Which of the listed properties is the same for the elements beryllium (be), magnesium (mg), and calcium (ca)? Number of valence electrons. How did mendeleev organize the elements in the periodic table? By increasing atomic mass from left to right and top to bottom. Atomic number of calcium is 20. Its electronic configuration is 1s2 2s2 2p6 3s2. It has 2 electons in the valence shell i.e. It has 2+6= 8 (stable octet) electons in the penultimate shell i.e.
Learning Objectives
- To understand the basics of electron shielding and penetration
For an atom or an ion with only a single electron, we can calculate the potential energy by considering only the electrostatic attraction between the positively charged nucleus and the negatively charged electron. When more than one electron is present, however, the total energy of the atom or the ion depends not only on attractive electron-nucleus interactions but also on repulsive electron-electron interactions. When there are two electrons, the repulsive interactions depend on the positions of both electrons at a given instant, but because we cannot specify the exact positions of the electrons, it is impossible to exactly calculate the repulsive interactions. Consequently, we must use approximate methods to deal with the effect of electron-electron repulsions on orbital energies. These effects are the underlying basis for the periodic trends in elemental properties that we will explore in this chapter.
Electron Shielding and Effective Nuclear Charge
If an electron is far from the nucleus (i.e., if the distance (r) between the nucleus and the electron is large), then at any given moment, many of the other electrons will be between that electron and the nucleus (Figure (PageIndex{1})). Hence the electrons will cancel a portion of the positive charge of the nucleus and thereby decrease the attractive interaction between it and the electron farther away. As a result, the electron farther away experiences an effective nuclear charge ((Z_{eff})) that is less than the actual nuclear charge (Z). This effect is called electron shielding.
As the distance between an electron and the nucleus approaches infinity, (Z_{eff}) approaches a value of 1 because all the other ((Z − 1)) electrons in the neutral atom are, on the average, between it and the nucleus. If, on the other hand, an electron is very close to the nucleus, then at any given moment most of the other electrons are farther from the nucleus and do not shield the nuclear charge. At (r ≈ 0), the positive charge experienced by an electron is approximately the full nuclear charge, or (Z_{eff} ≈ Z). At intermediate values of (r), the effective nuclear charge is somewhere between 1 and (Z):
[1 ≤ Z_{eff} ≤ Z.]
Notice that (Z_{eff} = Z) only for hydrogen and only for helium are (Z_{eff}) and (Z) comparable in magnitude (Figure (PageIndex{2})).
Shielding
Shielding refers to the core electrons repelling the outer electrons, which lowers the effective charge of the nucleus on the outer electrons. Hence, the nucleus has 'less grip' on the outer electrons insofar as it is shielded from them.
(Z_{eff}) can be calculated by subtracting the magnitude of shielding from the total nuclear charge and the effective nuclear charge of an atom is given by the equation:
[ Z_{eff}=Z-S label{4}]
where (Z) is the atomic number (number of protons in nucleus) and (S) is the shielding constant. The value of (Z_{eff}) will provide information on how much of a charge an electron actually experiences.
We can see from Equation ref{4} that the effective nuclear charge of an atom increases as the number of protons in an atom increases (Figure (PageIndex{2})). Therefore as we go from left to right on the periodic table the effective nuclear charge of an atom increases in strength and holds the outer electrons closer and tighter to the nucleus. As we will discuss later on in the chapter, this phenomenon can explain the decrease in atomic radii we see as we go across the periodic table as electrons are held closer to the nucleus due to increase in number of protons and increase in effective nuclear charge.
The shielding constant can be estimated by totaling the screening by all electrons ((n)) except the one in question.
[ S = sum_{i}^{n-1} S_i label{2.6.0}]
where (S_i) is the shielding of the ith electron.
Electrons that are shielded from the full charge of the nucleus experience an effective nuclear charge ((Z_{eff})) of the nucleus, which is some degree less than the full nuclear charge an electron would feel in a hydrogen atom or hydrogenlike ion.
