How can two atoms have the same mass but different charges?

The answer is: two atoms have same number of protons and neutrons, but different number of electrons.

Mass number (A) is the total number of protons and neutrons in a nucleus.

Atomic mass (ma) is approximately equivalent to the number of protons and neutrons in the atom.

For example, chlorine atom (Cl) and chlorine anion (Cl⁻).

Similarities:

1) atomic number is the same. Both, chlorine atom and anion, have atomic number 17. They have 17 protons in nucleus of an atom.

2) both are the same element.

Difference:  

1) chlorine atom (Cl) has neutral change and chlorine anion (chloride ion Cl⁻) has negative charge, because chlorine anion has one electron more.

answer:

the answer is 6 valence electrons.

explanation:

valence is the # of electrons on the outer most shell of an atom.

oxygen has 6.

although, the valency of oxygen is 2, because it only needs to make 2 covalent bonds, or gain 2 electrons, to achieve the stable octet configuration of a noble gas.

that's it!

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