Questions tagged [periodic-trends]

Trends which are observed in the properties of elements as you move along the periodic table in a given direction.

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48
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5answers
79k views

Why does bond angle decrease in the order H2O, H2S, H2Se?

I know that bond angle decreases in the order $\ce{H2O}$, $\ce{H2S}$ and $\ce{H2Se}$. I wish to know the reason for this. I think this is because of the lone pair repulsion but how?
38
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3answers
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Why is the bond angle H-P-H smaller than H-N-H?

$\ce{N}$ & $\ce{P}$ are in the same group. Both $\ce{NH3}$ and $\ce{PH3}$ have one lone pair and according to VSEPR theory, both the central atoms are predicted to be $\ce{sp^3}$ hybridized. But ...
35
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3answers
39k views

Why does basicity of group 15 hydrides decrease down the group?

In my textbook it is written that the order of basic strength of pnictogen hydrides is $$\ce{NH3 > PH3 > AsH3 > SbH3 > BiH3}$$ I tried but could not find any explanation as to why this ...
40
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3answers
108k views

What is the inert pair effect?

I was reading about the p-block elements and found that the inert pair effect is mentioned everywhere in this topic. However, the book does not explain it very well. So, what is the inert pair effect? ...
6
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1answer
20k views

Trend in the melting point down the group

We know that the atomic radii increases down the group. So, less energy is required to pull out the outermost electron as we go down the group, hence, gradually melting point decreases down the group....
9
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3answers
13k views

Why is lanthanum a lanthanide and actinium an actinide?

I do know that the lanthanides start with the element lanthanum, but why? Lanthanum doesn't even have an $f$ orbital, so why isn't it considered a transition metal? It's the same way with actinium, ...
39
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7answers
23k views

Why is BCl3 a monomer whereas AlCl3 exists as a dimer?

What makes dimerization possible in $\ce{AlCl3}$? Are there 3c-2e bonds in $\ce{Al2Cl6}$ as there are in $\ce{B2H6}$?
23
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2answers
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Melting and boiling points of transition elements

The melting and boiling points of transition elements increases from scandium ($1530~\mathrm{^\circ C}$) to vanadium ($1917~\mathrm{^\circ C}$). They increase because as we go across the group, we ...
14
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2answers
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Bond angles for the hydrides

I noticed the fact that all the hydrides of the elements belonging to group IV has bond angle 109.5° while in group V it varies from 107.3° for $\ce{NH3}$ to 91.3° for $\ce{SbH3}$. Similarly we see in ...
23
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2answers
137k views

Why does chlorine have a higher electron affinity than fluorine?

Since fluorine has its valence electrons in the n=2 energy level, and since chlorine has its valence electrons in the n=3 energy level, one would initially expect that an electron rushing towards ...
21
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2answers
7k views

Why is the boiling point of stibane higher than that of ammonia?

I recently came across the fact that the boiling point of $\ce{SbH3}$ (stibane) is greater than that of $\ce{NH3}$ (ammonia). I was expecting $\ce{NH3}$ to have a greater boiling point as a ...
12
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2answers
6k views

Which has the largest bond angle between water, oxygen difluoride and dichlorine oxide?

Which one out of $\ce{H2O}, \ce{Cl2O}, \&\ \ce{F2O}$ will have largest bond angle? I think it should be $\ce{H2O}$ because oxygen is most electronegative in this case so electrons will be more ...
27
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3answers
24k views

Why is lithium the most reducing alkali metal, and not caesium?

Caesium has a larger size, and the effective nuclear charge that the valence electron experiences will be far less compared to that of lithium's, right? But lithium is still considered the strongest ...
12
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1answer
13k views

Thermal stability of alkali metal hydrides and carbonates

Why is it that thermal stability of alkali metal hydrides decreases down the group, but for carbonates, it increases? I used Fajan's rule to check for ionic character but somehow this is only ...
7
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1answer
4k views

Outermost electronic configuration of f block elements

Why is it $(n-2)\mathrm{f}^{1-14}(n-1)\mathrm{d}^{0-1}n\mathrm{s}^2$. The only exception I thought to this formula was Thorium $\ce{5f^{0} 6d^{2} 7s^{2}}$. So I feel it should be $(n-2)\mathrm{...
5
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3answers
6k views

Halogen Oxyacid Trend (acid strength)

Why is $\ce{HClO4}$ more acidic than $\ce{HBrO4}$? But $\ce{HCl}$ is less acidic than $\ce{HBr}$? What determines the acidity? Is it the concentration of $\ce{H+}$? If so, how do these trends develop ...
7
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2answers
10k views

Periodic trend in difference of energy between the s and p orbitals

Why does the difference of energy between the 2s and 2p orbitals of the second period elements increase with increasing atomic number? Does this difference increases by moving down a group, e.g. is ...
5
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3answers
18k views

Why do the melting points of Group 15 elements increase upto Arsenic but then decrease upto Bismuth?