From Equations ref{4} and ref{2.6.0}, (Z_{eff}) for a specific electron can be estimated is the shielding constants for that electron of all other electrons in species is known. A simple approximation is that all other electrons shield equally and fully:
[S_i=1 label{simple}]
This crude approximation is demonstrated in Example (PageIndex{1}).
Example (PageIndex{1}): Fluorine, Neon, and Sodium
What is the effective attraction (Z_{eff}) experienced by the valence electrons in the three isoelectronic species: the fluorine anion, the neutral neon atom, and sodium cation?
Solution
Each species has 10 electrons, and the number of nonvalence electrons is 2 (10 total electrons - 8 valence), but the effective nuclear charge varies because each has a different atomic number (A). This is an application of Equations ref{4} and ref{2.6.0}. We use the simple assumption that all electrons shield equally and fully the valence electrons (Equation ref{simple}).
The charge (Z) of the nucleus of a fluorine atom is 9, but the valence electrons are screened appreciably by the core electrons (four electrons from the 1s and 2s orbitals) and partially by the 7 electrons in the 2p orbitals.
- (Z_mathrm{eff}(mathrm{F}^-) = 9 - 2 = 7+)
- (Z_mathrm{eff}(mathrm{Ne}) = 10 - 2 = 8+)
- (Z_mathrm{eff}(mathrm{Na}^+) = 11 - 2 = 9+)
So the sodium cation has the greatest effective nuclear charge. This also suggests that (mathrm{Na}^+) has the smallest radius of these species and that is correct.
Exercise (PageIndex{1}): Sodium Species
What is the effective attraction (Z_{eff}) experienced by the valence electrons in the sodium anion, the neutral sodium atom, and sodium cation? Use the simple approximation for shielding constants. Compare your result for the sodium atom to the more accurate value in Table (PageIndex{1}) and proposed an origin for the difference.
- (Z_mathrm{eff}(mathrm{Na}^-) = 11 - 2 = 7+)
- (Z_mathrm{eff}(mathrm{Na}) = 11 - 2 = 8+)
- (Z_mathrm{eff}(mathrm{Na}^+) = 11 - 2 = 9+)
The (Z_{eff}) in Table (PageIndex{1}) for (Z_mathrm{eff}(mathrm{Na}) is 10.63 and appreciables larger than the 8 estimated above. This means the simple approximation (Equation ref{simple}) overestimates the shielding constant (S).
Calcium Number Of Valence Electrons
Electron Penetration
The approximation in Equation ref{simple} is a good first order description of electron shielding, but the actual (Z_{eff}) experienced by an electron in a given orbital depends not only on the spatial distribution of the electron in that orbital but also on the distribution of all the other electrons present. This leads to large differences in (Z_{eff}) for different elements, as shown in Figure (PageIndex{2}) for the elements of the first three rows of the periodic table.
Penetration describes the proximity to which an electron can approach to the nucleus. In a multi-electron system, electron penetration is defined by an electron's relative electron density (probability density) near the nucleus of an atom (Figure (PageIndex{3})). Electrons in different orbitals have different electron densities around the nucleus. In other words, penetration depends on the shell ((n)) and subshell ((l)).
For example, a 1s electron (Figure (PageIndex{3}); purple curve) has greater electron density near the nucleus than a 2p electron (Figure (PageIndex{3}); red curve) and has a greater penetration. This related to the shielding constants since the 1s electrons are closer to the nucleus than a 2p electron, hence the 1s screens a 2p electron almost perfectly ((S=1). However, the 2s electron has a lower shielding constant ((S<1) because it can penetrate close to the nucleus in the small area of electron density within the first spherical node (Figure (PageIndex{3}); green curve). In this way the 2s electron can 'avoid' some of the shielding effect of the inner 1s electron.
