Why is the first affinity energy negative?

So, I've found a number of somewhat obscure responses to this question but I'm looking for something concrete:

Why is the first affinity energy negative for most elements?

I appreciate that this is because energy is lost but why is energy being released rather than gained?

Am I also correct in thinking that the second affinity energy is positive because it's 'harder' to push an electron into an electron shell (due to electron shielding)?

• Could you state your level of education? If you are say a 5th grade student, one has to answer this completely different than if you are say a chemistry undergrad in their 1st year. The first step of solving your problem would be clarifying several of the words you used. If you are for example a Ph.D. student in organic chemistry, using concepts like electron shells and shielding, neglecting most of the advances in quantum mechanics in the last 100 years, is highly inappropriate. If you are middle school student however, one can only draw from what you should know – Raditz_35 Sep 14 '18 at 10:52
• Thank you for your reply, Raditz_35. I'm in my first year of medical school and this is for a sort of 'intro to the hard sciences' module. – Anil T. Sep 14 '18 at 12:19

The planetary model of orbitals would have the 2s orbital perfect shielding the three 2p orbitals of neon since the three 2p orbitals are degenerate. However the quantum model of the atomic orbitals would show that the 1s, 2s, and three 2p orbitals are all distributed over space rather than having specific lanes like the planets orbiting the sun. So the 1s orbitals doesn't perfect shield the 2s orbital, and the 2s orbital doesn't perfect shield the three 2p orbitals, and the three 2p orbitals don't perfectly shield each other. Slater's Rules give a rough approximation to the shielding of orbitals.

So if you look at the electron affinity data page for free atoms in space, you can see that most elements have a positive electron affinity. That means that the capture of an electron by the neutral atom releases energy.

An atom is composed of a positive nucleus and some number of electrons that are located around the nucleus. Regardless of the model, this is true.

The nucleus has a charge $+Z$ which there are $Z$ electrons, each with charge $-1$.

If you introduce a new electron, the electron is attracted by the positive charge in the nucleus and repelled by the negative charge of the electrons.

However, these are not perfectly balanced out. Generally speaking, the electrons are distributed such that roughly half of the electrons are closer than the nucleus and half are farther away. It works out that in many cases, the electrons farther away can't cancel out enough of the nuclear charge, so there is still a net attraction between the nucleus and the new electron.

You can arrive at this conclusion even without any fancy quantum mechanics.

• Thanks a lot, Zhe! This makes a great deal of sense. A follow-up question, if I may: how does adding an extra energy lead to a release in energy (e.g. a photon of light)? – Anil T. Sep 16 '18 at 11:18
• Sorry, I don't understand the follow-up... – Zhe Sep 16 '18 at 23:56