The molar mass of potassium (K) is 39.0983 g/mol. This means that 0.551 g of potassium is:

$$0.551 \text{ g} \times \frac{1 \text{ mol}}{39.0983 \text{ g}} = 0.0141 \text{ mol}$$

One mole of any substance contains 6.02214076 x 10^23 atoms (Avogadro's number). Therefore, 0.0141 mol of potassium contains:

$$0.0141 \text{ mol} \times 6.02214076 \times 10^{23} \text{ atoms/mol} = 8.50 \times 10^{21} \text{ atoms}$$

So, there are approximately 8.50 x 10^21 atoms in 0.551 g of potassium.