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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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41
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5answers
42k 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?
39
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3answers
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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? ...
35
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4answers
25k 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?
33
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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 ...
33
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7answers
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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 $\ce{B2H6}$?
31
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1answer
10k 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 ...
23
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3answers
32k views

Are there any major exceptions when comparing electron affinity?

I was tasked with figuring out whether carbon or nitrogen has a more negative electron affinity value. I initially picked nitrogen, just because nitrogen has a higher $Z_\mathrm{eff}$, creating a ...
23
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1answer
6k 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{...
22
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4answers
15k 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 ...
22
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2answers
118k 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 ...
22
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2answers
50k views

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 ...
21
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1answer
1k views

Why does tin form tin (II) compounds?

The stability of the +II oxidation state in lead has been widely attributed to the so-called “inert pair effect”: the 6s subshell is stabilized through relativistic contraction due to the near-c speed ...
20
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3answers
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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 ...
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 $m_\mathrm{a}$ ...
19
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2answers
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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 ...
17
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3answers
851 views

Have there been no advances in the determination of effective nuclear charges since Clementi and Raimondi in the 60s?

Effective nuclear charge is a very important concept in chemistry, and is the basis for the qualitative explanation of many observed chemical and physical properties, including several periodic trends....
17
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2answers
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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 ...
16
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2answers
11k views

Diagonal relationships in the Periodic Table

I have often wondered about diagonal relationships between elements on the periodic table, and the most often cited explanations revolve around charge-density considerations. But other than that, ...
13
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2answers
17k views

Why are the atomic radii of iron, cobalt, nickel, and copper almost the same?

We have to select the set of elements which will have almost the same atomic radii: $\ce{O, S, Se, Te}$ $\ce{Li, Be, B, C}$ $\ce{Fe, Co, Ni, Cu}$ I googled the atomic radii of the third set and ...
12
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1answer
5k views

Why is osmium the densest known element?

Why is osmium so dense despite there being heavier elements after it in the periodic table?
12
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2answers
14k views

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 ...
12
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1answer
1k views

Why are inert gas (especially Xenon) compounds powerful oxidizing agents?

I am curious as to why compounds with inert gases, such as $\ce{XeF4}$, $\ce{XeF2}$, and $\ce{XeO3}$ are considered powerful oxidizing agents. I would attribute the phenomenon to the highly oxidized ...
12
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1answer
794 views

Is it possible that atoms with 120 protons are possible, but that atoms with 119 protons aren't possible?

We currently know that there are atoms with atomic number up to 118 are possible. Is it possible that atoms with 120 protons are possible, but that atoms with 119 protons aren't possible? Or are ...
11
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3answers
24k 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 ...
10
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3answers
53k 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 ...
10
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1answer
8k views

Why does cobalt have a higher atomic mass than nickel?

I understand that this deviation from the general trend of increasing atomic number and atomic mass is because cobalt's isotopes tend to have more neutrons than nickel's. Why is that the case? How ...
10
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1answer
7k views

Why is arsenic more chemically similar to phosphorus than is nitrogen?

Why is arsenic more chemically similar to phosphorus than is nitrogen (to phosphorus)? I thought that it may be because both phosphorus and arsenic have d orbitals (albeit one has one that is filled ...
10
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1answer
1k views

Trend in atomic radius for noble gases

In an exam, we were given the following graph and asked to explain why the slope of the change in r vs. Z changes dramatically at a point along the curve. I understand that the d block causes the ...
9
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2answers
479 views

Did Mendeleev predict the existence of neutrinos?

According to this Wikipedia page, Mendeleev originally believed that the inert gases belonged in Group 0 (to the left of the alkali metals). Thus, helium would be placed in the second period to the ...
9
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3answers
4k 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 ...
9
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3answers
9k 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, ...
8
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5answers
85k views

Why does the melting point get lower going down the Alkali Metal Group with increase in atomic number?

