Question

1. choose the best answer.

what is the electron configuration for calcium?
$1s^22s^22p^6$
$1s^22s^22p^63s^23p^6$
$1s^22s^22p^63s^23p^64s^2$
$1s^22s^22p^63s^23p^64s^23d^2$

Explanation:

Step1: Find calcium's atomic number

Calcium (Ca) has an atomic number of 20, so it has 20 electrons.

Step2: Sum electrons of each option

• Option 1: $1s^22s^22p^6$ has $2+2+6=10$ electrons
• Option 2: $1s^22s^22p^63s^23p^6$ has $2+2+6+2+6=18$ electrons
• Option 3: $1s^22s^22p^63s^23p^64s^2$ has $2+2+6+2+6+2=20$ electrons
• Option 4: $1s^22s^22p^63s^23p^64s^23d^2$ has $2+2+6+2+6+2+2=22$ electrons

Step3: Match to 20 electrons

Only the third option has 20 electrons, following the Aufbau principle (4s fills before 3d).

Answer:

$\boldsymbol{1s^22s^22p^63s^23p^64s^2}$ (the third option)