Understanding atomic structure begins with separating what is in the nucleus from what is outside it. AP Physics 2 focuses on the atom’s central nucleus, its subatomic particles, and how nuclei are written symbolically.

Basic picture of the atom

An atom has a central nucleus and a surrounding region containing electrons.

Stylized atom diagram showing electrons in the surrounding region and a compact central nucleus containing protons and neutrons. Use it to visually anchor the key structural idea that electrons are outside the nucleus, while essentially all positive charge is concentrated at the center. Source

The nucleus is extremely small compared with the full size of the atom, but it contains the positively charged part of the atom. Electrons are found outside the nucleus, so the atom is not a single solid object with charge spread evenly everywhere.

The nucleus is positive because it contains protons, and each proton has positive charge. The nucleus also contains neutrons, which have no electric charge.

Schematic nucleus diagram showing protons (red) and neutrons (blue) packed together in the nuclear core. The color-coding makes it easy to connect “protons give positive charge” and “neutrons change mass/composition but not charge.” Source

Because neutrons are neutral, they do not change the sign of the nucleus. Together, protons and neutrons make up the nuclear core of the atom, while electrons remain outside that core.

A useful distinction is the difference between the atom and the nucleus. The atom includes everything: nucleus plus electrons. The nucleus is only the inner part. This matters because some questions ask about the whole atom, while others ask only about the nucleus. Because the nucleus is so compact, the atom’s positive charge is concentrated at the center rather than spread throughout the electron region.

The three subatomic particles

The basic subatomic particles relevant here are:

• Protons: found in the nucleus and carry positive charge

• Neutrons: found in the nucleus and carry no charge

• Electrons: found outside the nucleus and carry negative charge

In a neutral atom, the total positive charge from the protons is balanced by the total negative charge from the electrons. That means the number of electrons equals the number of protons for a neutral atom. The neutron count does not affect the atom’s net charge, because neutrons have zero charge.

When describing atomic structure, keep the location of each particle clear. Protons and neutrons are grouped together in the center, while electrons occupy the space around them. If you confuse the particle locations, it becomes harder to interpret nuclear notation correctly.

Why nuclear notation is useful

Physicists and chemists use a compact symbol to show the composition of a nucleus. This notation tells you how many protons and neutrons are present without writing a long sentence each time.

In this notation, the lower number $Z$ is the proton number. It tells you how many protons are in the nucleus. The upper number $A$ is the mass number. It tells you the total number of particles in the nucleus that are either protons or neutrons. Since electrons are outside the nucleus, they are not included in $A$. The symbol $X$ is the standard chemical symbol written with those numbers.

The neutron number is not usually written directly, but it can always be found from the other two numbers.

Diagram set of several isotope nuclei with protons labeled $p$ and neutrons labeled $n$, highlighting that isotopes share the same $Z$ (protons) but differ in $N$ (neutrons). This visual makes the relationship $A=Z+N$ feel concrete: changing neutron count changes mass number without changing the element. Source

Once you know the total number of particles in the nucleus and the number of protons, the remaining particles must be neutrons. Notice that the lower number is especially important because it determines the positive charge of the nucleus.

When you read nuclear notation, move carefully. First identify the element symbol $X$. Then read the lower number for protons and the upper number for the total number of particles in the nucleus. Finally, if needed, subtract to find neutrons. This process helps avoid one of the most common mistakes: treating the mass number as the neutron number. The mass number includes both protons and neutrons.

Interpreting the notation correctly

Nuclear notation describes the nucleus first. If the atom is neutral, the proton number also tells you the electron number. However, the notation itself is mainly focused on the nucleus, so the key quantities shown directly are the proton number and the mass number.

Another important point is that the nucleus is positive overall, not because it contains only positive particles, but because the positive charge of the protons is not canceled inside the nucleus by the neutrons. Neutrons add to the nuclear composition without changing the nuclear charge.

Frequent notation mistakes

• Thinking the nucleus contains electrons

• Treating $A$ as the neutron number instead of the total of protons and neutrons

• Forgetting that neutrons affect the nuclear composition but not the nuclear charge

• Reading the upper and lower numbers in the wrong roles

• Forgetting that a neutral atom has the same number of electrons as protons