3 M. 4 N s s.p s.p.d s.p.d.f s.p.d.f s.p.d s Incomp lete

Transcription

1 If each orbital contains two electrons, the second energy level can have four orbitals: one s orbital and three individual p orbitals. These three p orbitals are energetically equivalent to each other and are labeled 2p x, 2 p y and 2 p z to indicate their orientation in space. The symbols 3s 2, 3p 6, and 3d 10 illustrate the sublevel breakdown of electrons in the third energy level. From this line of reasoning, we can see that if there are sufficient electrons, f electrons first appear in the fourth energy level. Table 4 shows the type of sublevel electrons and maximum number of orbitals and electrons in each energy level. Table 4 Sublevel electrons in each principal energy level and the maximum number of orbitals and electrons in each energy level. Principal energy Level Sublevel Electrons 1 K 2 L 3 M 4 N s s.p s.p.d s.p.d.f s.p.d.f s.p.d s 5 * O 6 * P 7 * Q Maximum Number of Orbitals Incomp lete Incomp lete Incomp lete Total Number of Electrons * Insufficient electrons to complete the shell. Since the s p d f atomic orbitals have definite distribution in space, they are represented by particular spatial shapes. We will consider only the s and p orbilals, the d and f being much more complicated geometrical patterns. The s orbitals are spherically symmetrical about the nucleus. A 2s Page 16

2 orbital is a larger sphere than a 1s orbital. The p orbilals (p x, p y and p z ) are dumbbell-shaped and are oriented at right angles to each other along the x, y and z axes in space. The electron has equal probability of being located in either lobe of the p orbital. The boundaries of the orbitals enclose the region of greatest probability (about 90 percent chance) of finding an electron. In the ground state of a hydrogen atom this falls within a sphere having radius of 0.53 A o The elements are numbered consecutively from 1 to 106, coinciding with the number of protons in the nucleus. Hydrogen, element number I, has one proton in the nucleus; helium, number 2, has two protons; magnesium, number 12, has twelve protons in the nucleus. The atomic number of an element is the same as the number of protons in the nucleus. It tells us the amount of positive electrical charge in the nucleus and also the number of electrons in the neutral atom. The hydrogen atom, consisting of a nucleus containing one proton and an electron, orbital containing one electron, is the simplest known atom. (Some hydrogen atoms-are known to contain one or two neutrons in their nucleus. Page 17

3 The electron in hydrogen occupies an s orbital in the first energy level. The electron does not move in any definite path but rather in a random motion within its orbital, forming an electron cloud about the nucleus. The diameter of the nucleus is believed to be about cm, and the diameter of the electron orbital to be about 10-8 cm, The orbital occupied by the electron is considerably larger than the nucleus. What we have, then, is a positive nucleus surrounded by an electron cloud formed by an electron in an s orbital. The net electrical charge on the hydrogen atom is zero; it is called a neutral atom. Figure 3 shows two methods of representing a hydrogen atom. Figure 3 (a) shows a discrete electron moving around its nucleus; Figure 3 (b) shows the electron orbital surrounding the nucleus. a) b ) The hydrogen atom, (a) Represents the Bohr description, indicating a discrete electron moving around its nucleus of one proton, (b) Represents the quantummechanical concept, showing the electron orbital as a cloud" surrounding the nucleus. Page 18

4 Experimental work done soon after the Bohr-Rutherford concept of the atom was established showed that the masses of nearly all atoms were greater than could be accounted for by simply adding up the masses of all the protons and electrons that were known to be present. This fact led to the concept of the neutron, a particle with no charge but with a mass about the same as that of a proton. Since this particle has no charge, it was very difficult to detect, and the existence. Of the neutron was not proven experimentally until All atomic nuclei except that of the simplest hydrogen atom are now believed to contain neutrons. The atomic number corresponds to the number of protons in the nucleus and identifies an atom as being a particular kind of element. Hence, all atoms of a given element must have the same number of protons. But experimental evidence showed that, in most cases, all the atoms of a given clement did not have identical masses. These mass differences exist because all of the nuclei of a given element do not contain the same number of neutrons. Atoms of an element having the same atomic number but different atomic masses are called isotopes of that clement. Atoms of the isotopes of an element, therefore, have the same number of protons and electrons but different numbers of neutrons. There are three known isotopes of hydrogen. Each is atomic number 1 and has one proton in the nucleus and one electron in the first energy Page 19

