# Science

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What up, I really need some help on this, I'm trying not to fail - I need to fill in blanks....( [ ] = means I filled it in )

The element called [ chromium ] has an atomic number of 24. Its symbol is [ Cr ] . When an atom of this element has a mass number of 52, the atom contains _________ protons and ___________ neutrons. The most common ion charge of this element is [ 3+ ] .

How do I know how many protons and neutrons it has? I looked at periodic table and it doesn't say how many

Much appreciated.

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The periodic table actually does say how many, it just says it in a round about way. The atomic number of any element is also the number of protons that the element has. To determine the number of nuetrons you need to know the number of protons and the atomic mass of the element. A proton and a nuetron both have an atomic mass of 1 atomic mass unit (AMU) while an electron's mass is considered negligible. Therefore the weight of all the protons and nuetrons in the element must equal it's atomic mass, so if you know how many protons you have (24) and how much they weigh (24 AMU) and you know Cr's atomic mass (52 AMU), all you have to do is some simple arithmetic to find out how many nuetrons it has.

Hope that helps.

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