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Lewis Structure of Group 5A Central Atoms

General Chemistry • Chemical Bonds

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Group 15 (5A) Central Atoms – Trigonal Pyramidal Lewis Structures

Choose a molecule with a Group 15 central atom and inspect how 3 bonding pairs and 1 lone pair produce the classic AX3E trigonal pyramidal pattern.

The calculator counts valence electrons, shows the step-by-step electron placement, and draws interactive diagrams with hover details, zoom, and drag-to-pan support.

Group 15 central atoms usually contribute 5 valence electrons. In these examples, the central atom forms 3 single bonds and keeps 1 lone pair, so the electron-domain geometry is tetrahedral while the molecular geometry is trigonal pyramidal.

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Frequently Asked Questions

How many valence electrons do Group 15 (5A) central atoms have?

Group 15 elements such as N, P, As, Sb, and Bi have 5 valence electrons in a neutral atom. In many EX3 molecules, three electrons are used to form three single bonds and the remaining two electrons stay as one lone pair.

Why are NH3 and PCl3 trigonal pyramidal?

The central Group 15 atom has four electron domains: three bonding pairs and one lone pair (AX3E). The electron-domain geometry is tetrahedral, but the molecular shape is trigonal pyramidal because only three atoms are bonded to the center.

How do you calculate total valence electrons for an EX3 molecule like NF3 or PH3?

Add the valence electrons from the central atom (5 for a Group 15 element) and from the three outer atoms. For example, NF3 has 5 + 3x7 = 26 total valence electrons, while PH3 has 5 + 3x1 = 8.

What does AX3E mean in VSEPR notation?

A is the central atom, X is the number of atoms bonded to it, and E represents a lone pair on the central atom. AX3E means three bonded atoms and one lone pair, which predicts a trigonal pyramidal molecular shape.

What is the difference between electron-domain geometry and molecular shape for AX3E?

Electron-domain geometry counts both bonds and lone pairs, so AX3E has a tetrahedral electron-domain arrangement. Molecular shape ignores lone pairs, so with only three bonded atoms the shape is trigonal pyramidal.

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