Valence Electrons

1. 6 Valence Electrons Least Mass
2. 6 Valence Electrons Elements

Key Questions

Molybdenum has six valence electrons. The atomic number of molybdenum is 42, and its electron configuration is 1s22s22p63s23p63d104s24p64d55s1 or 2, 8, 18, 13, 1 electrons per shell. The electrons in the 4d55s1 constitute its valence electrons. The symbol for molybdenum is Mo. Valence Electrons, Ions, and the Periodic Table. Valence Electrons.Valence electrons are electrons in the outer energy level of an atom.Find the number of valence electrons for: 1.Li 2.P 3.I 4.Ar 5.Ca 6.O 7.He. 1.Li = 1 2.P = 5 3.I = 7 4.Ar = 8 5.Ca = 2 6.O = 6 7.

• The valence electrons are the electrons that determine the most typical bonding patterns for an element.

  These electrons are found in the s and p orbitals of the highest energy level for the element.

  Sodium #1s^2 2s^2 2p^6 3s^1#
  Sodium has 1 valence electron from the 3s orbital

  Phosphorus #1s^2 2s^2 2p^6 3s^2 3p^3#
  Phosphorus has 5 valence electrons 2 from the 3s and 3 from the 3p

  Lets take the ionic formula for Calcium Chloride is #CaCl_2#

  Calcium is an Alkaline Earth Metal in the second column of the periodic table. This means that calcium #s^2# has 2 valence electrons it readily gives away in order to seek the stability of the octet. This makes calcium a Ca+2 cation.

  Chlorine is a Halogen in the 17th column or #s^2p^5# group.
  Chlorine has 7 valence electrons. It needs one electron to make it stable at 8 electrons in its valence shells. This makes chlorine a #Cl^(−1)# anion.

  Ionic bonds form when the charges between the metal cation and non-metal anion are equal and opposite. This means that two #Cl^(−1)# anions will balance with one #Ca^(+2)# cation.

  This makes the formula for calcium chloride, #CaCl_2#.

  For the example Aluminum Oxide #Al_2O_3#

  Aluminum #s^2p^1# has 3 valence electrons and an oxidation state of +3 or #Al^(+3)#
  Oxygen #s^2p^4# has 6 valence electrons and an oxidation state of -2 or #O^(−2)#

  The common multiple of 2 and 3 is 6.
  We will need 2 aluminum atoms to get a +6 charge and 3 oxygen atoms to get a -6 charge. When the charges are equal and opposite the atoms will bond as #Al_2O_3#.

  In molecular (covalent) compounds these same valence electrons are shared by atoms in order to satisfy the rule of octet.

  I hope this is helpful.
  SMARTERTEACHER

  
  

• Answer:

  The number of electrons in an atom's outermost valence shell governs its bonding behaviour.

  Explanation:

  The valence electrons are the electrons in the outermost electron shell of an atom.

  That is why elements whose atoms have the same number of valence electrons are grouped together in the Periodic Table.

  Generally, elements in Groups 1, 2, and 13 to 17 tend to react to form a closed shell, corresponding to the electron configuration#s^2p^6#.

  This tendency is called the octet rule, because the bonded atoms have eight valence electrons.

  METALS

  The most reactive kind of metallic element is a metal from Group 1 (e.g., sodium or potassium).

  An atom in Group 1 has only a single valence electron. This one valence electron is easily lost to form a positive ion with an #s^2p^6# configuration (e.g., #'Na'^+# or #'K'^+#).

  A metal from Group 2 (e.g., magnesium) is somewhat less reactive, because each atom must lose two valence electrons to form a positive ion (e.g., #'Mg'^(2+)# with an #s^2p^6# configuration.

  Within each group of metals, reactivity increases as you go down the group.

  The valence electrons are less tightly bound and easier to remove, because they are farther away from the nucleus of the atom.

  NONMETALS

  A nonmetal tends to attract additional valence electrons to attain a full valence shell.

