THE PERIODIC TABLE HELPS US PREDICT PROPERTIES OF ELEMENTS

The composition of air passing through this membrane changes from 21 percent oxygen to up to 44 percent oxygen because the larger nitrogen molecules gpt left behind. 

Recall from Chapter 2 that a gradual change in properties as we move in any direction in the periodic table is called a periodic trend. Most periodic trends can be understood from the perspective of the simplified shell model, and underlying most trends are two important concepts inner-shell shielding effectiw nuclear charge. 

Because inner-.shell electrons diminish the attraction outer-shell electrons have for the nucleus, the nuclear charge sensed by outer-shell electrons is always less than the actual charge of the nucleus. This diminished nuclear charge experienced by outer-.shcll electrons is called the effective nuclear charge and is 

The two electrons in a helium atom have equal exposure to the nucleus hence, they experience the same degree of attraction. represented by the pink shading in the space between the nucleus and the shell boundary. 

Lithiums two first-shell electrons shield the second-shell electron from the nucleus. The nuclear attraction, again represented by pink shading, is Ie.ss intense in the second shell. 

THE PERIODIC TABLE HELPS US PREDICT PROPERTIES OF ELEMENTS 

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The Periodic Table Helps Us Predict Properties of Elements... 

Although atomic theory and electron configuration help us understand the arrangement and behavior of the elements, it s important to remember that the design of the periodic table is based on observing properties of the elements. Before we use the concept of atomic structure to explain how and why atoms combine to form compounds, we need to understand the characteristic properties of the elements and the trends that occur in these properties on the periodic table. These trends allow us to use the periodic table to accurately predict properties and reactions of a wide variety of substances. 

A closer look at the periodic table points out some interesting trends. These trends not only help us predict how one element might perform relative to another, but also give us some insight into the important properties of atoms and ions that determine their performance. For example, examination of the melting points of the elements in Table 1.3 shows that there is a general trend to decrease melting point as we go down... 

The periodic table is a handy tool that correlates the properties of the elements in a systematic way and helps us to make predictions about chemical behavior. We will take a closer look at this keystone of chemistry in Chapter 8. 

A modem periodic table usually shows the atomic number along with the element symbol. As you already know, the atomic nnmber also indicates the number of electrons in the atoms of an element. Electron confignrations of elements help to explain the recurrence of physical and chemical properties. The importance and usefulness of the periodic table lie in the fact that we can use our understanding of the general properties and trends within a group or a period to predict with considerable accuracy the properties of any element, even though that element may be unfamiliar to us. 

As we continue our study of chemistry, we will see that the periodic table is a valuable tool for organizing knowledge and that it helps us predict the properties we expect a given element to exhibit. We will also develop a model for atomic structure that will explain why various groups of elements have similar chemical properties. 

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