Since the 3s² electrons are the outermost electrons, magnesium has two valence electrons. The number of valence electrons of an element can be determined by the periodic table group (vertical column) in which the element is categorized. With the exception of groups 3–12 (the transition metals), the units digit of the group number identifies how many valence electrons are associated with a neutral atom of an element listed under.
The electrons in an atom fill up its atomic orbitals according to the Aufbau Principle; 'Aufbau,' in German, means 'building up.' The Aufbau Principle, which incorporates the Pauli Exclusion Principle and Hund's Rule prescribes a few simple rules to determine the order in which electrons fill atomic orbitals:
- Electrons always fill orbitals of lower energy first. 1s is filled before 2s, and 2s before 2p.
- The Pauli Exclusion Principle states no two electrons within a particular atom can have identical quantum numbers. In function, this principle means that if two electrons occupy the same orbital, they must have opposite spin.
- Hund's Rule states that when an electron joins an atom and has to choose between two or more orbitals of the same energy, the electron will prefer to enter an empty orbital rather than one already occupied. As more electrons are added to the atom, these electrons tend to half-fill orbitals of the same energy before pairing with existing electrons to fill orbitals.
Valency and Valence Electrons
The outermost orbital shell of an atom is called its valence shell, and the electrons in the valence shell are valence electrons. Valence electrons are the highest energy electrons in an atom and are therefore the most reactive. While inner electrons (those not in the valence shell) typically don't participate in chemical bonding and reactions, valence electrons can be gained, lost, or shared to form chemical bonds. For this reason, elements with the same number of valence electrons tend to have similar chemical properties, since they tend to gain, lose, or share valence electrons in the same way. The Periodic Table was designed with this feature in mind. Each element has a number of valence electrons equal to its group number on the Periodic Table. This table illustrates a number of interesting, and complicating, features of electron configuration.
First, as electrons become higher in energy, a shift takes place. Up until now, we have said that as the principle quantum number, increases, so does the energy level of the orbital. And, as we stated above in the Aufbau principle, electrons fill lower energy orbitals before filling higher energy orbitals. However, the diagram above clearly shows that the 4s orbital is filled before the 3d orbital. In other words, once we get to principle quantum number 3, the highest subshells of the lower quantum numbers eclipse in energy the lowest subshells of higher quantum numbers: 3d is of higher energy than 4s.
Second, the above indicates a method of describing an element according to its electron configuration. As you move from left to right across the periodic table, the above diagram shows the order in which orbitals are filled. If we were the actually break down the above diagram into groups rather than the blocks we have, it would show how exactly how many electrons each element has. For example, the element of hydrogen, located in the uppermost left-hand corner of the periodic table, is described as 1s1, with the s describing which orbital contains electrons and the 1 describing how many electrons reside in that orbital. Lithium, which resides on the periodic table just below hydrogen, would be described as 1s22s1. The electron configurations of the first ten elements are shown below (note that the valence electrons are the electron in highest energy shell, not just the electrons in the highest energy subshell).
The Octet Rule
Our discussion of valence electron configurations leads us to one of the cardinal tenets of chemical bonding, the octet rule. The octet rule states that atoms becomeespecially stable when their valence shells gain a full complement of valence electrons. For example, in above, Helium (He) and Neon (Ne) have outer valence shells that are completely filled, so neither has a tendency to gain or lose electrons. Therefore, Helium and Neon, two of the so-called Noble gases, exist in free atomic form and do not usually form chemical bonds with other atoms.
Most elements, however, do not have a full outer shell and are too unstable to exist as free atoms. Instead they seek to fill their outer electron shells by forming chemical bonds with other atoms and thereby attain Noble Gas configuration. An element will tend to take the shortest path to achieving Noble Gas configuration, whether that means gaining or losing one electron. For example, sodium (Na), which has a single electron in its outer 3s orbital, can lose that electron to attain the electron configuration of neon. Chlorine, with seven valence electrons, can gain one electron to attain the configuration of argon. When two different elements have the same electron configuration, they are called isoelectronic.
Diamagnetism and Paramagnetism
The electron configuration of an atom also has consequences on its behavior in relation to magnetic fields. Such behavior is dependent on the number of electrons an atom has that are spin paired. Remember that Hund's Rule and the Pauli Exclusion Principle combine to dictate that an atom's orbitals will all half-fill before beginning to completely fill, and that when they completely fill with two electrons, those two electrons will have opposite spins.
An atom with all of its orbitals filled, and therefore all of its electrons paired with an electron of opposite spin, will be very little affected by magnetic fields. Such atoms are called diagmetic. Conversely, paramagnetic atoms do not have all of their electrons spin-paired and are affected by magnetic fields. There are degrees of paramagnetism, since an atom might have one unpaired electron, or it might have four.
Valence ElectronsChemistry is a study of different atoms, an atom is the smallest particle in matter. An atom consists of three major subatomic particles, which are electrons, neutrons and protons. The protons are the positively charged particles. The neutrons are the neutral particles. The electrons are the negatively charged particles. The neutrons and protons are located inside the nucleus of the atom. An atom has nucleus in the center, around the nucleus are electrons arranged in various orbits depending on the atomic number.
