Beryllium, calcium, strontium, and radium are all alkaline earth metals in the same group of the periodic table.

The electron affinity, a measure of the energy released when an atom gains an electron (an exothermic reaction), decreases from the top of a group (column) to the bottom. The trends in electron affinity can be correlated with ionization energy. When a smaller atom gains an electron, the force between the electron and nucleus is greater than in a larger atom; thus, more energy is released when this “bond” between the nucleus and electron is formed in a smaller atom than in a larger atom, meaning that smaller atoms will have greater electron affinity. Radium is the farthest down the group of alkaline earth metals, and will have the largest atomic radius of the answer choices, giving it the lowest electron affinity.

Sodium, aluminum, phosphorus, and chlorine are all in the same row (period) of the periodic table.

The electron affinity, a measure of the energy released when an atom gains an electron (an exothermic reaction), increases from left to right across the periodic table because when a smaller atom gains an electron, the force between the electron and nucleus is greater than with a larger atom. More energy is released when this “bond” between the nucleus and electron is formed. Chlorine has the smallest atomic radius of the answer choices because it is located farthest to the right of the period; thus, chlorine will also have the greatest attractive force between its nucleus and electrons, giving it the highest electron affinity.

This question refers to electron affinity, which is defined as the energy given off when a neutral atom in the gas phase gains an extra electron.

Electron affinity increases for elements towards the top and right of the periodic table, so the elements in the top right lose the most energy when gaining an electron. Another way of thinking is that they lose energy, but gain stability. Of the available answers, the element to the most upper right of the periodic table is fluorine.

Electronegativity and electron affinity can be easily confused. Both terms describe resistance to electron gain, but they do so by different classifications. Electronegativity describes how readily an atom will become an anion, or how easily it will accept an electron. The halogens have extremely high electronegativities, while the noble gases have virtually zero electronegativity. In contrast, electron affinity describes the energy change when an electron is added to an atom. The halogens, again, have very high electron affinities. The noble gases will sometimes have negative electron affinities, indicating that it is an exothermic process to remove an electron from these elements.

Chlorine has a great thermodynamic desire to capture an electron, thus taking on the electronic structure of a stable noble gas. This causes chlorine to release energy when it captures an electron as it becomes more stable.

Sodium, on the other hand, would prefer to lose an electron and gain the configuration of a noble gas. Adding an electron would however award some stability to sodium, due to the complete s orbital that this would ensue.

Second electron affinity is usually encountered for such elements as oxygen and sulfur, which form anions with the addition of two electrons. The first electron affinity gives you O^- or S^-, and so it takes significant energy to add another electron to an already negative ion.