What is the mass of one atom of Ca? 40 amu or (40g / N_A)

The mass of one atom of calcium (Ca) can be calculated as follows:

1. **Atomic mass in amu**:
The atomic mass of calcium (Ca) is 40 atomic mass units (amu). In other words, one atom of Ca has a mass of 40 amu.

2. **Mass in grams**:
To find the mass of one atom of calcium in grams, we use Avogadro's number (\( N_A \)), which is \( 6.022 \times 10^{23} \) atoms/mol.

The molar mass of calcium is 40 grams per mole, meaning 1 mole of calcium (which contains \( 6.022 \times 10^{23} \) atoms) weighs 40 grams.

Therefore, the mass of one atom of calcium in grams is:
\[
\text{Mass of one Ca atom} = \frac{40 \, \text{g/mol}}{N_A} = \frac{40 \, \text{g}}{6.022 \times 10^{23}}
\]

Now, calculating this value:
\[
\text{Mass of one Ca atom} = 6.64 \times 10^{-23} \, \text{g}
\]

### Conclusion:
The mass of one atom of calcium is:
- 40 amu (in atomic mass units), or
- \( 6.64 \times 10^{-23} \, \text{g} \)