chemists also count atoms when naming a sigmatropic shift where a hydrogen atom moves. the carbon chain does not break in a hydrogen atom migration. so, chemists count across all atoms involved in the reaction rather than only across the closest atoms. for example, the following hydrogen atom migration is of order [1,5], attained by counting counterclockwise through the π system, rather than the [1,3] order through the ring ch2 group that would mistakenly result if counted clockwise.