![]() ![]() ![]() We use a set of guidelines called Slater’s rules to give a more accurate shielding value which takes into account the shielding from electrons in the same shell. Using a simpler definition of shielding works fine as an introduction, but eventually becomes problematic for more complex problems. But in reality shielding is more complex. Shielding for effective nuclear charge is often first introduced as just originating from core electron electrons. This should make sense since all electrons have negative charges, so an electron in feels a repulsive force from a nearby electron in the same shell. So valence electrons shield other valence electrons from the nucleus. Shielding happens not only from core electrons, but also from electrons in the same shell. So, while a chloride ion has the same electron configuration as a neutral argon atom, they have different radii because of the different number of protons in the nuclei. The chlorine ion example is keeping the same number of protons but adding an electron. If the effective nuclear charge for elements increases as you move to the right, the electrons feel a greater force of attraction for the nucleus and the valence electrons orbit closer resulting in a smaller atomic radius. While you are also adding an extra electron, the extra proton results in a net increase in the effective nuclear charge because the attractive pull of a proton is greater than the shielding of an extra electron in the same shell. As you move left to right, you’re changing the type of element the atom is which means you’re adding an extra proton each step to the right. This is different from the trend of decreasing atomic radii as you move left to right along a period. You’ve decreased the effective nuclear charge felt by the electrons towards the nucleus and so they feel less attractive force towards the nucleus and the valence electrons orbit farther from the nucleus resulting in a larger atomic radius. In other words, even though an element group might break a trend, the elements within the group display periodic properties.If you have a neutral chlorine atom and all you do is add an electron, then you’ve added to the repulsive force felt by the electrons. However, the behavior of the noble gases is periodic. The noble gases are an exception to the trend since these elements have filled electron valence shells and electron affinity values approaching zero. Nonmetals usually have higher electron affinities than metals. Electron affinity increases moving across a period and decreases moving down a group. ![]() Electron Affinity - This is a measure of readily an atom accepts an electron.Atom and ion sizes shrink moving across a period because the increasing positive charge of the nucleus pulls in the electron shell. Although it might seem like increasing the number of protons and electrons in an atom would always increase its size, the atom size doesn't increase until a new electron shell is added. Ionic radius is the distance for ions of the atoms and follows the same trend. Atomic radius decreases moving left to right across a period and increases moving down a group. Atomic Radius - This is half the distance between the middle of two atoms just touching each other.Electronegativity - A measure of how readily an atom forms a chemical bond. Electronegativity increases moving left to right across a period and decrease moving down a group.Ionization energy increases moving left to right across the table and decreases moving down a group. Ionization Energy - This is the energy needed to completely remove an electron from an atom or ion. ![]()
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