Question

State whether the following statements are true or false. In a period, the first element has the smallest size.

Short answer

Answer: False. The atomic size decreases across a period from left to right due to an increase in the effective nuclear charge, making the last element in the period have the smallest size.

Step-by-step solution

Understand the structure of the periodic table

The periodic table is organized into rows called periods and columns called groups. Elements within a period have increasing atomic numbers, which affect their size and other properties.

Analyze the trend of atomic size in a period

As we move from left to right across a period, the number of protons in the nucleus increases, which causes an increase in the positive charge. This leads to a stronger attraction between the positively charged nucleus and the negatively charged electrons. At the same time, electrons are added to the same energy level (the same principal quantum number), so the shielding effect remains constant. This means that the effective nuclear charge (which is the net positive charge experienced by an electron) increases across a period, making the electron cloud more tightly held by the nucleus.

Determine whether the statement is true or false

Based on the analysis in Step 2, we can conclude that atomic size decreases across a period from left to right due to an increase in the effective nuclear charge. Therefore, the statement "In a period, the first element has the smallest size" is False. The atomic size decreases across a period, making the last element in the period have the smallest size.

Key concepts

Atomic Size

Atomic size, also known as atomic radius, refers to the distance from the nucleus of an atom to the outermost shell of electrons. As we move across a period in the periodic table from left to right, atomic size decreases. This is because, with each successive element, the number of protons in the nucleus increases.

• More protons mean a greater positive charge in the nucleus.
• This increased positive charge pulls electrons closer to the nucleus.
• The size of the electron cloud becomes smaller.

Interestingly, electrons added across a period occupy the same principal energy level. Despite more electrons, their added repulsion does not overcome the increased nuclear charge. This subtle interplay leads to a smaller atomic size as we progress across the period. Hence, in any given period, the element furthest to the right tends to have the smallest atomic size.

Effective Nuclear Charge

Effective nuclear charge (\(Z_{ ext{eff}}\)) is a concept that helps explain the pull or attraction that outer electrons feel from the nucleus. It's not just the simple number of protons, but rather the net positive charge experienced by an electron in the outermost shell. This is because the inner electrons shield or reduce the full charge of the nucleus felt by the outer electrons.Here’s how this plays out in a period:

• While moving across a period, the actual nuclear charge increases because of the additional protons.
• However, the shielding effect by inner electrons remains relatively constant, as added electrons are mostly in the same shell.
• Consequently, \(Z_{ ext{eff}}\) increases, causing a stronger attraction on outer electrons, pulling them closer.

A higher effective nuclear charge smaller atom size, making it an important factor in understanding periodic trends.

Periodicity in Chemistry

Periodicity in chemistry refers to the repeating patterns in properties of elements as arranged in the periodic table. These patterns are driven by the organization of elements in periods and groups reflecting their atomic structure. One such pattern is the decrease in atomic size across a period, but there are other relevant periodic trends too:

• Electronegativity increases across a period.
• Ionization energy tends to be higher for elements further to the right.
• Elements toward the left have metallic character, while non-metallic character increases across a period.

These periodic trends in the periodic table are vital for predicting and understanding chemical behaviors. The interplay between atomic structure and periodic trends defines the reactivity and properties of elements, guiding chemists in practical applications and theoretical explorations.