  It can either share electrons with a neighboring atom to form a covalent bond or it can remove electrons from another atom to form an ionic bond.

  The most reactive kind of nonmetal is a halogen such as fluorine or chlorine.

  It has an #s^2p^5# electron configuration, so it requires only one additional valence electron to form a closed shell.

  To form an ionic bond, a halogen atom can remove an electron from another atom in order to form an anion (e.g., #'F'^'-', 'Cl'^'-'#, etc.).

  To form a covalent bond, one electron from the halogen and one electron from another atom form a shared pair (e.g., in #'H–F'#, the dash represents a shared pair of valence electrons, one from #'H'# and one from #'F'#).

  Within each Group of nonmetals, reactivity decreases from top to bottom, because the valence electrons are at progressively higher energies and the atoms do not gain much stability by gaining electrons.

  In fact, oxygen (the lightest element in Group 16) is more reactive than chlorine, even though it is not a halogen, because the valence electrons of oxygen are closer to the nucleus (at a lower energy).

  Therefore, a metal from the bottom of Group 1 (like potassium) and a nonmetal from the top of Group 17 (like fluorine) will react violently, because they both benefit greatly from the reaction.

  #'K'# loses one electron to #'F'# and forms the ionic compound potassium fluoride, #'K'^'+'F'^'-'#.

Questions

Learning Objectives

• Describe how electrons are grouped within atoms.

Although we have discussed the general arrangement of subatomic particles in atoms, we have said little about how electrons occupy the space about the nucleus. Do they move around the nucleus at random, or do they exist in some ordered arrangement?

The modern theory of electron behavior is called quantum mechanics. It makes the following statements about electrons in atoms:

• Electrons in atoms can have only certain specific energies. We say that the energies of the electrons are quantized.
• Electrons are organized according to their energies into sets called shells (labeled by the principle quantum number, n). Generally the higher the energy of a shell, the farther it is (on average) from the nucleus. Shells do not have specific, fixed distances from the nucleus, but an electron in a higher-energy shell will spend more time farther from the nucleus than does an electron in a lower-energy shell.
• Shells are further divided into subsets of electrons called subshells. The first shell has only one subshell, the second shell has two subshells, the third shell has three subshells, and so on. The subshells of each shell are labeled, in order, with the letters s, p, d, and f. Thus, the first shell has only a single s subshell (called 1s), the second shell has 2s and 2p subshells, the third shell has 3s, 3p, and 3dand so forth.

Table (PageIndex{1}): Shells and Subshells

Shell	Number of Subshells	Names of Subshells
1	1	1s
2	2	2s and 2p
3	3	3s, 3p and 3d
4	4	4s, 4p, 4d and 4f

• Different subshells hold a different maximum number of electrons. Any s subshell can hold up to 2 electrons; p, 6; d, 10; and f, 14.

Table (PageIndex{2}): Number of Electrons

Subshell	Maximum Number of Electrons
s	2
p	6
d	10
f	14

It is the arrangement of electrons into shells and subshells that most concerns us here, so we will focus on that.

We use numbers to indicate which shell an electron is in. As shown in Table (PageIndex{1}), the first shell, closest to the nucleus and with the lowest-energy electrons, is shell 1. This first shell has only one subshell, which is labeled 1s and can hold a maximum of 2 electrons. We combine the shell and subshell labels when referring to the organization of electrons about a nucleus and use a superscript to indicate how many electrons are in a subshell. Thus, because a hydrogen atom has its single electron in the s subshell of the first shell, we use 1s¹ to describe the electronic structure of hydrogen. This structure is called an electron configuration. Electron configurations are shorthand descriptions of the arrangements of electrons in atoms. The electron configuration of a hydrogen atom is spoken out loud as “one-ess-one.”

Helium atoms have 2 electrons. Both electrons fit into the 1s subshell because s subshells can hold up to 2 electrons; therefore, the electron configuration for helium atoms is 1s²(spoken as “one-ess-two”).

