Question

Which of the following statements about the periodic table is false? a. Elements in the same column have similar reactivities because their valence electrons tend to be located in the same types of orbitals. b. A series of ions that are isoelectronic (contain the same number of electrons) in their ground states must have the same electron configuration. c. The atomic size of the elements increases going across a period from left to right because the number of electrons increases, so the electrons are located further from the nucleus. d. It takes more energy to remove an electron from lithium than from cesium because the valence electrons in lithium are located closer to the nucleus.

Short answer

The false statement is statement c. The atomic size of elements actually decreases going across a period from left to right due to the increase in nuclear charge, which attracts electrons closer to the nucleus, resulting in a smaller atomic size.

Step-by-step solution

Examine statement a

Statement a says, "Elements in the same column have similar reactivities because their valence electrons tend to be located in the same types of orbitals." This statement is true; elements in the same group (vertical column) in the periodic table have the same number of valence electrons in their outermost shell, resulting in similar chemical properties and reactions.

Examine statement b

Statement b says, "A series of ions that are isoelectronic (contain the same number of electrons) in their ground states must have the same electron configuration." This statement is true as well. Isoelectronic ions or atoms possess the same number of electrons, and therefore their electron configurations are alike.

Examine statement c

Statement c says, "The atomic size of the elements increases going across a period from left to right because the number of electrons increases, so the electrons are located further from the nucleus." This statement is false. The atomic size (or atomic radius) generally decreases across a period from left to right due to an increase in the positive charge of the nucleus. The increased nuclear charge attracts electrons closer to the nucleus, resulting in a smaller atomic size.

Examine statement d

Statement d says, "It takes more energy to remove an electron from lithium than from cesium because the valence electrons in lithium are located closer to the nucleus." This statement is true. The ionization energy (energy required to remove an electron) generally decreases going down a group in the periodic table because the atomic size increases. As a result, valence electrons are further from the nucleus and less tightly held, requiring less energy to remove them.

Conclusion

The false statement in the given exercise is statement c, as the atomic size of elements decreases, not increases, going across a period from left to right due to the increase in nuclear charge.

Key concepts

Valence Electrons

Valence electrons are the electrons found in the outermost shell of an atom. These electrons are crucial because they determine how an element reacts chemically with other substances. Elements in the same group of the periodic table have the same number of valence electrons, which explains why they often have similar chemical properties.

For example, all elements in group 1, like lithium and sodium, have one valence electron. This makes them highly reactive metals. Conversely, elements in group 18, such as helium and neon, have a full set of valence electrons, making them noble gases that are very stable and generally unreactive.

Understanding valence electrons helps us predict how elements will interact and form bonds with each other in a chemical reaction.

Atomic Size

Atomic size, or atomic radius, refers to the distance from the nucleus of an atom to the outermost electron shell. A key aspect of atomic size is how it changes across the periodic table.

As you move across a period from left to right, atomic size decreases. This occurs because as protons are added to the nucleus, the increased positive charge pulls electrons closer, even as additional electrons are being added to the same shell. This issue can lead to misconceptions, as it’s easy to assume that more electrons simply means a larger size.

However, moving down a group in the periodic table, the atomic size increases because electrons are being added to higher energy levels further from the nucleus. Thus, the effect of increased electron shell outweighs the increase in nuclear charge.

Isoelectronic Ions

Isoelectronic ions are atoms or ions that have the same number of electrons. This identical electronic structure means they share the same electron configuration, despite possibly having different numbers of protons.

A classic example is the isoelectronic species of neon (Ne), sodium ion ( ext{Na}^+), and magnesium ion ( ext{Mg}^{2+}). All these particles contain ten electrons, leading to identical electron configuration of 1s² 2s² 2p⁶.

While they possess the same electron configuration, they differ in atomic size and reactivity. This is because the number of protons affects the strength of attraction between the nucleus and the electrons, thus altering atomic characteristics.

Ionization Energy

Ionization energy is the energy needed to remove an electron from an atom in its gaseous state. This property is instrumental in understanding an element’s reactivity and bonding behavior.

Ionization energy generally increases from left to right across a period, due to the rising positive charge of the nucleus attracting electrons more strongly. Conversely, as you descend a group, ionization energy decreases.

For example, removing an electron from lithium requires more energy than from cesium. In lithium, electrons are closer to the nucleus and more tightly bound, requiring more energy to dislodge. In contrast, cesium’s valence electron is farther away from the nucleus, making it easier and less energy-intensive to remove.

• High ionization energy indicates an element is less likely to lose an electron easily, remaining relatively stable.
• Low ionization energy implies that an element can lose its outer electrons more readily, increasing its reactivity.