# Which of the following statements is correct?

## The mass number is equal to the total number of protons and electrons. The number of electrons equals the number of neutrons in a neutral atom. The atomic number is equal to the mass number of an atom. The atomic number is the number of electrons in a neutral atom.

Mar 21, 2017

The correct statement is:

$\textcolor{red}{\underline{\textcolor{b l a c k}{\text{The atomic number is the number of electrons in a neutral atom.}}}}$

#### Explanation:

Examining each of the given statements:

$\textcolor{w h i t e}{}$
$\textcolor{red}{\cancel{\textcolor{b l a c k}{\text{The mass number is equal to the total number of protons and electrons.}}}}$

No: The mass of an atom mostly comes from the protons and neutrons - electrons contribute very little to the mass. So the mass number is equal to the total number of protons and neutrons.

$\textcolor{w h i t e}{}$
$\textcolor{red}{\cancel{\textcolor{b l a c k}{\text{The number of electrons equals the number of neutrons in a neutral atom.}}}}$

No: Neutrons have no charge. A neutral atom has equal numbers of electrons - which contribute negative charges - and protons - which contribute positive charge.

$\textcolor{w h i t e}{}$
$\textcolor{red}{\cancel{\textcolor{b l a c k}{\text{The atomic number is equal to the mass number of an atom.}}}}$

No: The mass number is the sum of the number of protons and neutrons, but the atomic number is just the number of protons. So this statement is only true for protium - i.e. the most common isotope of hydrogen $\text{^1"H}$ - which has no neutrons.

$\textcolor{w h i t e}{}$
$\textcolor{red}{\underline{\textcolor{b l a c k}{\text{The atomic number is the number of electrons in a neutral atom.}}}}$

Yes: In a neutral atom, the number of electrons is the same as the number of protons, which is the atomic number.

$\textcolor{w h i t e}{}$
Footnote

The type of element and its essential chemical properties are determined by the number of protons - that is the atomic number. For example, all atoms of sodium ($\text{Na}$) have $11$ protons.

Different isotopes of an element have the same number of protons, but different numbers of neutrons. For example, the only stable isotope of sodium is $\text{^23"Na}$, which has $11$ protons and $12$ neutrons, making a total mass number $23$.

The longest lived unstable isotope of sodium is $\text{^22"Na}$, with $11$ protons, $11$ neutrons and a half life of $2.6$ years.