Invisible neutrons can contribute to isotope mass

[phet-steele]

On the Make Isotopes screen, use one finger to grab a neutron from the bucket (don't release). With another finger, change the element. The original finger can now place an invisible neutron in the nucleus:

Continuing from here, manipulating the neutrons (adding or subtracting them) can lead to numerous issues like

• Nucleus shaking when the atom is shown as stable
• Neutrons shaking while in the bucket
• Multiple neutrons flying into the nucleus when only one was dragged in
• Neutrons floating in random places on the screen without returning to the bucket or nucleus
• Not being able to return all neutrons to the bucket

Tested on 1.0.0-dev.5 with iPad 2 iOS 9.2 & 9.3 beta (Witten & Tycho). For phetsims/tasks/issues/488.

[phet-steele]

Troubleshooting information (do not edit):
Name: ‪Isotopes And Atomic Mass‬
URL: http://www.colorado.edu/physics/phet/dev/html/isotopes-and-atomic-mass/1.0.0-dev.5/isotopes-and-atomic-mass_en.html
Version: 1.0.0-dev.5 2016-01-26 23:09:01 UTC
Features missing: fullscreen
User Agent: Mozilla/5.0 (iPad; CPU OS 9_2 like Mac OS X) AppleWebKit/601.1.46 (KHTML, like Gecko) Version/9.0 Mobile/13C75 Safari/601.1
Language: en-us
Window: 1024x671
Pixel Ratio: 1/1
WebGL: WebGL 1.0 (OpenGL ES 2.0 IMGSGX543-124.1)
GLSL: WebGL GLSL ES 1.0 (OpenGL ES GLSL ES 1.00)
Vendor: WebKit (WebKit WebGL)
Vertex: attribs: 16 varying: 8 uniform: 128
Texture: size: 4096 imageUnits: 8 (vertex: 8, combined: 8)
Max viewport: 4096x4096
OES_texture_float: true
Dependencies JSON: {"assert":{"sha":"d4909677","branch":"master"},"axon":{"sha":"5a39ec57","branch":"master"},"babel":{"sha":"db982666","branch":"master"},"brand":{"sha":"8c6f9c04","branch":"master"},"chipper":{"sha":"57c99807","branch":"master"},"dot":{"sha":"ddf82014","branch":"master"},"isotopes-and-atomic-mass":{"sha":"3d0c3bb9","branch":"master"},"joist":{"sha":"4c228a32","branch":"master"},"kite":{"sha":"4a947119","branch":"master"},"phet-core":{"sha":"874a703d","branch":"master"},"phetcommon":{"sha":"3db92af3","branch":"master"},"scenery":{"sha":"953fe860","branch":"master"},"scenery-phet":{"sha":"e903fad2","branch":"master"},"sherpa":{"sha":"bf82eb95","branch":"master"},"shred":{"sha":"c1d4863f","branch":"master"},"sun":{"sha":"45c7d334","branch":"master"},"tandem":{"sha":"5388a9e4","branch":"master"}}

[aadish]

currently not handling multitouch issues