diff --git a/js/common/GasPropertiesConstants.ts b/js/common/GasPropertiesConstants.ts
index 8e2b1416..9b73065e 100644
--- a/js/common/GasPropertiesConstants.ts
+++ b/js/common/GasPropertiesConstants.ts
@@ -94,6 +94,10 @@ const GasPropertiesConstants = {
   HEAVY_PARTICLES_RANGE: new RangeWithValue( 0, 1000, 0 ),
   LIGHT_PARTICLES_RANGE: new RangeWithValue( 0, 1000, 0 ),
 
+  // mass
+  HEAVY_PARTICLES_MASS: 28, // AMU, equivalent to N2 (nitrogen), rounded to the closest integer
+  LIGHT_PARTICLES_MASS: 4, // AMU, equivalent to He (helium), rounded to the closest integer
+
   // radii
   HEAVY_PARTICLES_RADIUS: 125, // pm
   LIGHT_PARTICLES_RADIUS: 87.5, // pm
diff --git a/js/common/model/HeavyParticle.ts b/js/common/model/HeavyParticle.ts
index 6ef146d3..9384edb9 100644
--- a/js/common/model/HeavyParticle.ts
+++ b/js/common/model/HeavyParticle.ts
@@ -13,15 +13,12 @@ import Particle from './Particle.js';
 
 export default class HeavyParticle extends Particle {
 
-  // equivalent to N2 (nitrogen), in AMU, rounded to the closest integer
-  public static readonly MASS = 28;
-
   public constructor() {
     super( {
 
       // ParticleOptions
-      mass: HeavyParticle.MASS,
-      radius: GasPropertiesConstants.HEAVY_PARTICLES_RADIUS, // pm
+      mass: GasPropertiesConstants.HEAVY_PARTICLES_MASS,
+      radius: GasPropertiesConstants.HEAVY_PARTICLES_RADIUS,
       colorProperty: GasPropertiesColors.heavyParticleColorProperty,
       highlightColorProperty: GasPropertiesColors.heavyParticleHighlightColorProperty
     } );
diff --git a/js/common/model/LightParticle.ts b/js/common/model/LightParticle.ts
index d5215e54..fe4c546d 100644
--- a/js/common/model/LightParticle.ts
+++ b/js/common/model/LightParticle.ts
@@ -13,14 +13,12 @@ import Particle from './Particle.js';
 
 export default class LightParticle extends Particle {
 
-  public static readonly MASS = 4; // equivalent to He (helium), in AMU, rounded to the closest integer
-
   public constructor() {
     super( {
 
       // ParticleOptions
-      mass: LightParticle.MASS,
-      radius: GasPropertiesConstants.LIGHT_PARTICLES_RADIUS, // pm
+      mass: GasPropertiesConstants.LIGHT_PARTICLES_MASS,
+      radius: GasPropertiesConstants.LIGHT_PARTICLES_RADIUS,
       colorProperty: GasPropertiesColors.lightParticleColorProperty,
       highlightColorProperty: GasPropertiesColors.lightParticleHighlightColorProperty
     } );
diff --git a/js/common/model/ParticleSystem.ts b/js/common/model/ParticleSystem.ts
index ec361830..573008dd 100644
--- a/js/common/model/ParticleSystem.ts
+++ b/js/common/model/ParticleSystem.ts
@@ -85,7 +85,8 @@ export default class ParticleSystem {
       numberType: 'Integer',
       range: GasPropertiesConstants.HEAVY_PARTICLES_RANGE,
       tandem: tandem.createTandem( 'numberOfHeavyParticlesProperty' ),
-      phetioDocumentation: `Number of heavy particles (mass = ${HeavyParticle.MASS} AMU) in the container.`,
+      phetioDocumentation: 'Number of heavy particles in the container. ' +
+                           `(mass = ${GasPropertiesConstants.HEAVY_PARTICLES_MASS} AMU, radius = ${GasPropertiesConstants.HEAVY_PARTICLES_RADIUS} pm)`,
       hasListenerOrderDependencies: true // TODO: https://github.com/phetsims/gas-properties/issues/186
     } );
 
@@ -93,7 +94,8 @@ export default class ParticleSystem {
       numberType: 'Integer',
       range: GasPropertiesConstants.LIGHT_PARTICLES_RANGE,
       tandem: tandem.createTandem( 'numberOfLightParticlesProperty' ),
-      phetioDocumentation: `Number of light particles (mass = ${LightParticle.MASS} AMU) in the container.`,
+      phetioDocumentation: 'Number of light particles in the container. ' +
+                           `(mass = ${GasPropertiesConstants.LIGHT_PARTICLES_MASS} AMU, radius = ${GasPropertiesConstants.LIGHT_PARTICLES_RADIUS} pm)`,
       hasListenerOrderDependencies: true // TODO: https://github.com/phetsims/gas-properties/issues/186
     } );