Question

As the expanding universe cooled, which particles combined to form the first atoms?

Options

• Ions, plasma, and dust grains

• Molecules, compounds, and mixtures

• Nuclei, isotopes, and photons

• Neutrons, protons, and electrons

Answer 1

The cooling universe allowed the subatomic particles — neutrons, protons, and electrons — to combine and form the first atoms. These atoms were mainly hydrogen and helium; heavier molecules formed much later.