Atomic Structure

Atomic Structure

The concept of the atom was created by early Greek philosophers who believed that all matter was composed of indivisible particles. They called these particles atomos, meaning “uncuttable.” It wasn't until the early nineteenth century that John Dalton formulated a theory based on scientific investigation that characterized the nature of atoms. Further discoveries in the nineteenth and twentieth centuries led to the knowledge that atoms possess an internal structure of smaller subatomic particles.

Subatomic particles. The major subatomic particles were found to be protons, electrons, and neutrons. Protons are positively charged particles that have weight.Electrons are negatively charged particles of little weight, while neutrons are just slightly heavier than protons but have no charge. Investigations revealed that protons and neutrons are located in the central core, or nucleus, of the atom, while electrons exist outside of the nucleus in areas of high probability called orbits, or shells. Orbits are further divided into more precise regions of electron probability called orbitals, or subshells.

Niels Bohr proposed the concept of the solar-system atom, in which the nucleus of the atom is like the sun and the electrons are like the planets, revolving in circular orbits. The farther an orbit is from the nucleus, the larger the orbit becomes and the more electrons it can hold.

Because all atoms are electrically neutral, the number of protons and electrons must be equal. Neutrons add weight but no charge to an atom, so additional neutrons do not change an element but merely convert it to one of its isotopic forms. Theatomic number ( Z) of an atom is equal to the number of protons in the nucleus or the number of electrons in its orbits. The atomic mass ( A) is equal to the sum of the protons and neutrons in the atom. (A proton and neutron each have a mass of 1 atomic mass unit, while an electron has virtually no mass.)

Atoms are capable of both losing and gaining electrons to achieve a stable state. If an atom loses one or more electrons, it becomes a positively charged ion called acation. If an atom gains one or more electrons, it becomes a negatively charged ion called an anion. The charge on an ion is equal to the number of electrons lost or gained.

Orbits and orbitals. Electrons fill orbits in an organized fashion based on energy factors. The order of electron fill-in, called the aufbau buildup, is 1 s, 2 s, 2 p, 3 s, 3 p, 4 s, …, where the numerals represent the principal quantum number of the orbit, and the lowercase letters represent the orbitals within a given orbit. The numbering begins with 1 for the orbit closest to the nucleus of the atom. The lower the orbit number, the smaller the orbit size and fewer electrons the orbit can hold.

The first principal orbit is large enough to hold just two electrons in an s orbital. The second principal orbit is large enough to contain one s and three p orbitals, while the third principal orbit, which is larger still, contains an s orbital, three p orbitals, and five d orbitals. When electrons are added to equivalent orbitals, which are orbitals of the same principal level and type, one electron must occupy each equivalent orbital before any of these orbitals can contain two electrons. Thus carbon, Z = 6, has six electrons distributed in these orbitals:

The orbitals can also be shown in the following fashion. In this diagram, the arrows represent electrons. Notice that single electrons are filling the 2 p orbitals one at a time and not pairing first in 2 p_x .

For two electrons to occupy the same orbital, they must have opposite spins, or paired spins, which generate orbital stability by creating opposite magnetic poles.Between equivalent orbitals, the spins of the electrons must be parallel, that is, spinning in the same direction, for the orbitals to be stable. Parallel spins create the same magnetic pole, causing repulsion between the orbitals. This repulsion gives the orbitals maximum separation and the greatest stability.

Orbitals within a given orbit have different shapes and sizes. The s orbitals are spherical, while the p orbitals are hourglass shaped. The s orbital is smaller than thep orbital.

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Three‐Dimensional Shapes of Molecules

Three‐Dimensional Shapes of Molecules

The overall shape of an organic molecule is fixed by the shape of the central carbon atoms, which compose the backbone of the molecule. The shape of this backbone is determined by the types of hybrid orbitals making up the bonds between the central carbon atoms. If the central carbon atoms are sp³ hybridized, the molecule will possess a tetrahedral shape. Central carbon atoms that are sp² hybridized lead to trigonal-planar shapes, while sp hybridization produces linear molecules. Three-dimensional representations of methane ( sp³ hybridization), ethene ( sp¹hybridization), and ethyne ( sp hybridization) molecules are shown in Figure 1 . 