For the same shell value ((n)) the penetrating power of an electron follows this trend in subshells (Figure (PageIndex{3})):
[s > p > d approx f. label{better1}]
Valence Electrons Ca
for different values of shell (n) and subshell (l), penetrating power of an electron follows this trend:
[ce{1s > 2s > 2p > 3s > 3p > 4s > 3d > 4p > 5s > 4d > 5p > 6s > 4f ...} label{better2}]
Penetration
Penetration describes the proximity of electrons in an orbital to the nucleus. Electrons that have greater penetration can get closer to the nucleus and effectively block out the charge from electrons that have less proximity.
| Atom | Sublevel | Z | Zeff |
|---|---|---|---|
| H | 1s | 1 | 1 |
| He | 1s | 2 | 1.69 |
| Li | 1s, 2s | 3 | 2.69, 1.28 |
| Be | 1s, 2s | 4 | 3.68, 1.91 |
| B | 1s, 2s, 2p | 5 | 4.68, 2.58, 2.42 |
| F | 1s, 2s, 2p | 9 | 8.65, 5.13, 5.10 |
| Na | 1s, 2s, 2p, 3s | 11 | 10.63, 6.57, 6.80, 2.51 |
Data from E. Clementi and D. L. Raimondi; The Journal of Chemical Physics 38, 2686 (1963).
Because of the effects of shielding and the different radial distributions of orbitals with the same value of n but different values of l, the different subshells are not degenerate in a multielectron atom. For a given value of n, the ns orbital is always lower in energy than the np orbitals, which are lower in energy than the nd orbitals, and so forth. As a result, some subshells with higher principal quantum numbers are actually lower in energy than subshells with a lower value of n; for example, the 4s orbital is lower in energy than the 3d orbitals for most atoms.
A Better Estimation of Shielding: Slater Rules
The concepts of electron shielding, orbital penetration and effective nuclear charge were introduced above, but we did so in a qualitative manner (e.g., Equations ref{better1} and ref{better2}). A more accurate model for estimating electron shielding and corresponding effective nuclear charge experienced is Slater's Rules. However, the application of these rules is outside the scope of this text.
Summary
The calculation of orbital energies in atoms or ions with more than one electron (multielectron atoms or ions) is complicated by repulsive interactions between the electrons. The concept of electron shielding, in which intervening electrons act to reduce the positive nuclear charge experienced by an electron, allows the use of hydrogen-like orbitals and an effective nuclear charge ((Z_{eff})) to describe electron distributions in more complex atoms or ions. The degree to which orbitals with different values of l and the same value of n overlap or penetrate filled inner shells results in slightly different energies for different subshells in the same principal shell in most atoms.
| Group | 1 | 2 | 18 |
|---|---|---|---|
| Period | |||
| 1 | 1 H | 2 He | |
| 2 | 3 Li | 4 Be | |
| 3 | 11 Na | 12 Mg | |
| 4 | 19 K | 20 Ca | |
| 5 | 37 Rb | 38 Sr | |
| 6 | 55 Cs | 56 Ba | |
| 7 | 87 Fr | 88 Ra |
The s-block is one of four blocks of elements in the periodic table. The element of s- group have a common property. The electron in their most outward electron shell are in the s-orbital.[1] Elements in the s- are in the first two periodic table groups.[2] The elements in group one are called the alkali metals. The elements in group two are called the alkaline earth metals.
The modern periodic law says that 'The properties of elements are periodic function of their atomic number.' This means that some properties of elements are repeated as the atomic number of the elements gets larger. These repeating properties have been used to separate the elements into four s. These s are s-, p-, d-, and f-.
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Properties of s- elements[change | change source]
All of the s- elements are metals (except Hydrogen). In general, they are shiny, silvery, good conductors of heat and electricity. They lose their valenceelectrons easily. In fact, they lose their trademark s orbital valence electrons so easily that the s- elements are some of the most reactive elements on the periodic table.
The elements in group 1, known collectively as the alkali metals (except hydrogen), always lose their one valence electron to make a +1 ion. These metals are characterized by being silvery, very soft, not very dense and having low melting points. These metals react extremely vigorously with water and even oxygen to produce energy and flammablehydrogen gas. They are kept in mineral oil to reduce the chance of an unwanted reaction or worse, an explosion.