The 1s subshell cannot hold 3 electrons (because an s subshell can hold a maximum of 2 electrons), so the electron configuration for a lithium atom cannot be 1s³. Two of the lithium electrons can fit into the 1s subshell, but the third electron must go into the second shell. The second shell has two subshells, s and p, which fill with electrons in that order. The 2s subshell holds a maximum of 2 electrons, and the 2p subshell holds a maximum of 6 electrons. Because lithium’s final electron goes into the 2s subshell, we write the electron configuration of a lithium atom as 1s²2s¹. The shell diagram for a lithium atom is shown below. The shell closest to the nucleus (first shell) has 2 dots representing the 2 electrons in 1s, while the outermost shell (2s) has 1 electron.

The next largest atom, beryllium, has 4 electrons, so its electron configuration is 1s²2s². Now that the 2s subshell is filled, electrons in larger atoms start filling the 2p subshell. Thus, the electron configurations for the next six atoms are as follows:

• B: 1s²2s²2p¹
• C: 1s²2s²2p²
• N: 1s²2s²2p³
• O: 1s²2s²2p⁴
• F: 1s²2s²2p⁵
• Ne: 1s²2s²2p⁶

With neon, the 2p subshell is completely filled. Because the second shell has only two subshells, atoms with more electrons now must begin the third shell. The third shell has three subshells, labeled s, p, and d. The d subshell can hold a maximum of 10 electrons. The first two subshells of the third shell are filled in order—for example, the electron configuration of aluminum, with 13 electrons, is 1s²2s²2p⁶3s²3p¹. However, a curious thing happens after the 3p subshell is filled: the 4s subshell begins to fill before the 3d subshell does. In fact, the exact ordering of subshells becomes more complicated at this point (after argon, with its 18 electrons), so we will not consider the electron configurations of larger atoms. A fourth subshell, the f subshell, is needed to complete the electron configurations for all elements. An f subshell can hold up to 14 electrons.

Electron filling always starts with 1s, the subshell closest to the nucleus. Next is 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, etc., shown in the electron shell filling order diagram in Figure (PageIndex{2}). Follow each arrow in order from top to bottom. The subshells you reach along each arrow give the ordering of filling of subshells in larger atoms.

Example (PageIndex{1}): Electronic Configuration of Phosphorus Atoms

Using Figure (PageIndex{2}) as your guide, write the electron configuration of a neutral phosphorus atom. The atomic number of P is 15.

Solution

A neutral phosphorus atom has 15 electrons. Two electrons can go into the 1s subshell, 2 can go into the 2s subshell, and 6 can go into the 2p subshell. That leaves 5 electrons. Of those 5 electrons, 2 can go into the 3s subshell, and the remaining 3 electrons can go into the 3p subshell. Thus, the electron configuration of neutral phosphorus atoms is 1s²2s²2p⁶3s²3p³.

Exercise (PageIndex{1}): Electronic Configuration of Chlorine Atoms

Using Figure (PageIndex{2}) as your guide, write the electron configuration of a neutral chlorine atom. The atomic number of Cl is 17.

Answer

A neutral chlorine atom has 17 electrons. Two electrons can go into the 1s subshell, 2 can go into the 2s subshell, and 6 can go into the 2p subshell. That leaves 7 electrons. Of those 7 electrons, 2 can go into the 3s subshell, and the remaining 5 electrons can go into the 3p subshell. Thus, the electron configuration of neutral chlorine atoms is 1s²2s²2p⁶3s²3p⁵.

Since the arrangement of the periodic table is based on the electron configurations, Figure (PageIndex{3}) provides an alternative method for determining the electron configuration. The filling order simply begins at the top left, with hydrogen (Z=1) and includes each subshell as you proceed in increasing atomic number (Z) order.

