Question

which is a characteristic of the part of the atom marked \a\?
it contains most of the mass.
it is negatively charged.
it is mostly empty space.
it is composed of electrons.

Explanation:

Step1: Identify Part A

Part A is the nucleus (protons + neutrons) of the atom.

Step2: Analyze Options

• "It contains most of the mass": Nucleus has protons (mass ≈ 1u) and neutrons (mass ≈ 1u), electrons (mass ≈ 0.0005u) contribute little. So nucleus holds most mass.
• "It is negatively charged": Nucleus has protons (+) and neutrons (0), so positive, not negative. Eliminate.
• "It is mostly empty space": The electron cloud (Part B) is mostly empty. Nucleus is dense. Eliminate.
• "It is composed of electrons": Electrons are in the cloud (Part B). Nucleus has protons/neutrons. Eliminate.

Answer:

It contains most of the mass.