ALEKS: Writing Lewis structures for a molecule with one central atom and no octet rule exceptions

Summary

This video tutorial guides viewers through solving an ALEKS problem on writing Lewis structures for molecules with a single central atom, including cases without octet rule exceptions. It explains step-by-step how to count valence electrons based on periodic table positions, identify the central atom (noting that hydrogen cannot be central), and arrange surrounding atoms with single bonds. The tutorial demonstrates distributing remaining electrons as lone pairs on the central atom to satisfy the octet rule and emphasizes checking that all atoms have the correct number of electrons. Practical tips help avoid common mistakes and ensure hydrogen and sulfur atoms are 'happy' in the final structure.

Takeaways

• 😀 Always start by counting the total number of valence electrons for all atoms in the molecule.
• 😀 Valence electrons are determined by an atom's column in the periodic table, with hydrogen being an exception.
• 😀 Hydrogen has 1 valence electron and typically forms only one bond.
• 😀 The first atom in a molecular formula (except hydrogen) is usually the central atom.
• 😀 Hydrogen cannot be the central atom and should be placed as a terminal atom.
• 😀 Surround the central atom with all other atoms and connect them using single bonds.
• 😀 Each single bond uses 2 valence electrons, which must be subtracted from the total count.
• 😀 After bonding, remaining electrons are placed as lone pairs on the central atom to complete its octet.
• 😀 Always check that each atom satisfies its preferred electron configuration (hydrogen: 2 electrons, other atoms: 8 electrons if applicable).
• 😀 Avoid trying to create non-standard shapes; atoms naturally arrange around the central atom like spokes on a wheel.

Q & A

What is the first step in drawing a Lewis structure according to the video?

The first step is to count the total number of valence electrons for all the atoms in the molecule based on their position in the periodic table.

How many valence electrons does hydrogen have?

Hydrogen has one valence electron.

How many valence electrons does sulfur have?

Sulfur has six valence electrons because it is in the sixth column of the periodic table.

What is the total number of valence electrons for a molecule with two hydrogens and one sulfur?

The total number of valence electrons is eight—two from hydrogen atoms and six from sulfur.

How do you determine the central atom in a molecule?

The central atom is usually the first atom listed in the molecular formula, except when the first atom is hydrogen, because hydrogen can never be a central atom.

How are the outer atoms connected to the central atom in a Lewis structure?

The outer atoms are connected to the central atom with single bonds, and each bond contains two electrons.

After forming single bonds with outer atoms, what should you do with the remaining electrons?

The remaining electrons should be placed on the central atom as lone pairs to complete its octet.

How do you verify that the central atom satisfies the octet rule?

Count all electrons around the central atom, including shared electrons in bonds and lone pairs, to ensure it totals eight.

Do hydrogen atoms follow the octet rule?

No, hydrogen atoms do not follow the octet rule; they only need two electrons to be stable.

What common mistake do students make when arranging atoms in a Lewis structure?

A common mistake is trying to create unconventional shapes, such as triangles, instead of placing the central atom in the middle with other atoms arranged around it like spokes on a wheel.

How many electrons are used when connecting two hydrogen atoms to a sulfur atom with single bonds?

Four electrons are used—two electrons for each single bond connecting hydrogen to sulfur.

Why is it important to place remaining electrons as lone pairs on the central atom?

Placing remaining electrons as lone pairs ensures the central atom achieves a stable octet, making the molecule chemically stable.