He law of constant composition says that the composition of a compound is always the same that the composition of a homogeneous mixture is always the same that the composition of an element is always the same that the composition of a heterogeneous mixture is always the same that all substances have the same composition

The law of constant composition says that the composition of a homogeneous mixture is always the same

Explanation:

The law of constant composition that is also known as Proust's Law says that when two substances react with each other, to form a compound they will always do in a constant weight ratio, that is, the ratio of the masses between two substances that form a compound, must be proportional.

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.  

they it depends on if one believes about radioactivity, they make a geiger counter beep but maybe it was going to beep anyway electronics do wierd things all the time or it was programmed to beep near certain metals.