Figure 1

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Hybridization of Atomic Orbitals

Hybridization of Atomic Orbitals

Physical studies of the simplest organic compound, methane (CH₄), have shown the following:

• all of the carbon-hydrogen bond lengths are equal


• all of the hydrogen-carbon-hydrogen bond angles are equal


• all of the bond angles are approximately 110°


• all of the bonds are covalent

The ground state, or unexcited state, of the carbon atom ( Z = 6) has the following electron configuration.

Covalent bonds are formed by the sharing of electrons, so ground-state carbon cannot bond because it has only two half-filled orbitals available for bond formation. Adding energy to the system promotes a 2 s electron to a 2 p orbital, with the resulting generation of an excited state. The excited state has four half-filled orbitals, each capable of forming a covalent bond. However, these bonds would not all be of the same length because atomic 5 orbitals are shorter than atomic porbitals.

To achieve equal bond lengths, all the orbitals would have to be the same type. The creation of identical orbitals occurs in nature by a hybridization process.Hybridization is an internal linear combination of atomic orbitals, in which the wave functions of the atomic s and p orbitals are added together to generate new hybrid wave functions. When four atomic orbitals are added together, four hybrid orbitals form. Each of these hybrid orbitals has one part s character and three parts pcharacter and, therefore, are called sp³ hybrid orbitals.

In the hybridization process, all bond lengths become equal. Bond angles can be explained by the valence-shell electron-pair repulsion theory (VSEPR theory).According to this theory, electron pairs repel each other; therefore, the electron pairs that are in bonds or in lone pairs in orbitals around an atom generally separate from each other as much as possible. Thus, for methane, with four single bonds around a single carbon, the maximum angle of repulsion is the tetra-hedral angle, which is 109°28″, or approximately 110°.

In a similar fashion, the atomic orbitals of carbon can hybridize to form sp² hybrid orbitals. In this case, the atomic orbitals that undergo linear combination are one sand two p orbitals. This combination leads to the generation of three equivalent sp²hybrid orbitals. The third p orbital remains an unhybridized atomic orbital. Because the three hybrid orbitals lie in one plane, the VSEPR theory predicts that the orbitals are separated by 120° angles. The unhybridized atomic p orbital lies at a 90° angle to the plane. This configuration allows for the maximum separation of all orbitals.

Last, the atomic orbitals of carbon can hybridize by the linear combination of one sand one p orbital. This process forms two equivalent sp hybrid orbitals. The remaining two atomic p orbitals remain unhybridized. Because the two sp hybrid orbitals are in a plane, they must be separated by 180°. The atomic p orbitals exist at right angles to each other, one in the plane of the hybridized orbitals and the other at a right angle to the plane.

The type of hybrid orbital in any given carbon compound can be easily predicted with the hybrid orbital number rule.

A hybrid orbital number of 2 indicates sp hybridization, a value of 3 indicates sp²hybridization, and a value of 4 indicates sp³ hybridization. For example, in ethene (C₂H₄), the hybrid orbital number for the carbon atoms is 3, indicating sp²hybridization.

All the carbon-hydrogen bonds are σ, while one bond in the double bond is σ and the other is π.

Thus, the carbons have sp² hybrid orbitals.

Using the hybrid orbital number rule, it can be seen that the methylcarbocation contains sp² hybridization, while the methylcar-banion is sp³ hybridized.

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Adding Tables in HTML

Creating tables with HTML

Topics

I. Introduction
II. Creating a basic table
III. Adding a border, title, and headings
IV. Polishing your table
V. Creating links
Full Document

I. Introduction

You may want to consider using HTML tables in your website. In addition to creating HTML tables to present data in rows and columns, you can also create HTML tables to organize information on your web page.

The process of creating an HTML table is similar to the process that you used to create your web page and any elements that you may have already included in your page, such as links or frames. Coding HTML tables into your web page is fairly easy since you need only understand a few basic table codes.