Why does the melting point get lower going down the Alkali Metal Group with increase in atomic number?
8
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2answers
6k views

Why is the melting point of magnesium oxide higher than aluminium oxide?

There's a graph of the melting points of period three oxides. The melting point of magnesium oxide is several hundred Kelvin higher than aluminiumoxide. I can't find any explanations for this on the ...
8
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2answers
13k views

Solubility of Calcium Fluoride in water

$\ce{CaF2}$ is insoluble in water, but $\ce{CaCl2}$, $\ce{CaBr2}$, $\ce{CaI2}$ are soluble. Why is this so?
8
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2answers
6k views

Why do alkyl bromides and iodides develop colour when exposed to light?

My book says alkyl halides are colourless when pure. However, bromides and iodides develop colour when exposed to light. Why do they develop colour when exposed to light? It is important to note that ...
8
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1answer
2k views

Why is lutetium part of the lanthanide series?

I’ve been looking at the periodic table and began to wonder - why is lutetium part of the lanthanide series? Or, for that matter, why is lawrencium part of the actinide series? This isn’t readily ...
8
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2answers
838 views

Which element has a greater second electron affinity, sulfur or oxygen?

I found a question asking the above which states that sulfur has a lesser second electron affinity than oxygen. But since the inter-electronic repulsion in sulfur is lesser, shouldn't it be willing to ...
8
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3answers
4k views

Why is the boiling point of heavy water more than that of normal water?

In class we learnt that the London forces become stronger as the relative molecular mass increases. Not just as in organic chemistry but in things like the halogens. However, as I understand, the ...
7
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2answers
7k 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 ...
7
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1answer
267 views

On the oxyacids of +III pnictogens and +IV chalcogens

Phosphorus in its $\mathrm{+III}$ oxidation state is known to exist as the phosphonic acid tautomer rather than phosphorous acid one. All salts isolated containing $\ce{H2PO3-}$ are (to the best of my ...
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{...
7
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1answer
6k 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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1answer
304 views

Second ionization potential of Gadolinium

I notice that Gd has a second ionization potential which is significantly higher than would be expected from the general trend in the lanthanides (see this paper p. 945 for a graph). What is the ...
7
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1answer
1k views

Why does hydration of ions depend on their charge to area ratio?

The following is written in my book Concise Inorganic Chemistry by J. D. Lee. Hydration energy, hydrated radius and hydration number of a particular ion depends upon charge per unit area. Hence ...
7
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1answer
154 views

Instability of heavy hydrides

Question: Why do hydrogen compounds from the third period down (that are electron deficient or have completed octets) readily decompose? Thoughts: Apparently, heavier group 14 elements form ...
7
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1answer
3k views

Hydrolysis products of chlorides of group 15

$\ce{NCl3}$ on hydrolysis produces $\ce{NH4OH}$ and $\ce{HOCl}$. $\ce{PCl_3}$ when hydrolyzed produces $\ce{P(OH)3}$ i.e. $\ce{H3PO3}$. $\ce{AsCl_3}$ when hydrolyzed produces $\ce{As(OH)3}$. ...
7
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1answer
2k views

Why does attractive forces of nucleus increase more than shielding across groups?

When you study the electronegativity of the elements, the general trend is that it rises with increasing group number, and decreasing period. Supposedly this is because the attractive forces of the ...
7
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1answer
1k views

Brittle d-block metal trend

I remember seeing a periodic table that had the top-left corner of the d-block shaded and marked as "brittle". If I recall correctly, the elements were $\ce{Sc,Ti,V,Cr,Mn,Y,Zr,Nb,La}$. I think (sorry, ...
7
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1answer
540 views

Why doesn't D block contraction affect the other P block groups 14 15 16 etc

Usually we come across anomalies in the trends of periodic properties for example the size [Atomic radius value from Raymond Chang Chemistry 9th Edition] of Gallium (135 pm) is smaller than Aluminium (...
7
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1answer
2k 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 ...