discussed in the next chapter. 

O element a substance contaming only one type of atom O compound a substance made of two. or more, elements chemically confined together O Periodic Table the table in which the elements are organised in order of increasing proton number and electron arrangement... 

The first periodic table was developed in 1869 by Dmitri Mendeleev several decades before the nature of electron energy states in the atom was known. Mendeleev arranged the elements in order of increasing atomic mass into columns of similar physical and chemical properties. He then boldly predicted the existence and the properties of undiscovered elements to fill the gaps in his table. These interpolations were initially treated with skepticism until three of Mendeleev s theoretical elements were discovered and were found to have the properties he predicted. It is the correlation with properties—not with electron arrangements—that have placed the periodic table at the beginning of most chemistry texts. 

Draw boxes to represent the first 20 elements in the periodic table. Using Figure 2.9 as a guide, sketch the electron arrangements for these elements. 

B> What is the relationship between electron arrangement and the organization of elements in the periodic table ... 

SUV How does your understanding of electron arrangement and forces in atoms help you explain the following periodic trends ... 

In this section, you will describe how electron arrangement and forces in atoms can explain the periodic trend associated with electronegativity... 

One of the most striking things about the chemistry of the elements is the periodic repetition of properties. There are several groups of elements that show great similarities in chemical behavior. As we saw in Chapter 2, these similarities led to the development of the periodic table of the elements. In this chapter we will see that the modern theory of atomic structure accounts for periodicity in terms of the electron arrangements in atoms. 

For our purposes the main significance of the electron spin quantum number is connected with the postulate of Austrian physicist Wolfgang Pauli (1900-1958), which is often stated as follows In a given atom no two electrons can have the same set of four quantum numbers (n, , me, and ms). This is called the Pauli exclusion principle. Since electrons in the same orbital have the same values of n, i, and mc, this postulate requires that they have different values of ms. Since only two values of ms are allowed, we might paraphrase the Pauli principle as follows An orbital can hold only two electrons, and they must have opposite spins. This principle will have important consequences when we use the atomic model to relate the electron arrangement of an atom to its position in the periodic table. 

A current version of the periodic table is shown inside the front cover of this book. The only fundamental difference between this table and that of Mendeleev is that the current table lists the elements in order by atomic number rather than by atomic mass. The reason for this will become clear later in this chapter as we explore the electron arrangements of the atom. 

We can use the quantum mechanical model of the atom to show how the electron arrangements in the atomic orbitals of the various atoms account for the organization of the periodic table. Our main assumption here is that all atoms have orbitals similar to those that have been described for the hydrogen atom. As protons are added one by one to the nucleus to build up the elements, electrons are similarly added to these atomic orbitals. This is called the aufbau principle. 

The element lithium, located in group 1A of the periodic table, has just one valence electron in its second shell. If this electron is lost, lithium forms the cation Li having no electrons in the second shell. However, it will have a stable electronic arrangement with two electrons in the first shell like helium. 

The rows of the periodic table are called the periods (hence periodic table). The position that an element occupies in a period is determined by the number of electrons in its outermost shell. Each time the atomic number increases by one, the number of protons in the nucleus increases by one, and the new element requires one more electron for a neutral atom. The electrons arrange themselves in shells around the nucleus. Each time a shell is filled, a row is finished. A filled shell is represented on the periodic table as a filled row, as shown in figure 1.2.4. 

In previous chapters, you learned about atomic structure, electron arrangement, and periodic properties of the elements. The elements within a group on the periodic table have similar properties. Many of these properties are due to the number of valence electrons. These same electrons are involved in the formation of chemical bonds between two atoms. 

In this section, we introduce Hartree s method and use it to describe the electron arrangements and energy levels in many-electron atoms. Later sections detail how this approximate description rationalizes periodic trends in atomic properties and serves as a starting point for descriptions of chemical bond formation. 

Alkali Metals Look at the element family in Group 1 on the periodic table at the back of this book, called the alkali metals. The first members of this family, lithium and sodium, have one electron in their outer energy levels. You can see in Figure 8 that potassium also has one electron in its outer level. Therefore, you can predict that the next family member, rubidium, does also. These electron arrangements are what determines how these metals react. 

But we should consider these only as a guide to predicting electron arrangements. The observed electron configurations of lowest total energy do not always match those predicted by the Aufbau guide, and we will see a number of exceptions, especially for elements in the B groups of the periodic table. 

Valence shell electron arrangements and the periodic table... 

Give the atomic number, number of electrons, electron arrangement, and the Lewis dot structure for the elements helium, boron, chlorine, neon, and phosphorus. Use the periodic table as a source of information. 

It s easy to see how chlorine has achieved a stable octet of electrons, but how does sodium become stable by losing an electron Look at the position of sodium on the periodic table. By losing its lone valence electron, sodium will have the outer electron arrangement of neon. Sodium s stable octet consists of the eight electrons in the energy level below the level of the lost electron. Figure 4.12 summarizes the way that sodium and chlorine react. 

---