The boiling points of group 15 elements increase on going down the group (or, as size increases) but the same is not true for the melting points. The melting points increase from $\ce{N}$ to $\ce{As}$ ...
36
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4answers
37k views

Why do atoms generally become smaller as one moves left to right across a period?

It seems to me that the addition of electrons and protons as you move across a period would cause an atom to become larger. However, I'm told it gets smaller. Why is this?
6
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2answers
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Why is Aluminium oxide amphoteric?

I was reading my book, in which it mentioned that aluminium and gallium oxides are amphoteric and and indium and thalium oxide are basic in their properties. But no explanation was given. My main ...
13
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1answer
6k views

Why is osmium the densest known element?

Why is osmium so dense despite there being heavier elements after it in the periodic table?
7
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3answers
8k views

Why do the trends in reactivity not apply for francium?

Why is francium not included in the reactivity series? Why is potassium considered more reactive than francium? I know that reactivity increases down the group, but why does it not apply here?
7
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1answer
1k views

What trends exist in the Periodic Table? [closed]

I wish to know the trends in the periodic table and why that trend exists, for the following parameters: Atomic size Ionization energy Metallic character Positive ion size Negative ion size ...
13
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4answers
35k views

Why does screening effect decrease due to d-orbital?

In 13th group, atomic radius increases from boron to aluminium. From aluminium to gallium, atomic radii decreases. From gallium to indium, atomic radii increases. And from indium to thallium, atomic ...
35
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1answer
10k views

"Middle row anomaly" of the periodic table

I was studying about the periodic table recently, and was reading a topic associated with oxides of halogens, and came across the following line The bromine oxides, $\ce{Br2O}$, $\ce{BrO2}$, $\ce{...
20
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2answers
2k views

Is there a function to approximate atomic mass from the atomic number?

The product of $2Z$ generally approximates the value of $m_\mathrm{a}$ of atoms that comprise relatively few protons. Whereas, the product of $2.5Z$ generally approximates the value of $m_\mathrm{a}$ ...
11
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3answers
75k views

What is charge density?

For example, Magnesium has a higher charge density compared to Sodium, therefore its metallic lattice is stronger and it has a higher melting point. Does that mean that Magnesium has more protons and ...
18
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2answers
86k views

Why are there peaks in electronegativities in d-block elements?

Looking at the Pauling electronegativities in the Periodic Table (below, from ChemWiki): Asides from the overall trend of increasing electronegativity across and up the Periodic Table (towards ...
14
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1answer
1k views

Why doesn't ionization energy decrease from O to F or F to Ne?

I know that in general, the first ionization energy increases across a period due to increasing nuclear charge, reasonably constant shielding & decreasing atomic radius. From N to O, however, the ...
7
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1answer
4k views

Why color of alkali metal peroxides, superoxides and ozonides deepens down the group?

This is basically a continuation of the question-"Why is potassium monoxide (K2O) coloured?" I knew that color of alkali metal oxides deepens down the group: Lithium oxide ($\ce{Li2O}$) is the ...
9
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1answer
14k views

Why is fluorine more reactive than iodine despite the weaker I-I bond?

The atomic radius of halogens increases as we go down the group due to the addition of new shells. As a result, the bond length of halogen $\ce{X-X}$ increases down the group. So, less energy is ...
7
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1answer
11k views

Why does the second electron affinity has an opposite sign of the first one?

Many first electron affinities are positive, indicating a favourable process, but the corresponding second electron affinities are negative. For example, the first and second electron affinities of ...
5
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2answers
9k views

Why do heavier transition metals show higher oxidation states?

In p-block elements, higher oxidation states are less stable down the group due to the inert pair effect. This is not the case for transition metals. Why do heavier transition metals show higher ...
35
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1answer
11k views

Why are there two Hydrogen atoms on some periodic tables?

