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THE ACTUAL ATOM MODEL. ELECTRON ARRANGEMENT IN AN ATOM
Electrons orbit the nucleus in atomic orbitals : they are zones of space, round the nucleus, with a high probability (90 %) to contain an electron with a specific energy. Two electrons at most can be inside each orbital, and there are four different kinds of orbitals, called s, p, d and f.
Orbitals are arranged in 7 energy levels. Each energy level has one or several sub-levels called s , p , d and f. Each type of sublevel holds a different number of orbitals, and therefore, a different number of electrons: Sublevel Number of orbitals Maximum number of electrons s 1 2 p 3 6 d 5 10 f 7 14
We call each sublevel with the number of the level and the corresponding letter. So, 4p refers to the sublevel with three p orbitals in level 4. Each level hasn’t got all the types of sublevels. Look at this:
Levels Sublevels Maximum number of electrons in sublevel
Maximum number of electrons in level 1 1s 2 2
2 2s 2 8 2p 6
3s 2 3p 6 18 3d 10
4s 2
32
4p 6 4d 10 4f 14
The position of the electrons, and in particular the number of them in each energy level, affects the way the substance behaves. Electrons always fill the lowest available energy levels first, so the inner levels are always filled before the outer levels.
Electron configuration
The way the electrons are arranged is called the electron structure or electron configuration. The electron configuration of an atom is a form of notation which shows how the electrons are distributed among the various atomic orbitals and energy levels. The format consists of a series of numbers, letters and superscripts as shown below:
1s^2
Here we see the electron configuration for the element helium. This electron configuration provides us with the following information: The large number "1" refers to the energy level. It tells us that the electrons of helium occupy the first energy level of the atom. The letter "s" stands for the sublevel "s", which only has one orbital. The exponent "2" refers to the total number of electrons in that orbital or sublevel. In this case, we know that there are two electrons in the orbital at the first energy level.
The next thing that you need to remember is the fact that the energy sublevels are filled in a specific order as shown by this diagram, called Möeller diagram:
Remember to start at the beginning of each arrow, and then follow it all of the way to the end, filling in the sublevels that it passes through. In other words, the order for filling in the sublevels becomes: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d,7p
Some examples of electron configuration are:
O (Z=8) 1s^2 2s^2 2p^4
Na (Z=11) 1s^2 2s^2 2p^6 3s^1
Ar (Z=18) 1s^2 2s^2 2p^6 3s^2 3p^6
Fe (Z=26) 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^6
As (Z=33) 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^10 4p^3
Sr (Z=38) 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^10 4p^6 5s^2
I (Z=53) 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^10 4p^6 5s^2 4d^10 5p^5
Pb (Z=82) 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^10 4p^6 5s^2 4d^10 5p^6 6s^2 4f^14 5d^10 6p^2
ACTIVITIES
- What is the atomic number? What other particles in the atom are equal in number too?
- What is the mass number?
- What are isotopes? What number is equal in isotopes? What number is different in them?
- Define atomic mass unit, u.
- The mass of an isotope is practically equal to … what number?
- Write in order the symbol and the name of alkali metals. The same for the carbon family.
- Complete the following table: Symbol Name Group Z A p+^ n e- K 19 20
Barium 137 56
Mo 42 54
Copper 29 63
Ga 70 31
Lead 208 126
P 16 15
Tellurium 52 76
I 127 53
Krypton 84 48
- What are orbitals? How many electrons can be in an orbital?
- What is electron configuration of an atom? Write the Möeller diagram.
- Write electron configurations of: Na, Mg, Al, Si, P, S, Cl, Ar
- Write electron configurations of: C, Si, Ge, Sn, Pb
- What is the valence level in an atom?
- What determines the chemical behaviour of an atom?
- Why elements in the same group have a similar chemical behaviour?
- Write the endings of electron configurations of each group of the periodic table.
- Write four applications of radioactivity.
- What are the dangers of radioactivity?
- Why atoms are neutral?
- Why atoms are mostly empty space?
- Calculate the relative atomic mass of silver, that occurs in nature as two following isotopes: Ag- 107 (51,9 %), and Ag-109 (48,1 %).
- Write in order the symbol and the name of alkaline earth metals. The same for chalcogens.
- Write in order the symbol and the name of halogens. Then, write their electron configurations and check they finish equal in the last level.
- Write in order the symbol and the name of noble gases. The same for pnictogens and the boron family.
