Question

a potassium atom has 20 neutrons. what is the mass number of the atom?
answer

2
multiple answer 1 point
select the cations from the list below.
cl-
p3-
na+
cu2+
o2-
mn7+
mg
li
h+

Explanation:

Step1: Determine the atomic number of potassium

The atomic number of potassium ($K$) is $19$. This is a known value from the periodic table.

Step2: Calculate the mass number

The formula for mass number ($A$) is $A =$ number of protons ($p$)+ number of neutrons ($n$). Since the atomic number is equal to the number of protons, $p = 19$ and $n=20$. Then $A=19 + 20$.

Cations are positively - charged ions.

• $Cl^{-}$ is a negatively - charged ion (anion).
• $P^{3 - }$ is a negatively - charged ion (anion).
• $Na^{+}$ is a positively - charged ion (cation).
• $Cu^{2+}$ is a positively - charged ion (cation).
• $O^{2 - }$ is a negatively - charged ion (anion).
• $Mn^{7+}$ is a positively - charged ion (cation).
• $Mg$ is a neutral atom.
• $Li$ is a neutral atom.
• $H^{+}$ is a positively - charged ion (cation).

Answer:

$39$