GS-1 Physical Sciences Question 50

[richardtap6915]

"As a result, atoms will accept electrons more readily as we go across the periodic table and the electron affinity becomes more negative (less positive)."

Now, I got this problem right because I chose the only answer stating "increasing effective nuclear charge". However, I know that electron affinity (in most sources I've seen) is positive and increases across the periodic table. While the atomic radius becomes more negative across the table, "electron affinity" becomes more positive -- am I correct? Please clarify.

Thank you,
Rich

[admin]

You used the right strategy to get the correct answer. However, just as it is described in the Explanation, electron affinity becomes increasingly negative across a period.

Reference: www.sparknotes.com/chemistry/fundamentals/atomicstructure/section3.rhtml

Electron Affinity

An atom's electron affinity is the energy change in an atom when that atom gains an electron. The sign of the electron affinity can be confusing. When an atom gains an electron and becomes more stable, its potential energy decreases: upon gaining an electron the atom gives off energy and the electron affinity is negative. When an atom becomes less stable upon gaining an electron, its potential energy increases, which implies that the atom gains energy as it acquires the electron. In such a case, the atom's electron affinity is positive. An atom with a negative electron affinity is far more likely to gain electrons.

[shaun.w.mc6504]

So your saying when an atom gains an electron it becomes more stable, gives off energy and the electron affinity is negative (or becomes more negative). Yet you say that an atom with a negative electron affinity is more likely to gain an electron. If the stability of the atom increases then why would it be more likely to gain an electron?

[admin]

Please note: the further we dig into this question is the further away that we get from the type of knowledge or reasoning required for the new MCAT. The answer is dependent on the 'increasing nuclear charge' across a period.

To read more about stability, the sign convention and electron afinity (more that what is required for the MCAT) consider the link mentioned above or: en.wikipedia.org/wiki/Electron_affinity

[newno558362]

I also agree that this question was confusing. I understand that the effective nuclear charge increases as you go across the periodic table (addition of more protons etc.)

I do NOT however understand that moving from left to right DECREASES Ea. Hopefully that is not an important concept, because I don't necessarily understand or agree with the first part of that answer.

This is a quote from that wiki page: "Eea generally increases across a period (row) in the periodic table."

[geoffn045751]

Yeah I agree this one should be changed or thrown out. An atom's affinity for an electron becomes greater as the electronegativity increases in the same direction across the periodic table. I don't think it can be justified by referencing spark notes or wikipedia lmao

[hellogoodb7913]

[leonreshko6365]

I second the idea of throwing this question out or rephrasing it.

[mcat_premed3832]

The expression "electron affinity increases from left to right across the periodic table" is a true, qualitative expression meaning that the force of attraction is indeed increasing. However, numerically, alkaline earths have positive electron affinity (EA) values and halogens have very negative values. Thus, quantitatively, the trend is from positive to negative as we go from left to right across the periodic table.