For example, the first row (Period 1) contains H and He only, because only two electrons are required to fill the 1s subshell. The second row s-block, contains only two elements, Li and Be, to fill the 2s subshell. This is followed by the second row p-block, containing 6 elements (B through Ne) since six electrons are required to fill the 2p subshell. The third row is similar to the second row elements. Two electrons are needed (Na and Mg) to fill the 3s subshell and six electrons are required (Al through Ar) to complete the 3p subshell. After filling the 3p block up to Ar, we see the next subshell will be 4s (K, Ca), followed by the 3d subshell, which are filled by ten electrons (Sc through Zn). The 4p subshell is filled next by six electrons (Ga through Kr). As you can see, the periodic table shown in Figure (PageIndex{3}) provides a simple way to remember the order of filling the subshells in determining the electron configuration. The order of filling subshells is the same: 1s,2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, etc.

Example (PageIndex{2}): Aluminum

Using Figure (PageIndex{3}) as your guide, write the electron configuration of neutral aluminum atom. The atomic number of Al is 13.

Solution

Aluminum has 13 electrons.

Start at Period 1 of the periodic table, Figure (PageIndex{3}). Place two electrons in the 1s subshell (1s²).

6 Valence Electrons Least Mass

Proceed to Period 2 (left to right direction). Place the next two electrons in the 2s subshell (2s²) and the next six electrons in the 2p subshell (2p⁶).

Proceed to Period 3 (left to right direction). Place the next two electrons in the 3s subshell (3s²) and the last one electron in the 3p subshell (3p¹).

The electron configuration of Aluminum is 1s²2s²2p⁶3s²3p¹

Exercise (PageIndex{2})

Using Figure (PageIndex{3}) as your guide, write the electron configuration of the atom that has 20 electrons

Answer

Start at Period 1 of Figure (PageIndex{3}). Place two electrons in the 1s subshell (1s²).

Proceed to Period 2 (left to right direction). Place the next two electrons in the 2s subshell (2s²) and the next six electrons in the 2p subshell (2p⁶).

Proceed to Period 3 (left to right direction). Place the next two electrons in the 3s subshell (3s²) and the next six electron in the 3p subshell (3p⁶).

Proceed to Period 4. Place the remaining two electrons in the 4s subshell (4s²).

The electron configuration is 1s²2s²2p⁶3s²3p⁶4s²

Valence Electrons

In the study of chemical reactivity, we will find that the electrons in the outermost principal energy level are very important and so they are given a special name. Valence electrons are the electrons in the highest occupied principal energy level of an atom.

In the second period elements, the two electrons in the (1s) sublevel are called inner-shell electrons and are not involved directly in the element's reactivity or in the formation of compounds. Lithium has a single electron in the second principal energy level and so we say that lithium has one valence electron. Beryllium has two valence electrons. How many valence electrons does boron have? You must recognize that the second principal energy level consists of both the (2s) and the (2p) sublevels and so the answer is three. In fact, the number of valence electrons goes up by one for each step across a period until the last element is reached. Neon, with its configuration ending in (2s^2 2p^6), has eight valence electrons.

The alkali metal sodium (atomic number 11) has one more electron than the neon atom. This electron must go into the lowest-energy subshell available, the 3s orbital, giving a 1s²2s²2p⁶3s¹ configuration. The electrons occupying the outermost shell orbital(s) (highest value of n) are called valence electrons, and those occupying the inner shell orbitals are called core electrons ( Figure PageIndex4). Since the core electron shells correspond to noble gas electron configurations, we can abbreviate electron configurations by writing the noble gas that matches the core electron configuration, along with the valence electrons in a condensed format. For our sodium example, the symbol [Ne] represents core electrons, (1s²2s²2p⁶) and our abbreviated or condensed configuration is [Ne]3s¹.

Similarly, the abbreviated configuration of lithium can be represented as [He]2s¹, where [He] represents the configuration of the helium atom, which is identical to that of the filled inner shell of lithium. Writing the configurations in this way emphasizes the similarity of the configurations of lithium and sodium. Both atoms, which are in the alkali metal family, have only one electron in a valence s subshell outside a filled set of inner shells.

