What does the chemical formula for a compound tell?

the chemical formula shows what element make up a chemical.

Explanation:

Example,

water or H₂O

is composed of two hydrogen atoms, H₂ and one oxygen, O.

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answer by yourhope:

how do models compare to real-world objects?

d. most models represent a real-world object in a simpler way.

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explanation:

let the different atoms be x and y. each with four protons and same mass will be represented as follows.

mass number is number of protons + number of neutrons.

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since, it is given that one has a negative charge while the other is neutral. therefore, there will be increase in number of protons and the atom will be represented as follows.

according to periodic table, mass number of beryllium is 9 and it has atomic number 4 so, the atoms will be and .  

the protons and neutrons will be located inside the nucleus whereas the electrons will be revolving around the nucleus.