II. Creating a basic table

The basic structure of an HTML table consists of the following tags:

Table tags:
Row tags:
Cell tags:
Constructing an HTML table consists of describing the table between the beginning table tag, , and the ending table table tag, . Between these tags, you then construct each row and each cell in the row. To do this, you would first start the row with the beginning row tag, , and then build the row by creating each cell with the beginning cell tag, , adding the data for that cell, and then closing the cell with the ending cell tag, . When you finish all of the cells for a row, you would then close the row with the ending row tag, .Then, for each new row, you would repeat the process of beginning the row, building each cell in the row, and closing the row.

The following table is an example of a basic table with three rows and two columns of data.

Data 1 Data 2
Data 3 Data 4
Data 5 Data 6
The codes that generated this table look like this:

Data 1	Data 2
Data 3	Data 4
Data 5	Data 6

This table contains no border, title, or headings. If you wish to add any of these elements to your table, you need to include additional HTML codes. The codes for these elements are explained in the next section.

III. Adding a border, title, and headings

In addition to the basic table tags, several options are available for adding additional elements to your table. For example, if you add a border, title, and column headings to the table in the previous section, the table would then resemble the following:


TABLE TITLE

Column A Column B
Data 1 Data 2
Data 3 Data 4
Data 5 Data 6
The following codes generated the border, TABLE TITLE, and Column A and Column B headings for this table:

Note: If you wish to view the codes that generated the Data 1 through Data 6 cells, refer to the previous section. Notice that the beginning table tag,

TABLE TITLE
Column A	Column B

, now includes the border tag, BORDER="5", which places a border around the table and frames each cell. The number that you ascribe to the border tag, BORDER=n, sets the width of the table border. Depending on how you design your table, you can then determine the border size that best suits your table and the overall design of your web page. To add a title to your table, you would place the title and the attributes of that title between the row commands, and . The heading codes, , define a heading cell and, by default, these codes center the heading and set it in bold type. However, if you want the title to span across the columns below it, you need to include the COLSPAN=n code. Since this table has two columns, the COLSPAN="2" code was necessary. To add emphasis to the header, you can use the header commands to make the text larger. In this table, notice that the

and

commands made the title larger. Finally, the
tag created a space above the title. The individual column headings are also described by the heading codes, . Since these codes, by default, center the heading and set it in bold type, no additional commands or attributes were included in the heading commands. IV. Polishing your table To give your table a more polished look, you can include commands that will adjust the size of your table, add space in the cell, add space between rows, and align the data in a cell. Working with these commands is basically a process of trial and error to create the most appealing presentation of your information. The type of table that you create and the overall design of your web site will help you determine what works best for your table. Some of the commands that enable you to customize your table include: The WIDTH=n% command sets the width of your table as a percentage of the screen. The letter n designates the percentage that you assign to this command. For example, if you want the width of your table to be one half the width of the screen, you would include the WIDTH="50%" command in the beginning table command. The CELLPADDING=n command adjusts the vertical dimension of the cells. The letter n designates the numerical value that you assign to this command. The CELLSPACING=n command sets the space or border around the cells. The letter n designates the numerical value that you assign to this command. The ALIGN=(LEFT, RIGHT, or CENTER) command will horizontally align the data in a cell. For example, if you wish to place the data in the center of each cell in a row, you would include the ALIGN=CENTER command within the row command. The VALIGN=(TOP, MIDDLE, or BOTTOM) command will vertically align the data in a cell. For example, if you wish to place the data in the center of each cell in a row, you would include the ALIGN=MIDDLE command within the row command. In addition to the codes that were explained in the previous sections, the table below now includes some of these commands. TABLE TITLE Column A Column B Data 1 Data 2 The following codes, along with codes previously discussed, created this table:

and	and

TABLE TITLE
Column A	Column B
Data 1	Data 2

Notice that the TABLE command now includes the WIDTH="50%" command. This command extends the table across one half of the width of the text. Also, the CELLPADDING="4" command increases the vertical dimension of the cells, and the CELLSPACING="3" command increases the border around the cells. Finally, the ALIGN="CENTER" command places Data 1 and Data 2 in the center of the cell.

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