Valence electrons definition:Every atom has a unique atomic number. The number of electrons an atom has is equal to the atomic number of the atom. Initially the atom fills electrons in the inner most orbit and then the outer orbits. Theses electrons are arranged in orbits around the nucleus. The electrons that are present in the outermost shell of the atom are called as the valence electrons. Identifying the number of valence electrons for each atom is a very useful concept which can be applied in chemical bonding and many other chemistry topics.
The following is a table to understand the number of valence electrons the atoms in the periodic table have:
|Periodic Table Group||Number of valence electrons|
|The Alkali metals (Group I)||1|
|The Alkaline earth metals (Group II)||2|
|The transition metals (Group 3 to 12)||3-12|
|Noble Gases Group (VIII)||8|
Writing Electronic Configuration for finding the valence electrons:The electronic configuration of an atom can be written according its atomic number. The number of electrons of an atom are arranged in sub energy levels at the ground state of the atoms. The electronic configuration of an atom starts by filling the lower sub energy levels. Given below is the chart that helps to identify how the electrons are filled and the order in which the electronic configuration is written:
So the order in which the electrons are filling is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s and so on.
Valence electrons of few atoms:
The following are examples of finding the number of valence shell electrons using the electronic configuration of the given atom.
Valence electrons of Hydrogen:
· The atomic number of Hydrogen is 1.
· The electronic configuration for Hydrogen is 1s1.
· The number of valence electrons is 1. Since the 1s shell is the outermost shell it has only one electron in it. The orbits for the Hydrogen atom looks like this:
Valence electrons of Helium:
· The atomic number of Helium is 2.
· The electronic configuration for Helium He is 1s2.
· The number of valence electrons is 2. The number of electrons it has is 2.
The diagram for the Helium atom looks like this:
Valence electrons of Lithium:
· The atomic number of Lithium is 3.
· The electronic configuration for Lithium Li is 1s2, 2s1.
· The outermost shell of Lithium is 2s. The number of valence electrons is has 1.
The diagram for the Helium atom looks like this:
Valence electrons of Beryllium:
· The atomic number of Beryllium is 4.
· The electronic configuration for Beryllium Be is 1s2, 2s2.
· The outermost shell of Beryllium is 2s. The number of valence electrons is has 2. The diagram for the Beryllium atom looks like this:
Valence electrons of Boron:
· The atomic number of Boron is 5.
· The electronic configuration for Boron B is 1s2, 2s2, 2p1.
· The outermost shell of Boron is n = 2. The total number of valence electrons it has is 3. The diagram for the Boron atom looks like this:
Valence electrons of Carbon:
· The atomic number of Carbon is 6.
· The electronic configuration for Carbon C is 1s2, 2s2, 2p2.
· The outermost shell of Carbon is n = 2. The total number of valence electrons it has is 4. The diagram for the Carbon atom looks like this:
Valence electrons of Nitrogen:
· The atomic number of Nitrogen is 7.
· The electronic configuration for Nitrogen is N 1s2, 2s2, 2p3.
· The outermost shell of Nitrogen N is n = 2. The total number of valence electrons it has is 5. The diagram for the Nitrogen atom looks like this:
Valence electrons of Oxygen:
· The atomic number of Oxygen is 8.
· The electronic configuration for Oxygen O is 1s2, 2s2, 2p4.
· The outermost shell of Oxygen is n = 2. The total number of valence electrons it has is 6. The diagram for the Oxygen atom looks like this:
Valence electrons of Fluorine:
· The atomic number of Fluorine is 9.
· The electronic configuration for Fluorine F is 1s2, 2s2, 2p5.
· The outermost shell of Fluorine is n = 2. The total number of valence electrons it has is 7. The diagram for the Fluorine atom looks like this:
Valence electrons of Neon:
· The atomic number of Neon is 10.
· The electronic configuration for Neon Ne is 1s2, 2s2, 2p6.
· The outermost shell of Neon is n = 2. The total number of valence electrons it has is 8. The orbits of Neon atom looks like this:
Valence electrons of Sodium:
· The atomic number of Sodium is 11.
· The electronic configuration for Sodium Na is 1s2, 2s2, 2p6, 3s1.
· The outermost shell of Sodium Na is n = 3. The total number of valence electrons it has is 1. The orbits of Sodium atom looks like this:
Valence electrons of Magnesium:
· The atomic number of Magnesium is 12.
· The electronic configuration for Magnesium Mg is 1s2, 2s2, 2p6, 3s2.
· The outermost shell of Magnesium Mg is n = 3. The total number of valence electrons it has is 2. The orbits of Magnesium atom looks like this:
Valence electrons of Aluminum:
· The atomic number of Aluminum is 13.
Number Of Valence Electrons Magnesium Has· The electronic configuration for Aluminum Al is 1s2
Periodic Table Valence Electrons, 2s2, 2p6, 3s2, 3p1.
· The outermost shell of Aluminum Al is n = 3. The total number of valence electrons it has is 3. The orbits of Aluminum atom looks like this:
Shown above are the valence electrons for atoms with atomic number 1 to 13. The same concept applies for finding the valence electrons of all the atoms in the periodic table.