Every atom is made of three types of subatomic particles: protons, neutrons, and electrons. Protons carry a positive charge and neutrons carry no charge — together they form the dense central core called the nucleus. Electrons carry a negative charge and move rapidly in the space surrounding the nucleus. In a neutral atom, the number of protons equals the number of electrons, so the charges balance out.
Use a hands-on model (even colored beads or clay) to build a few different atoms, placing protons and neutrons in the center and electrons on the outside. Comparing a hydrogen atom (1 proton, 0 neutrons, 1 electron) to a carbon atom (6 protons, 6 neutrons, 6 electrons) makes the pattern concrete.
Now that you know matter is made of atoms, the next question is: what are atoms themselves made of? It turns out that every atom contains three kinds of subatomic particles — particles smaller than an atom. These are protons, neutrons, and electrons.
At the center of every atom is a dense core called the nucleus. The nucleus contains protons, which carry a positive electrical charge (+1 each), and neutrons, which carry no charge at all. Despite being incredibly small — if an atom were the size of a football stadium, the nucleus would be about the size of a marble on the 50-yard line — the nucleus holds almost all of the atom's mass. That is because protons and neutrons are relatively heavy particles.
Surrounding the nucleus is a much larger region where electrons move. Electrons carry a negative electrical charge (-1 each) and have very little mass compared to protons and neutrons. You might see diagrams showing electrons traveling in neat rings around the nucleus, like planets orbiting the sun. In reality, electrons move so fast and unpredictably that scientists describe their location as a "cloud" of probability rather than a fixed path. What matters most right now is that electrons are on the outside of the atom, and their arrangement determines how atoms interact with each other.
In a neutral atom — one that has no overall electrical charge — the number of electrons exactly equals the number of protons. For example, a carbon atom has 6 protons in its nucleus and 6 electrons surrounding it. The 6 positive charges and 6 negative charges cancel out, leaving the atom electrically neutral. If an atom gains or loses electrons, it becomes charged and is called an ion, but that is a topic for later.
Here is a helpful comparison: imagine a basketball court. Place a bag of heavy marbles at center court — that is your nucleus, holding protons and neutrons. Now imagine a few gnats flying around somewhere in the building — those are your electrons. The gnats weigh almost nothing compared to the marbles, but they take up a huge amount of space. That size difference between the nucleus and the electron cloud is what makes atoms mostly empty space, even though we experience solid matter all around us.