5 level. The first isotope (protium) does not have any neutrons in the nucleus, giving it a mass of 1; the second (deuterium) has one neutron in the nucleus, giving it a mass of 2; the third (tritium) has two neutrons, giving it a mass of 3. The atomic structures of the isotopes of hydrogen are shown in the next Figure. These isotopes may be represented by the symbols 1 H, 2 H and 3 H, indicating an atomic number of 1 and a mass of 1, 2, and 3 mass units, respectively. This method of representing atoms is called isotopic notation. The subscript number to the left of the symbol indicates the atomic number of the atom (number of protons in the nucleus). The superscript number to the left of the symbol is the mass number (the total number of protons and neutrons). Diagram of the isotopes of hydrogen A = Mass number (sum of the protons and neutrons in the nucleus) Z = Atomic number (number of protons in the nucleus) Using nuclear symbols to determine the number of p, n, e, and total charge Mass Number = 16 Atomic Number = 8 Page 20

6 # protons = atomic number = 8 # neutrons = Mass # - Atomic # = 16-8 = 8 # electrons = # protons = 8 Mass Number = 16 Atomic Number = 8 # protons = atomic number = 8 # neutrons = Mass # - Atomic # = 16-8 = 8 # electrons = # protons - charge = 8 - (-2) = 10 Mass Number = 137 Atomic Number = 56 # protons = atomic number = 56 # neutrons = Mass # - Atomic # = = 81 # electrons = # protons - charge = 56 - (+2) = Example: Write the nuclear symbol for the following atoms: 1) 50 p, 70 n 2) 17 e -, 20 n 2. Example: Write the nuclear symbol for the following ions: 1) 53 p, 74 n, 54 e - 2) 23 e -, 30 n, net charge = +3 All the elements occur as two or more isotopes. However, not all isotopes are stable. Some isotopes have unstable nuclei and are continually decomposing into other element. For example, of the seven known isotopes of carbon, only two, carbon 12 and carbon 13 are stable. Of the seven known isotopes of oxygen, three are stable 6 8 O 7 8 O and I8 8 O. Of the 15 known isotopes of arsenic, As is the only stable one. Page 21

7 The structure of the atoms of the first 20 elements, arranged in order of increasing atomic number (number of protons). We start with hydrogen, and as we progress to helium, lithium, beryllium, etc., the atoms of each succeeding element contain one more proton and one more electron than the atoms of the preceding element. This sequence, without exception, continues throughout the entire list of known elements. (See periodic Table of element). This sequence is an impressive example of the orderly arrangement of nature- If you recognize at this time that the elemental building blocks of matter are arranged in a systematic fashion, and then you may begin to appreciate the orderly arrangement of the universe. The number of neutrons in an atom also increases as we progress from the simpler elements to the more complex ones, but not in as uniform a manner as do the protons and electrons. For example, the predominant isotope of helium has two protons, two electrons, and also two neutrons. In the helium atom the protons and the neutrons are found in the nucleus; the electrons occupy the 1s orbital in the first energy level, which is now filled to capacity. The electron structure of helium is written as 1s 2. Page 22

8 The most predominant isotope of lithium, atomic number 3, has three protons, four neutrons, and three electrons..since the first energy level can contain no more than two electrons, the third electron is located in the 2s sublevel of the second energy level. The electron structure of lithium is 1s 2 2s 1. In succession, the atoms of beryllium (4), boron (5), carbon (6), nitrogen (7), oxygen (8), fluorine (9), and neon (10) have one more proton and one more electron than the preceding element. Both the first and second energy levels are filled to capacity by the ten electrons of neon, which has two electrons in the first and eight electrons in the second energy level. Element 11, sodium (Na), has two electrons in the first energy level and eight electrons in the second energy level, with the remaining electron occupying the 3s orbital in the third energy level. The electron structure of sodium is 1s 2 2s 2 2p 6 3s 1. Magnesium (12), aluminum (13), silicon (14), phosphorus (15), sulfur (16), chlorine (17), and argon (18) follow in order, Page 23

