Lesson 3: Molecules in 3D

Why bonds spread out

Section 2 of 4

Every bond is a pair of electrons, and electrons are negative. Negative charges push each other away, so the bonds around an atom spread out as far apart as they can.

For four bonds, the furthest-apart arrangement points them at the corners of a tetrahedron: a pyramid with a triangle for a base. The carbon sits in the middle, not at the top, and every bond is 109.5° from each of the others. That is wider than the 90° the flat drawing suggests.

Methane's four bonds point to the corners of a tetrahedron A carbon atom in the middle with four hydrogen atoms at the corners of a tetrahedron, its edges drawn as dashed lines. The two upper bonds lie flat in the page and the angle between them is marked as 109.5 degrees. One lower hydrogen is closer to you and drawn larger; the other is further away. 109.5°
Methane's four bonds point to the corners of a tetrahedron, 109.5° apart. The dashed lines show the tetrahedron's edges; nothing joins the hydrogen atoms.

Chemists have a way to draw 3D shapes on flat paper. Plain lines lie flat in the page. A solid wedge points toward you, out of the page, and a hashed wedge points away from you, behind it. Each wedge starts narrow at the carbon and gets wider toward the atom at its end.

Methane drawn with solid and hashed wedges A carbon atom with four bonds to hydrogen. Two plain lines form a V above the carbon; they lie flat in the page, 109.5 degrees apart. Below, a solid wedge, narrow at the carbon and wide at the hydrogen, points toward you, and a hashed wedge points away from you. C H H H H toward you away from you plain lines lie in the page
Methane drawn with wedges: two bonds in the page, one coming toward you and one going away.

Quick check

Why do the bonds around a carbon atom spread out as far apart as they can?