Problem 24 - An Introduction to Physical Science

Question

How does the size of atoms change (a) across a period and (b) down a group?

Step-by-Step Solution

Answer

Atomic size decreases across a period but increases down a group.

Step 1: Understanding Atomic Size

Atomic size refers to the distance between the nucleus of an atom and its outermost electron shell. It can also be perceived as the size of the 'cloud' where the electrons are found.

Step 2: Analyzing Atomic Size Across a Period

When examining elements across a period (from left to right on the periodic table), the atomic size generally decreases. This occurs because, as we move across a period, protons are added to the nucleus, increasing the positive charge. Although electrons are also added, they enter the same energy level (or shell). The increased positive charge pulls the electrons closer to the nucleus, resulting in a smaller atomic size.

Step 3: Analyzing Atomic Size Down a Group

When moving down a group (from top to bottom on the periodic table), the atomic size increases. As new periods are added, new electron shells are populated, despite the increase in nuclear charge. The shield effect, where inner electrons shield the outer electrons from the nucleus, reduces the nucleus' pull, allowing the outer shell to spread out more, hence increasing atomic size.

Key Concepts

Periodic Table

The periodic table is a tabular arrangement of chemical elements, organized on the basis of atomic number. Elements are presented in rows called periods and columns known as groups. This layout is designed to reflect recurring trends or periodicity in the properties of the elements.
The periodic table helps predict the electronic configuration of atoms, as well as their chemical behavior.
When you move from left to right across a period, the properties of elements change systematically. Elements gain protons and electrons across a period, affecting atomic size. Conversely, moving down a group displays parallel properties more predictably, although atomic size increases due to added electron shells.

Electron Shell

Electron shells, also known as energy levels, surround an atom's nucleus and house electrons. Electrons in these shells are negatively charged particles that orbit the nucleus, containing positively charged protons. The arrangement of electrons within these shells is crucial for determining how atoms interact with each other.
Each shell can hold a specific number of electrons. For example, the first shell can hold up to 2 electrons, the second up to 8, and so on. As you progress to higher shells, the capacity increases following the formula 2n², where n is the shell number.

• First Shell: 2 electrons
• Second Shell: 8 electrons
• Third Shell: 18 electrons

Within these shells, electrons fill in specific sub-levels or orbitals (s, p, d, f), which further affect how electrons interact and bind in chemical reactions. The outermost shell, known as the valence shell, primarily determines an element's chemical properties.

Atomic Structure

Atomic structure refers to the organization of protons, neutrons, and electrons within an atom. Electrons orbit the nucleus in various energy levels, whereas the nucleus itself contains protons and neutrons.
Protons carry a positive charge, neutrons are neutral, and electrons possess a negative charge. The number of protons, or the atomic number, defines the element and its position on the periodic table.

• Nucleus: Contains protons and neutrons
• Electrons: Orbit in shells around the nucleus
• Atomic Number: Number of protons in the nucleus

The balance between protons and electrons determines the neutral charge of an atom, unless it gains or loses electrons, becoming an ion. These fundamental particles and their interactions dictate the chemical and physical properties of elements.

Nuclear Charge

Nuclear charge is the total charge of an atom's nucleus, calculated as the number of protons. Since protons are positively charged, the nuclear charge directly influences electron behavior in an atom.
As you move across a period in the periodic table, the nuclear charge increases. This happens because each succeeding element has an additional proton in the nucleus, enhancing the nucleus' pull on electrons.
Although electrons are added to the same energy level, the stronger nuclear charge pulls them closer, reducing atomic size. Meanwhile, when moving down a group, although nuclear charge increases with more protons, the added electron shells overshadow its effect. This results in larger atomic sizes due to the increased distance created by additional electron shells. The inner-shell electrons shield the outer electrons from the full force of the nuclear charge, allowing the atom to expand.