The elements in group 2, known as the alkaline earth metals (except helium), always lose their two valence electrons to make a +2 ion. Like the alkali metals, the alkaline earth metals are silvery, shiny and relatively soft. Some of the elements in this column also react vigorously with water and must be stored carefully.
S- elements are famous for being ingredients in fireworks. The ionic forms of potassium, strontium and barium make appearances in firework displays as the brilliant purples, reds and greens.
Francium is considered to be the most rare naturally occurring element on earth. It is estimated that there is only ever one natural atom of Francium present on earth at a time. Francium has a very unstable nucleus and undergoes nuclear decay rapidly.
Chemical properties of alkali metals
1.Alkali metals react with dry hydrogen to form hydrides.
a.These hydrides are ionic in nature
b.These hydrides of alkali metals react with water to form corresponding hydroxides and hydrogen gas.
LiH+ H2O->LiOH+H2
c.These hydrides are strong reducing agents and their reducing nature increases down the group.
d.Alkali metals also form complex hydrides such as LiAlH4 which is a good reducing agent.Alkali metal hydrides do not exist in water and this reaction with any other agent is carried out in protic solvent.
e.Fused alkali metal hydrides on electrolysis produce H2 gas at anode.
2.Formation of oxides and hydroxides.
a.These are most reactive metals and have strong affinity towards O2 ,they form oxides on surface.They are kept under kerosene or paraffin oil to protect them from air.
b.When burnt in air (O2) ,li forms Li20 , Na forms Na 2O2 and other alkali metals form superoxides.
3. They are purely metallic , as they lose the electrons from the outermost shell readily , they are highly reactive metals and they have low ionization energy .
4. Beryllium is amphoteric in nature .
Diagonal relationship[change | change source]
| Group | 1 | 2 | 13 | 14 |
|---|---|---|---|---|
| Period 2 | Li | Be | B | C |
| Period 3 | Na | Mg | Al | Si |
The first element in group one, Lithium, and the first in group two, Beryllium, behave differently to other members of their groups. Their behaviour is like the second element of the next group. So lithium is similar to magnesium, and beryllium is similar to aluminum.[source?]
In the periodic table this is known as a 'diagonal relationship'. The diagonal relationship is because of similarities in ionic sizes and charge/radius ratio of the element. The similarity between lithium and magnesium is because of their similar sizes:[source?]
- Radii, Li=152pm Mg=160pm
Lithium[change | change source]
Lithium has many different behaviours to other elements in group one. This difference caused by:
- the small size of the lithium atom and its ion.
- the higher polarization power of li
+ (i.e. charge size ratio). This means increased covalent character of its compounds which is responsible for their solubility in organic solvents - high ionisationenthalpy and high electronegative character of lithium as compared to other alkali metals
- non availability of d-orbitals in its valence shell
- strong intermetallic bonding
Some of the ways in which lithium behaves differently from other members of are:[source?]
Total Number Of Valence Electrons In Calcium Atom In The Ground State
Calcium Total Number Of Valence Electrons
- Lithium is harder than sodium and potassium which are so soft that they can be cut by a knife.
- The melting and boiling points of lithium are higher.
- Lithium forms monoxide with oxygen, other alkali form peroxide and superoxide.
- Lithium combines with nitrogen to form nitrides, while other alkali metals do not.
- Lithium Chloride is deliquescent and crystallizes as a hydrate LiCl.2H2O. Other alkali metal chlorides do not form hydrates.
References[change | change source]
- ↑'Electron Configuration for all the elements in the Periodic Table'. Retrieved 22 October 2016.CS1 maint: discouraged parameter (link)
- ↑'Archived copy'(PDF). Archived from the original(PDF) on 2013-01-23. Retrieved 2012-02-16.CS1 maint: archived copy as title (link)
Related pages[change | change source]
- [{ NCERT SOLUTIONS THE S-BLOCK ELEMENTSArchived 2020-07-06 at the Wayback Machine}]
Number Of Valence Electrons In A Calcium Atom