The number of electrons in an atom depends on its neutrality, cation or anion.
- All atoms of elements found in the periodic table have neutral atoms.
- A neutral atom always has the same number of protons as the number electrons.
- Number of protons in an Atom = Atomic number for the atom
- Also, the number protons can be used to determine the number electrons.
- Number of protons = Number electrons
- Ex: Sodium (Z = 11)
So, 11 = Number of protons + Number of electrons
Cation (positively charged ion)
- When a neutral atom loses an electron, it forms cations. The number of lost electrons determines the amount of positive charge.
- If the ion’s value is positive, it means that there are more protons than electrons.
- Add the number positive charges to the atomic number to find the number electrons.
- Ex: Na+ ion forms from Sodium (Z =11) when it loses an electron.
The charge for the Na + =+1 is shown here
Number of electrons in Na+ Ion = Z – number positive charges
= 11 – 1 =10 electrons
Ex: Manganese (Z = 25) loses 2 electrons and forms the Mg+2 Ion.
The charge for Mg+2 is +2
Therefore, the number of electrons in Mg+2 Ion = Z – 2.
= 25 – 2 = 23 Electrons
Anion (a negatively charged ion)
- An electron is created when it is added to a neutral atom. The number of electrons that have been added will determine the number of negative charges.
- An anion can be formed if the number of electrons in the ion exceeds that of protons.
- To find the number electrons in an anion add the number negative charges to the atomic numbers.
- Ex: Fluorine (Z = 9) gains an electron, and forms F-ion (Z = 9)
The charge on the F ion is =-1
Thus, the total number of electrons within F-ion = Z + total positive charges
= 9 + 1 = 10 electrons
Ex: Oxygen (Z = 8) gains 2 electrons and forms O-2.
The total O-2 charges are shown below = -2
Thus, the total number of electrons in O-2 = Z+ 2
= 8 + 2 = 10 electrons
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