[ce{Li:[He]},2s^1 ce{Na:[Ne]},3s^1]

A chemical reaction results from electron removal, electron addition, or electron sharing of the valence electrons of the different atoms. The path a specific element will take depends on where the electrons are in the atom and how many there are. Thus, it is convenient to separate electrons into two groups. Valence shell electrons (or, more simply, the valence electrons) are the electrons in the highest-numbered shell, or valence shell, while core electrons are the electrons in lower-numbered shells. We can see from the electron configuration of a carbon atom—1s²2s²2p²—that it has 4 valence electrons (2s²2p²) and 2 core electrons (1s²). You will see in the next chapters that the chemical properties of elements are determined by the number of valence electrons.

Example (PageIndex{3})

Examine the electron configuration of neutral phosphorus atoms in Example (PageIndex{1}), 1s²2s²2p⁶3s²3p³ and write the abbreviated notation.

Solution

Phosphorus has electron configuration, 1s²2s²2p⁶3s²3p³.

The highest-numbered shell is the third shell (3s²3p³): 2 electrons in the 3s subshell and 3 electrons in the 3p subshell. That gives a total of 5 valence electrons.

The 10 inner shell (core) electrons, 1s²2s²2p⁶ can be replaced by [Ne] (see Figure (PageIndex{3})). Abbreviated notation is : [Ne]3s²3p³

Exercise (PageIndex{3})

Examine the electron configuration of neutral calcium atom (Exercise (PageIndex{2})), 1s²2s²2p⁶3s²3p⁶4s², and write the abbreviated notation.

Answer

The highest-numbered shell is the fourth shell 4s², which has 2 electrons in the 4s subshell. Hence, Calcium has 2 valence electrons.

The 18 inner-shell (core) electrons, 1s²2s²2p⁶3s²3p⁶, can be replaced by [Ar], see Figure (PageIndex{3}). The abbreviated notation is: [Ar]4s²

Example (PageIndex{4})

Based on their respective locations in the periodic table (use Figure (PageIndex{3})), determine the number of valence electrons and the valence shell configuration of elements A, B and C.

Solution

Element A is located in Period 2, the 5th position in 2p-block. Before the electrons are placed in 2p subshell, the 2s subshell must be filled first. This means that A has two valence electrons in 2s (2s²) and five valence electrons in 2p (2p⁵). Answer: 2s²2p⁵. It has 2 + 5 = 7valence electrons.

Element B is located in Period 3, the 2nd position in 3s-block. This means that B has two valence electrons in 3s (3s²). Answer: 3s².

Element C is located in Period 5, the 1st position in 5s-block). This means that there is only one valence electron in 5s (5s¹). Answer: 5s¹.

Exercise (PageIndex{4})

Using the location of Na is the periodic table (Figure (PageIndex{3})), draw the shell diagram of sodium atom.

Answer

Sodium (Na) is the first element in the 3rd row (Period 3) in the periodic table. This means that the first shell and second shells of Na atom are filled to the maximum number of electrons.

The first shell (1s) is filled with 2 electrons. The second shell(2s and 2p) has a total of 8 electrons. And, the third (last) shell has1 electron.

The shell diagramof the Na atom is shown below. The shell nearest the nucleus (first shell) has 2 electrons (2 dots), the second shell has 8 electrons and the last (outermost) shell has 1 electron. (2.8.1)

Concept Review Exercises

1. How are electrons organized in atoms?
2. What information does an electron configuration convey?
3. What is the difference between core electrons and valence electrons?

Answers

1. Electrons are organized into shells and subshells around nuclei.
2. The electron configuration states the arrangement of electrons in shells and subshells.
3. Valence electrons are in the highest-numbered shell; all other electrons are core electrons.

Key Takeaway

• Electrons are organized into shells and subshells about the nucleus of an atom.
• The valence electrons determine the reactivity of an atom.

6 Valence Electrons Elements

Exercises