To calculate the number of atoms in 0.5 kg of sodium, we need to use the molar mass of sodium and Avogadro's number.

1. Molar mass of sodium (Na) = 22.99 g/mol

2. Avogadro's number (Nₐ) = 6.022 × 10^23 atoms/mol

3. Convert 0.5 kg to grams:

0.5 kg = 0.5 × 1000 g = 500 g

4. Calculate the number of moles of sodium in 500 g:

Moles of Na = Mass / Molar mass

Moles of Na = 500 g / 22.99 g/mol

Moles of Na ≈ 21.76 mol

5. Calculate the number of atoms of sodium using Avogadro's number:

Number of atoms = Moles of Na × Avogadro's number

Number of atoms = 21.76 mol × 6.022 × 10^23 atoms/mol

Number of atoms ≈ 1.31 × 10^25 atoms

Therefore, 0.5 kg of sodium contains approximately 1.31 × 10^25 atoms.