6.3. Trends in the Periodic Table http://www.ck12.org
b. Pb- How do you determine the number of valence electrons for a given atom?
Introduction
In the last section, we studied the relationship between an element’s behavior and its location on the periodic table.
The characteristics of an element are largely described by the configuration of its valence electrons. In this section,
we are going to look at specific properties that can be predicted by an element’s position on the periodic table.
Additionally, we will look at the formation of ions, including how to predict which ions are likely to form and which
are not.
Atomic Radius
One important characteristic that determines the way in which elements behave is the total size of each atom. Free
atoms are spherical in shape, so the relative sizes of the elements can be compared by looking at each atom’satomic
radius, which is the distance from an atom’s nucleus to the electrons in the outermost orbitals. You might expect
atoms to generally grow larger as they go up in atomic number (which is equal to the total number of electrons in the
neutral atom). Indeed, if you look at a single group of the periodic table, this trend holds true. Iodine is larger than
bromine, which is in turn larger than chlorine and fluorine. In the case of a single group, each successive row places
electrons in a higher principal energy level. Since higher energy levels are farther from the nucleus on average, this
results in a larger total volume occupied by the atom.
However, when going across a period from left to right, the atomic radius actually tends to decrease. Why is this
so? Each successive electron is going into the same principal energy level as the previous one, so the total amount
of occupied space does not really go up significantly. Additionally, because protons are also added to the nucleus as
you go across the row, the pull of the positively charged nucleus on the negatively charged electrons increases. This
tighter pull leads to a slight decrease in atomic radius. As a result, the atomic radii of the elements exhibit a periodic
trend, gradually tending downward, but with a sharp spike up whenever electrons are added to a new principal energy
level (Figure6.9).
Forming Ions
Anionis an atom or group of bonded atoms that has a positive or negative charge. Ions are formed when an atom
gains or loses electrons from its valence shell (Figure6.10). This process causes an imbalance between the number
of positively charged protons and negatively charged electrons, so the overall ion will carry a net positive or negative
charge.
When an atom loses one or more electrons, it becomes positively charged, because it now has more protons than
electrons. A positively charged ion is called acation. The charge for a cation is written as a numerical superscript
after the chemical symbol, followed by a plus sign. If the ion carries a single unit of charge, the number “1” is
assumed and is not written. For example, a sodium atom that loses one electron becomes a sodium ion, which is
written as Na+. A magnesium atom that loses two electrons becomes a magnesium ion, which is written as Mg^2 +.
This magnesium ion carries a 2+ charge because it now has two more protons than electrons.
When an atom gains one or more electrons, it becomes negatively charged, because it now has more electrons than
protons. A negatively charged ion is called ananion. The charge of an anion is written in the same way as the charge