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Electronegativity
Fluorine has an electronic signature of 1s22s22px22py22pz1 . It has just 9 protons in the nucleus, with an incoming electron entering the 2-level of orbitals. It has only 2 shielding electrons. Chlorine has an electronic signature of 1s22s22p63s23px 23py
23pz
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1 with 17 protons and 10 shielding electrons. The reason why the electron affinity is higher for chlorine than for fluorine is because fluorine is so small that it is crowded with electrons already so there is slightly more repulsion of these electrons in fluorine than it is with chlorine. The same thing happens between oxygen and sulfur, which are of the same group but reverse the expected trend.
The issue of second electron affinity is only seen in Group 16 elements oxygen and sulfur. Both of these will form a -2 (negative two) anion. The second electron affinity is described as the energy required to add a second electron to a mole of gaseous ions to produce a mole of negative two anions. This takes energy because, in a sense, one is adding a negative electron to something that is already negative. For oxygen, the first electron affinity is -142 kj per mole. The second electron affinity is +844 kJ per mole, which makes it an endothermic, energy-requiring process.
ELECTRONEGATIVITY
Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons. Fluorine is the most electronegative element, having an assigned value of 4.0. This is the value given to it by the Pauling Scale. When two atoms of the same element come together, they have the same electronegativity and therefore have electrons that are on average halfway between the two atoms. This is seen in H2 gas or Cl2 gas. In most cases, however, there is not an equal sharing of electrons between two substances bonded together.
If one element or atom is slightly more electronegative than another element, they are described as not sharing electrons completely equally. The way this looks is that there will be one atom that is called delta-positive (meaning slightly positive) and one that is delta-negative (meaning slightly negative). In such cases, there will be a “covalent” bond, which is a bond that shares electrons from between the two atoms. On the other