Most periodic tables only feature one Hydrogen atom, on the top of the first group. But some, like the one I was given, also show Hydrogen in the 7th group, to left of Helium. Why are there two ...
8
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1answer
31k views

Why do the boiling and melting points decrease as you go down group 1 and vice versa for group 7?

I used to think that because an alkali metal needs to lose one electron to complete its outer shell, when the atom increases in size (atomic radius), the electron would be easier to lose as the ...
8
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2answers
15k views

Why the radius of noble gases is more than the halogens or the previous groups?

Noble gases have larger radii than that of halogens. Sometimes it is greater than the radius of group I elements. Why is it like that? When we talk about radii of noble gases, what type of radius is ...
7
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1answer
8k views

Why does color of halogen deepens and the boiling point decreases down the group?

Why does the color of the halogens deepens as we go down the group? also why there is a change in physical state of halogens down the group? \begin{array}{c|c} \mathbf{Gas} & \mathbf{Color} \\\...
7
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2answers
20k views

Why is the ionization energy of sulfur anion more than that of oxygen anion?

Which of the following are in the correct order of their ionization energies? (multi-answer question) $\ce{O > S > S- >O-}$ $\ce{F > F- > Cl- > Cl}$ $\ce{O > O- &...
6
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1answer
1k views

Reorganization during ionisation for d block elements

This is a quote from my textbook: The irregular trend in the first ionisation enthalpy of 3d lmetals,can be accounted for by considering that the removal of one electron alters the relative ...
3
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1answer
158 views

Is there an opposite to shielding effect? [closed]

I recently read about shielding effect and lowering of effective nuclear charge due to penetration of other electrons. I wonder while doing calculations involving Slater's rules the electrons from ...
1
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1answer
14k views

What is the reason for the different solubility of silver halides in ammonia?

According to my knowledge, I know that $\ce{AgCl}$ dissolves in dilute ammonia, $\ce{AgBr}$ dissolves in concentrated ammonia and $\ce{AgI}$ does not dissolve even with concentrated ammonia. What is ...
-3
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2answers
4k views

Why is caesium considered the most reactive element and not fluorine? [closed]

Some people say caesium is most reactive element. I thought it to be fluorine as it is the element that reacts with almost all elements (except couple of inert gases). But caesium won't react many of ...
4
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2answers
16k views

Where hydrogen belongs in the Periodic Table

Why do some people say that hydrogen should be above lithium in the periodic table and others argue it should be above fluorine?
4
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6answers
63k views

Why does electronegativity increase across a period?

What explains why electronegativity increases as you move across a period? Does it have something to do with the shielding effect of added electrons?
4
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2answers
4k views

How adding one more electron increases the ionization energy?

Elements of group 6A, compared to 5A, require less ionization energy due to the paired electrons of 6A. But a question arises: why does group 7A, compared to group 6A, require more ionization energy ...
2
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0answers
2k views

Why there is an exception in melting and boiling point in p block?

Why is the boiling point and melting point of 15th group and 16th group has an exception? We know that as molecular mass increases boiling point and melting point also increase. So, down the group 15 ...
2
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1answer
353 views

Scale to be considered for comparing electronegativities of nitrogen and chlorine

The Pauling scale gives the $\chi$ values of $\ce{N}$ and $\ce{Cl}$ to be $3.04$ and $3.16,$ respectively, but the Allen scale gives the $\chi$ values of $\ce{N}$ and $\ce{Cl}$ to be $3.066$ and $2....
1
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1answer
3k views

Why the ionization energy of Au is more than that of Cu?

As we go down the group from 3d to 5 d, size increases. Vanderwaal radius of Cu and Au is 140pm and 166pm respectively. Since Cu is smaller in size, ionization energy of Cu must be greater. Then ...
0
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1answer
563 views

Periodic table and p block elements

I have two questions: It's said that in p block there's gradation in physical and chemical properties of elements. What is meant by this statement? Why are the different blocks named s, p, d and f?
0
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1answer
1k views

Why is the melting point of hydrogen fluoride lower than that of ammonia?

The melting point of hydrogen fluoride is -83.6°C, as compared to that of ammonia, which is -77.73°C. How does this make any sense? Both hydrogen fluoride and ammonia show hydrogen bonding, which "...