Atom:

* Definition: The basic unit of a chemical element. It's the smallest unit of matter that retains the chemical properties of that element.

* Composition:

* Nucleus: The dense, positively charged center of the atom. Contains protons and neutrons.

* Electron cloud: A region surrounding the nucleus where electrons are found. These electrons are negatively charged and orbit the nucleus.

* Size: Atoms are extremely small, measured in nanometers (1 nanometer = 1 billionth of a meter).

* Overall Charge: Atoms are generally electrically neutral, meaning they have an equal number of protons (positive) and electrons (negative).

Nucleus:

* Definition: The central part of an atom that contains protons and neutrons.

* Composition:

* Protons: Positively charged particles. The number of protons determines the element.

* Neutrons: Neutrally charged particles. The number of neutrons can vary within an element, leading to isotopes.

* Size: The nucleus is even smaller than the atom, occupying a tiny fraction of its volume.

* Charge: The nucleus is positively charged due to the presence of protons.

In simpler terms:

Imagine an atom like a tiny solar system:

* The nucleus is like the sun: It's the dense center and contains the most mass.

* Electrons are like planets: They orbit the nucleus in a cloud.

Key Differences:

* Size: The nucleus is much smaller than the atom.

* Charge: The atom is generally neutral, while the nucleus is positively charged.

* Composition: The atom includes both the nucleus and electrons, while the nucleus contains only protons and neutrons.

Let me know if you'd like any more details or examples!