- Home
- Reference
- Education
- Directories
- Science eStore
- Job Board
- Ask The Experts
- eGreetings
- Fun
- About Us

Welcome to PhysLink.com - Your physics and astronomy online portal. Stay a while! Check out our extensive library of educational and reference materials. Also, check out our fun section!

QuestionApproximately how many atoms are in a grain of salt? Asked by: Roger Bevels (teacher) AnswerThis is mainly an estimation problem -- there is no exact measure of how big a 'grain of salt' is. So, the answer will be only correct within an order of magnitude (a factor of 10). The first step is to estimate how large a grain of salt is. I spilt some salt onto the table, and visually estimated that about three grains of salt placed end-to-end are about a millimeter long. (Your mileage may vary.) So, as a simple estimate, I'll assume that salt grains are 0.3 millimeters long. The next assumption is that salt crystals are cubes, with 0.3 millimeter sides. In fact they tend to be, since the crystal structure of NaCl is cubic. However, most have broken corners, but we will ignore that. The density of NaCl is about 2.165 gr/cm ^{3}. With the cube
assumption, we find that a grain of salt is about
5.85x10^-5 grams. (We could have arrived at this result by
weighing an individual grain, or by weighing a gram and
then counting the number of grains in it, but this is
left as an exercise for the PhysLink reader.)
The next thing we need is the weight of a 'salt atom'. There is no such thing as a salt atom, it consists of Na (sodium) and Cl (chlorine) atoms. So, we need to use an average value. The atomic mass of Na is 23 gr per mole, and the atomic mass of Cl is 35.5 gr per mole. So, the average 'atomic weight of salt' is 29.25 grams per mole. Now it is a simple matter to find how many atoms there are in a grain of salt. (Note that one mole contains Avogadro's number of atoms, which is 6.02x10 ^{23}.)
So a grain of salt contains about: 5.85x10 ^{-5} gr/ (29.25 gr / 6.02x10^{23})
= 1.2x10, half of which are sodium atoms.
(The other half is chlorine atoms, of course.)^{18} atomsAnswered by: Yasar Safkan, Ph.D. M.I.T., Software Engineer, Istanbul, Turkey |

Loading

a FREE ride with Uber!

sign up for Birchbox

(

10 Ways to Keep Your Kids Interested In Science

Young children are natural scientists: they ask questions, pick up sticks and bugs outside, and are curious about the world around them. But as they get a bit older, many kids gradually lose their interest in science. They might see it as just another task at school, something that doesn't apply to their lives. Of course nothing could be further from the truth, so here are ten ways you can remind your kids that science is everywhere. Most of these are fun for adults, too! Continue reading ...

Here are our physics & astronomy bestsellers:

Mini Plasma Ball

KonusScience 5 Way Microscope Kit

3D Magnetic Field Tube

Scorpion, Ant, Wasp and Flower Bug

Alnico Bar Magnet - 6 inch Long

Weather Station 4M Kit

Solar Radiometer

Cherry Wood Levitron

12 inch Galileo Thermometer

Revolving Multi-Color Fiberoptic Light