Question

Draw Lewis symbols showing the valence electrons of the following atoms. (a) \(\mathrm{C}\) (c) Se (e) \(\mathrm{Cs}\) (b) I (d) \(\mathrm{Sr}\) (f) \(\mathrm{Ar}\)

Short answer

The Lewis symbols for the atoms are as follows: \(\mathrm{C}\) has 4 dots, Se has 6 dots, \(\mathrm{Cs}\) has 1 dot, I has 7 dots, \(\mathrm{Sr}\) has 2 dots, and \(\mathrm{Ar}\) has 8 dots surrounding the respective atomic symbols.

Step-by-step solution

Determine the number of valence electrons

Using the periodic table, determine the group number of each atom. Atoms in groups 1 (e.g. Cs) and 2 (e.g. Sr) have valence electrons equal to their group number. Atoms in groups 13-18 have valence electrons equal to their group number minus 10. C has 4, Se has 6, Cs has 1, I has 7, Sr has 2, and Ar has 8.

Draw the Lewis symbols

Draw a symbol for each atom, then draw dots for each valence electron. Distribute the electrons around the symbol. One dot is drawn on each side of the atomic symbol for each atom until each side has one dot (up, down, right, left). After that, if there are still more electrons, a second dot is added to each side. Continue this process until all the valence electrons have been accounted for.

Checking the Results

Check that the Lewis symbols correctly represent the number of valence electrons for each atom. C has 4 dots, Se has 6 dots, Cs has 1 dot, I has 7 dots, Sr has 2 dots, and Ar has 8 dots. A complete outer shell is represented by a circle of dots, and Argon has a complete shell.

Key concepts

Valence Electrons

Valence electrons are the electrons that reside in the outermost shell of an atom. These are the electrons primarily involved in chemical bonding and reactions. Understanding valence electrons helps us predict an atom's chemical properties and its reactivity.
To find the number of valence electrons in an atom, we look at its position in the periodic table:

• For elements in groups 1 and 2, the number of valence electrons is equal to the group number. For example, cesium (Cs) in group 1 has 1 valence electron.
• For elements in groups 13-18, the number of valence electrons is equal to the group number minus 10. For example, iodine (I) in group 17 has 7 valence electrons.

Valence electrons can also be represented visually through Lewis symbols, where the element symbol is surrounded by dots denoting valence electrons.

Periodic Table Groups

The periodic table is divided into vertical columns known as groups. Each group contains elements with similar chemical properties. This is because the elements in a group have the same number of valence electrons, which largely determine their behavior in reactions.
The groups are numbered from 1 to 18, and they provide essential information about the electronic configuration of the atoms within them:

• Group 1 elements, like lithium and sodium, have 1 valence electron.
• Group 2 includes elements like magnesium and strontium, with 2 valence electrons.
• Groups 13-18 are often referred to in terms of their number of valence electrons, such as group 17, which includes elements like chlorine and fluorine, having 7 valence electrons.

Understanding which group an element belongs to helps in predicting its chemical behavior and the types of bonds it can form.

Electron Distribution

Electron distribution refers to how electrons are organized around the nucleus of an atom into different energy levels known as shells. Each shell can hold a specific number of electrons, and these are filled in a particular order:

• The first shell can hold up to 2 electrons.
• The second shell can hold up to 8 electrons.
• The third shell can hold up to 18 electrons, but often fills up to 8 for lighter elements.

Lewis symbols provide a simple way to represent valence electron distribution. Each dot represents a valence electron, placed around the symbol of the element in a systematic way. This helps visualize pairing arrangements and potential for forming covalent bonds.
Consistent electron distribution ensures chemical stability, with most atoms aiming to reach a stable configuration similar to the nearest noble gas.

Complete Outer Shell

A complete outer shell, or valence shell, means an atom has the maximum number of electrons that can be accommodated in its outermost electron shell. For most elements, this means having 8 valence electrons, which is often referred to as the octet rule.
Noble gases like helium, neon, and argon naturally have complete outer shells, making them very stable and mostly unreactive under normal conditions. For helium, a complete shell is 2 electrons, while for others, it's 8 electrons.
This concept is crucial in understanding why atoms form ions or covalent bonds. Atoms will often gain, lose, or share electrons to achieve a complete outer shell and energy stability. This drive influences the formation of molecules, ionic compounds, and network structures, ultimately underpinning much of chemistry's foundational principles.