Problem 110 - Chemistry: An Atoms-Focused Approach

Question

Why does atomic size tend to decrease with increasing atomic number across a row of the periodic table?

Step-by-Step Solution

Answer

Answer: As we move across a row in the periodic table, the atomic size tends to decrease. This trend is primarily due to the increase in atomic number, which leads to a greater effective nuclear charge experienced by the electrons. The increased nuclear charge attracts the outermost electrons more strongly, pulling them closer to the nucleus and resulting in a smaller atomic size. The shielding effect of inner core electrons remains relatively constant across a row, causing the effective nuclear charge to be the main factor affecting atomic size.

Step 1: Understanding atomic size

Atomic size refers to the distance from the center of the nucleus to the outermost shell of electrons in an atom. It is determined by the number of energy levels or electron shells, and the effective nuclear charge (attraction between the nucleus and electrons) experienced by the electrons.

Step 2: Understanding atomic number and electron configuration

Atomic number is the number of protons in an atom, which determines its position in the periodic table. It also determines the number of electrons in a neutral atom. As we move across a row of the periodic table, the atomic number increases. The electron configuration determines how electrons are distributed in various electron shells around the nucleus. Typically, the electrons fill lower-energy shells or orbitals first before occupying higher-energy ones.

Step 3: Examining the periodic table and trends

The periodic table displays elements in increasing order of atomic number from left to right and top to bottom. Each row (period) represents a new energy level where electrons are located in the same principal shell. As we move from left to right in a period, the number of protons and electrons both increase, while the energy level of the outermost electrons generally remains the same.

Step 4: Understanding effective nuclear charge

Effective nuclear charge is the net positive charge experienced by the outermost electrons in an atom. It is influenced by the actual nuclear charge (number of protons) and the shielding effect of the inner core electrons, which partially block the attractive force between the nucleus and the outermost electrons. The shielding effect remains relatively constant across a row in the periodic table, while the nuclear charge increases from left to right.

Step 5: Relating effective nuclear charge to atomic size

As we move across a row of the periodic table, the increased nuclear charge exerts a stronger attractive force on the electrons, drawing them closer to the nucleus. With the shielding effect remaining relatively constant and the atomic number increasing, the effective nuclear charge becomes greater. As a result, the outermost electrons are pulled closer to the nucleus, and the atomic size tends to decrease.