Again, we can summarize this trend as follows:įigure 8.24 “Atomic Radii Trends on the Periodic Table” shows spheres representing the atoms of the s and p blocks from the periodic table to scale, showing the two trends for the atomic radius. The increasing positive charge leads to a larger effective nuclear charge which casts a tighter grip on the valence electrons, so as you go across the periodic table, the atomic radii decrease. This is because although the valence shell maintains the same principal quantum number, the number of protons - and hence the nuclear charge - is increasing as you go across the row. Where PT stands for “periodic table.” Going across a row on the periodic table, left to right, the trend is different.

This is because the valence electron shell is getting a larger and there is a larger principal quantum number, so the valence shell lies physically farther away from the nucleus. Such radii can be estimated from various experimental techniques, such as the x-ray crystallography of crystals.Īs you go down a column of the periodic table, the atomic radii increase. Although the concept of a definite radius of an atom is a bit fuzzy, atoms behave as if they have a certain radius. The atomic radius is an indication of the size of an atom. Figure 8.23 “Increasing Effective Nuclear Charge.” The periodic trend for effective nuclear charge. These core electron clouds that are more diffuse do not screen as well, giving a slight increase to Z eff (see Figure 8.23 “Increasing Effective Nuclear Charge”). As the principal quantum number ( n) increases, the orbital size increases making the core electron clouds more spread out. However, we actually see a slight increase in Z eff moving down a column of the periodic table. Moving from top to bottom down a column of the periodic table, we might expect the elements to have a similar effective nuclear charge as they all have the same number of valence electrons. This results in a trend that in general the effective nuclear charge increases from left to right across any period of the periodic table. Moving left to right across a period on the periodic table, each subsequent element has an additional proton and valence electron, but the core electrons which are responsible for the majority of screening remain the same. Magnesium is element 12, so it has 12 protons and a nuclear charge of 12: Therefore, magnesium has 10 core electrons from its 1 s 2, 2 s 2, and 2 p 6 orbitals. Where Z is the nuclear charge (equal to the number of protons), and S is the screening constant which can be approximated to the number of non-valence or “core” electrons.įor example: try to approximate the effective nuclear charge of magnesium.įirst, we must determine the electron configuration of magnesium to determine the number of core electrons: The effective nuclear charge is always less than the actual nuclear charge, and can be roughly estimated using the following equation: This net nuclear charge felt by valence electrons is known as its effective nuclear charge, Z eff (pronounced “zed-effective”). Valence electrons are simultaneously attracted to the positive charge of the nucleus and screened (repelled) by the negative charges of other electrons. Many of the periodic properties of atoms depend on electron configuration in particular, the valence electrons and their level of attraction to the nucleus.

There may be a few points where an opposite trend is seen, but there is an overall trend when considered across a whole row or down a whole column of the periodic table. There is no other tool in science that allows us to judge relative properties of a class of objects like this, which makes the periodic table a very useful tool. The variation of properties versus position on the periodic table is called periodic trends. One of the reasons the periodic table is so useful is because its structure allows us to qualitatively determine how some properties of the elements vary versus their position on the periodic table.

Be able to state how certain properties of atoms vary based on their relative position on the periodic table.

Periodic table cl