# Are isoelectronic ions the same size?

An Isoelectronic Series is a group of atoms/ions that have the same number of electrons. A typical question about isoelectronic series usually involve size comparisons. Since the number of electrons are the same, size is determined by the number of protons.
A.

### What determines the size of the charge of an ion?

In general, ionic radius decreases with increasing positive charge. As the charge on the ion becomes more positive, there are fewer electrons. The ion has a smaller radius. In general, ionic radius increases with increasing negative charge.
• #### Which is greater in size fe2+ or fe3+?

Fe3+ ionic radius is 63 pm, while Fe2+ has an ionic radius of 77 pm. This is because the outermost electron in the Fe2+ ion is pulled off to form Fe3+ ion. As the electrons are removed, it reduces the repulsion increasing nuclear charge experienced by each of the other d electrons and decreases the size of the ion.
• #### How do you find the charge of an ion?

To find the ionic charge of an element you'll need to consult your Periodic Table. On the Periodic Table metals (found on the left of the table) will be positive. Non-metals (found on the right) will be negative. But you need to know the specific ionic charge elments.
• #### Are cations or anions bigger?

Cations and Anions. These cations are smaller than their respective atoms; this is because when an electron is lost, electron-electron repulsion (and therefore, shielding) decreases and the protons are better able to pull the remaining electrons towards the nucleus (in other words, (Z_{eff})increases).
B.

### What is the trend of ion size?

The Periodic Trend. Due to each atom's unique ability to lose or gain an electron, periodic trends in ionic radii are not as ubiquitous as trends in atomic radii across the periodic table. Therefore, trends must be isolated to specific groups and considered for either cations or anions.
• #### What is the radius of an ion?

The ionic radius is the measure of an atom's ion in a crystal lattice. It is half the distance between two ions that are barely touching each other. Since the boundary of the electron shell of an atom is somewhat fuzzy, the ions are often treated as though they were solid spheres fixed in a lattice.
• #### Are positive ions larger or smaller than their neutral atom?

Because the nucleus can't hold the 18 electrons in the Cl- ion as tightly as the 17 electrons in the neutral atom, the negative ion is significantly larger than the atom from which it forms. For the same reason, positive ions should be smaller than the atoms from which they are formed.
• #### Is a positive ion larger or smaller than its parent atom?

Because the nucleus can't hold the 18 electrons in the Cl- ion as tightly as the 17 electrons in the neutral atom, the negative ion is significantly larger than the atom from which it forms. For the same reason, positive ions should be smaller than the atoms from which they are formed.

Updated: 27th September 2018