9 each adding one electron to the third energy level up to argon, which has eight electrons in the M shell Table 5 Electron configurations of the first 11 elements, in subshell notation. Notice how configurations can be built by adding one electron at a time. atom Z ground state electronic configuration H 1 1s 1 He 2 1s 2 Li 3 1s 2 2s 1 Be 4 1s 2 2s 2 B 5 1s 2 2s 2 2p 1 C 6 1s 2 2s 2 2p 2 N 7 1s 2 2s 2 2p 3 O 8 1s 2 2s 2 2p 4 F 9 1s 2 2s 2 2p 5 Ne 10 1s 2 2s 2 2p 6 Na 11 1s 2 2s 2 2p 6 3s 1 The placement of the last electron in potassium and calcium, elements number 19 and 20, defects somewhat from the expected order. One might expect that if the third energy level can contain a maximum of 18 electrons, electrons would continue to fill this shell until the maximum capacity was reached. However, this is not the case. The 4S sublevel is at a lower energy. Hence, in elements 19 and 20 the last element is found in the 4s level. The electron structure for potassium is Is 2 2s 2 2p 6 3s 2 3p 6 4S 1 Calcium has an electron structure similar to potassium, except that it has two 4s electrons. This break in sequence does not invalidate the formula 2n 2 which merely prescribes the maximum number of electrons that each shell may contain, but does not state the order in which the shells are filled. Page 24

10 The elements following calcium have a less regular pattern of adding electrons. The lowest energy level available for the twenty-first electron is the 3d level, thus, scandium (21) has the following electron arrangement: first energy level, two electrons; second energy level, eight electrons; third energy level, nine electrons; fourth energy level, two electrons. The last electron is located in the 3d level. The structure for scandium is Is 2 2s 2 2p 6 3s 2 3p 6 3d 1 4s 2 The elements following scandium, titanium (22) to copper (29), continue to add d electrons until the third energy level has its maximum of 18. Two exceptions in the orderly electron addition are chromium (24) and copper (29. The third energy level of the electrons is first completed in the element copper. The next table shows the order of filling of the electron orbitals and the electron configuration of all the known elements. Example: Determine the electron structure of chlorine atom? chlorine atom has 17 electrons we begin by placing two electrons in the 1s orbital, then two electrons in the 2s orbital, and six electrons in the 2p orbitals. We now have used ten electrons. Finally, we place the next two electrons in the 3s orbital and the remaining five electrons in the 3p orbitals, which use all 17 electrons. The electron structure for a chlorine atom is ls 2 2s 2 2p 6 3s 2 3p 5. The sum of the superscripts equals 17, the number of electrons in the atom. Page 25

11 Table 6 Electron configurations of most elements, in subshell notation. Notice how configurations can be built by adding one electron at a time. Page 26

12 Several methods can be used to diagram atomic structures of atoms, depending on what we are trying to illustrate. When we want to show both the nuclear makeup and the total electron structure of each energy level, we can use a diagram such as Atomic structure diagrams of sodium atom. The number of protons (p) and neutrons (n) in the nucleus are shown in the shaded circle; outside the nucleus is shown the number of electrons (e) in each principal energy level A method of diagramming energy sublevels is shown in next table. Each orbital is represented by a circle. When the orbital contains one electron, an arrow is placed in the circle. A second arrow, pointing downward indicates the second electron in that orbital. The diagram for hydrogen is Helium with two electrons is drawn as both electrons are 1s electrons. Lithium has three electrons in two energy levels, ls 2 2s 1. All four electrons of beryllium are s electrons ls 2 2s 2. Boron has the first p electron, which is located in the 3p, orbital. Since it is energetically more difficult for the next p electron to pair up with the electron in the p orbital than to occupy an empty p orbital, the second p electron in carbon, is located in the 2p. orbital. The third p electron in nitrogen is still unpaired and is found in the 2p. orbital. Page 27

13 The next three electrons pair with each of the 2p electrons up to the element neon. Table 7 Examples of ground state electron configurations in the orbital box notation that shows electron spins. atom orbital box diagram B C N O F Cl Mn 1s 2s 2p 1s 2s 2p 1s 2s 2p 1s 2s 2p 1s 2s 2p 1s 2s 2p 3s 3p 1s 2s 2p 3s 3p 3d 4s Page 28