1. Convert kilograms to grams:

* 1 kg = 1000 g

* 12.3 kg CO₂ = 12.3 kg * 1000 g/kg = 12300 g CO₂

2. Calculate the moles of carbon dioxide:

* Molar mass of CO₂ = 12.01 g/mol (C) + 2 * 16.00 g/mol (O) = 44.01 g/mol

* Moles of CO₂ = (mass of CO₂) / (molar mass of CO₂) = 12300 g / 44.01 g/mol = 279.5 mol CO₂

3. Determine the moles of oxygen atoms:

* There are 2 moles of oxygen atoms per 1 mole of CO₂.

* Moles of O atoms = 279.5 mol CO₂ * (2 mol O / 1 mol CO₂) = 559 mol O

4. Calculate the number of oxygen atoms:

* Avogadro's number = 6.022 x 10²³ atoms/mol

* Number of O atoms = 559 mol O * 6.022 x 10²³ atoms/mol = 3.37 x 10²⁶ atoms

Therefore, there are approximately 3.37 x 10²⁶ atoms of oxygen in 12.3 kg of carbon dioxide.