![]() And atomic radius can be measured by x-ray diffraction. If we consider an atom to be a sphere, we can describe its size by its atomic radius. This increase in nuclear charge cause the nucleus to attract the electrons and their orbitals more strongly, shrinking the size of the atom. The added electron is slightly repelled by the other electrons in the shell, while the added proton increases the positive charge of the nucleus. ![]() Why do atoms get smaller as you go across the period?Īs you go across a period from left to right and atom to atom, each step adds another electron to the same energy level (shell) and a proton to the nucleus. As you can see, the +1 effective nuclear charge experienced by the 1s electron is far less than the nuclear charge. That is: 3 – 2= +1, where 3 is the number of protons (nuclear charge) and 2, the inner electrons. Because of this, we can predict the effective nuclear charge (Z*) experienced by the electron in the 2s orbital by subtracting the two inner electrons from the number of protons. The two inner electrons in the 1s weakens the nucleus (3 protons) attraction (Z) for the one electron in the 2s orbital. And we can write its electron configuration as :1s 22s 1įrom the electron configuration notice that Li has only one electron in the 2s orbital and 2 inner electrons in the 1s orbital. And since Li is neutral, it immediately follows that, it has 3 protons and 3 electrons. Where Z* is the effective nuclear charge, Z, the nuclear charge, and δ, the screening effect by the inner electrons.įor instance, if you look for lithium on your periodic table, you will notice that it has an atomic number of 3. We can write a relationship that describes the effective nuclear charge as: Effective nuclear charge decreases because the inner electrons repel the outer electrons, weakening the nucleus pull for the outer electrons. ![]() The size increase because the effective nuclear charge (positive charge of nucleus) experienced by the outer electrons decreases down a group. Another way of saying the same thing is that as you move down a group, the size of the atoms increase. This repulsion, called screening by the inner electrons usually cause the nucleus to attract the outer electrons less strongly, resulting in an increase in the size of the atom. In addition to that the electrons buried deep inside the atom (inner electrons) usually repel the outer electrons. How atomic size trends across main group elements Why do atoms get larger as you go down a group?Īs you go down a group one atom at a time each step puts valence electron(s) in the highest and largest shell called energy level. Let’s use the following condensed periodic table to show the relative sizes of the main group elements. How does atomic size vary across and down the periodic table for main group or representative elements?įor main group elements, atomic size gets larger as you go down a group (column) and atomic size gets smaller as you go across a period (row). ![]()
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