What Is the Lewis Dot Structure for PH3? Easy Steps Explained

The Lewis dot structure for PH3 (phosphine) shows a central phosphorus atom bonded to three hydrogen atoms, with one lone pair of electrons on the phosphorus. To draw it, count 8 total valence electrons (5 from phosphorus + 1 from each of three hydrogens), place P in the center, connect the three H atoms with single bonds, and put the remaining 2 electrons as a lone pair on the phosphorus. The result is a trigonal pyramidal molecule, and every atom satisfies the octet rule (H with 2 electrons, P with 8).

Phosphine is a simple molecule, but it’s a good test case because it breaks the flat, symmetric pattern many people expect from a three-bonded central atom. If you’re asking what is the lewis dot structure for PH3, the short answer is above — the rest of this guide walks through each step so you can reproduce it on your own.

Lewis Structure Basics

A Lewis dot structure is a simple two-dimensional diagram that shows how valence electrons are arranged in a molecule. Valence electrons are the outermost electrons of an atom — the ones that participate in bonding. Dots represent individual electrons, and lines represent shared pairs (bonds).

For PH3, the diagram tells you three things at once: which atoms bond to which, how many lone pairs exist, and whether the molecule is stable. The key feature is that phosphorus keeps a lone pair, which is what makes the molecule pyramidal rather than flat.

If you want more practice with the underlying electron-counting rules, Khan Academy has free lessons on Lewis diagrams and bonding that pair well with this walkthrough.

What “octet rule” means

The octet rule says most atoms are most stable when surrounded by eight valence electrons. Hydrogen is the main exception — it’s stable with just two, because its first shell only holds two electrons. Keep both numbers in mind as you build PH3.

Count the Valence Electrons

Before drawing anything, add up every valence electron in the molecule. You can read the count straight off the periodic table for main-group elements.

For PH3:

• Phosphorus (P): group 15, so it has 5 valence electrons.
• Hydrogen (H): group 1, so each H has 1 valence electron. Three hydrogens give 3.

Total: 5 + 3 = 8 valence electrons.

That’s the entire electron budget. Every bond and every dot in the final structure must come from these 8 electrons — no more, no less. Write the total at the top of your page before you start.

Draw the PH3 Structure

Now place the atoms and distribute those 8 electrons. Work through it in order:

Step 1: Put phosphorus in the center

Phosphorus is the least electronegative atom here, so it goes in the middle. Electronegativity is simply an atom’s pull on shared electrons — the central atom is usually the one with the weaker pull. Hydrogen can only form one bond, so it always sits on the outside.

Step 2: Bond the three hydrogens to P

Draw a single line from P to each H. Each line counts as 2 electrons, so three bonds use 3 × 2 = 6 electrons.

You’ve now used 6 of your 8. Two remain.

Step 3: Add the lone pair

Place the last 2 electrons as a pair of dots on the phosphorus. This is phosphorus’s lone pair — electrons that are not shared with any other atom.

Your finished structure: P in the center, three single bonds to H, and two dots (one lone pair) on P. That’s the complete Lewis dot structure for PH3.

Quick worked check

Bonds: 3 × 2 = 6 electrons. Lone pair: 2 electrons. Total = 8. The count matches your valence electron budget exactly, which confirms the structure is correct.

Check Octet and Shape

Verify each atom one at a time.

• Each hydrogen: 2 electrons in its single bond. That satisfies hydrogen’s two-electron rule.
• Phosphorus: 6 electrons from the three bonds plus 2 in the lone pair = 8 electrons. Octet satisfied.

So every atom in PH3 is stable as drawn. No double bonds are needed, and you have no leftover electrons.

Why PH3 is trigonal pyramidal

The shape comes from the lone pair, not just the bonds. Phosphorus has four electron domains around it — three bonding pairs and one lone pair. Those four domains spread out to minimize repulsion, which gives a tetrahedral electron arrangement.

But when you describe the molecular shape, you look only at the atoms. The lone pair pushes the three P–H bonds away from it, so the hydrogens form a pyramid with phosphorus at the apex. That’s a trigonal pyramidal geometry.

A useful comparison: PH3 looks like ammonia (NH3), which is also trigonal pyramidal for the same reason. It does not look like BF3, which has no lone pair and is flat and trigonal planar. That single lone pair is the difference.

Bond angles in PH3 are approximately 93°, noticeably smaller than the 109.5° of a perfect tetrahedron — the lone pair squeezes the H–P–H angles inward. The Royal Society of Chemistry maintains periodic table data you can check for phosphorus’s electron configuration and group, which is the source of that 5-electron count.

Key Takeaways

• PH3 has 8 total valence electrons: 5 from phosphorus plus 1 from each of three hydrogens.
• The phosphorus atom sits in the center with three single bonds to hydrogen and one lone pair.
• Every atom satisfies the octet rule: H has 2 electrons, P has 8.
• Three bonding pairs plus one lone pair give PH3 a trigonal pyramidal shape, with bond angles near 93°.
• The lone pair — not the bonds — is what makes the molecule pyramidal instead of flat.
• To verify any Lewis structure, confirm that bonding electrons plus lone-pair electrons equal your total valence count.