Question

As we move across a period of the periodic table, why do the sizes of the transition elements change more gradually than those of the representative elements?

Short answer

The sizes of transition elements change more gradually than those of representative elements because the additional electrons in the inner d-orbital of transition elements provide poor shielding compared to the constant shielding effect in the representative elements. This leads to increased effective nuclear charge on outer electrons and a less abrupt decrease in atomic size as we move from left to right across a period in the d-block of the periodic table.

Step-by-step solution

Definition of Atomic Size

Atomic size refers to the distance between the nucleus and the outermost electrons of an element. It depends on various factors such as atomic number, screening or shielding effect, effective nuclear charge, and the number of energy levels.

Understanding Periodic Trends

In general, atomic size decreases as we move from left to right across a period of the periodic table. This is mainly due to the increase in atomic number (number of protons in the nucleus) which causes a greater effective nuclear charge attracting the electrons, pulling them closer to the nucleus.

Review Representative Elements

Representative elements or main group elements are those elements that belong to groups 1, 2, and 13-18 of the periodic table. They exhibit a more predictable trend in their properties, including atomic size, compared to transition elements. Their outermost electrons are present in the same energy level or shell, which directly affects the atomic size.

Analyze Transition Elements

Transition elements are found in the d-block of the periodic table. As we move from left to right across a period of transition elements, the additional electrons go into the inner d-orbital, which is closer to the nucleus and have a slightly poor shielding effect. Due to this poor shielding effect, the electrons in the outermost shell experience a higher effective nuclear charge that gradually pulls the outer electrons closer to the nucleus.

Comparing Transition and Representative Elements

Transition elements change more gradually in size compared to representative elements because the additional electrons in the inner d-orbital of transition elements provide poor shielding. For representative elements, the outermost electrons are in the same energy level, and the shielding effect remains constant across the period. In summary, the sizes of the transition elements change more gradually than those of the representative elements due to the effect of inner d-orbital electrons providing poorer shielding compared to the representative elements. This leads to increased effective nuclear charge on outer electrons and the atomic size decreases less abruptly as we move from left to right across a period in the d-block of the periodic table.

Key concepts

Atomic Size

Atomic size is essentially how big an atom is, measured as the distance from the nucleus to the outermost electrons. It's influenced by several factors:

• Atomic number, which is the number of protons in the nucleus.
• Shielding or screening effect, where inner electrons block outer electrons from feeling the full pull of the nucleus.
• Effective nuclear charge, the net positive charge experienced by an electron.
• The number of energy levels or shells, affecting how spread out the electrons are.

Generally, as you move across a period on the periodic table, atomic size decreases. This happens because there are more protons in the nucleus, which pull the electrons closer due to stronger positive charge.

Effective Nuclear Charge

The concept of effective nuclear charge ( Z_{eff} ) is crucial to understanding why atoms shrink as you move across a period. It refers to the actual pull that an electron experiences from the nucleus, once the shielding by inner electrons is considered.

• As you add more protons, the nucleus pulls more on the electrons.
• However, the increase in inner electrons does some blocking, reducing the full impact of the protons' positive charge on the outer electrons.
• Z_{eff} increases as you move across a period, pulling outer electrons closer and reducing atomic size.

The play between the number of protons and shielding effect determines how tightly electrons are held.

Transition Elements

Transition elements are situated in the d-block of the periodic table. Unlike representative elements, electrons are added to an inner d-orbital rather than to the outermost shell.

• These d-electrons have a poorer shielding effect.
• As a result, the outermost electrons feel a stronger pull from the nucleus, due to an increased effective nuclear charge.
• Because of this, the atomic size changes more gradually across a period.

The poor shielding means the size reduction isn't as stark, leading to the gradual change in size.

Representative Elements

Representative elements, also known as main group elements, are found in groups 1, 2, and 13-18.

• Their outer electrons are added to the same outermost shell.
• This creates a predictable trend where the shielding effect provided by inner electrons stays constant across the period.
• Because of this consistency, these elements show more noticeable changes in size when moving left to right.

The new electrons in representative elements feel a more pronounced effect of the additional protons in the nucleus, making atomic size decrease more sharply than in transition elements.