AP Syllabus focus: 'Each element has a unique proton number; isotopes depend on protons and neutrons, ions have nonzero charge, and nuclear mass dominates atomic mass.'
Understanding how proton number, neutron number, and electron number define atoms is essential for identifying elements, distinguishing isotopes, describing ions, and explaining why almost all atomic mass is concentrated in the nucleus.
Element Identity
An element is identified by its number of protons, not by its neutrons or electrons. If two atoms have the same proton count, they are atoms of the same element, even if other particle counts differ. The proton number sets the positive charge of the nucleus, so it is the property that fixes elemental identity.
Element: A substance whose atoms all have the same number of protons.
That proton count is called the atomic number.
It is the fundamental label for an element. Changing the number of protons changes the nucleus itself, so it creates a different element rather than a different version of the same one.
Atomic number: The number of protons in the nucleus of an atom.
In a neutral atom, the number of electrons equals the number of protons. Neutrality, however, does not determine the element. Two neutral atoms can still be different elements if their proton numbers differ, and two particles can be the same element even if one is neutral and one is charged.
Isotopes
Atoms of the same element can contain different numbers of neutrons. These variants are called isotopes. Because neutron count can vary while proton count stays fixed, one element can exist in several isotopic forms without changing into a different element.
Isotope: One of two or more atoms of the same element that have the same number of protons but different numbers of neutrons.
Isotopes of an element have the same atomic number but different mass numbers, because mass number counts both protons and neutrons. The neutron number affects the mass of the atom without changing the element’s identity. This is why isotopes are especially important when discussing atomic mass.
= mass number, count of protons and neutrons, no unit
= atomic number, count of protons, no unit
= neutron number, count of neutrons, no unit
This relation shows why isotopes differ in mass: they have different values while stays the same. A larger neutron count gives a larger mass number. Since neutrons are part of the nucleus, they contribute significantly to the atom’s mass.
Two isotopes of the same element still have the same number of protons, so they remain the same element. However, they are not identical particles, because the different neutron count changes the nucleus and changes the mass. In AP Physics 2, this distinction matters whenever you compare particles by composition or by mass.
Ions
An ion is an atom with a nonzero net charge. Ions form when the number of electrons does not equal the number of protons. Unlike isotopes, which involve neutrons, ions are about electron imbalance.
Ion: An atom with a net electric charge because its number of electrons is not equal to its number of protons.
If an atom loses electrons, it has more protons than electrons and becomes positively charged. If it gains electrons, it has more electrons than protons and becomes negatively charged. In both cases, the nucleus is unchanged, so the atom remains the same element.
The key idea is that changing electrons changes charge, not element identity. An atom and its ion still represent the same element because both have the same proton number. Only a change in proton number would make them different elements. This is why proton number is always the first quantity to check when identifying a particle.
Atomic Mass and Nuclear Dominance
Almost all atomic mass comes from the nucleus.
This table compares the charge, location, and especially the masses of the proton, neutron, and electron in atomic mass units. It makes the “nuclear dominance” idea quantitative by showing that protons and neutrons are about each, while the electron is about , so electrons contribute negligibly to atomic mass. Source
Protons and neutrons are far more massive than electrons, so the total mass of an atom is controlled mainly by how many protons and neutrons it contains.
Atomic mass: The mass of an atom, determined almost entirely by the mass of its protons and neutrons.
This is why the syllabus emphasizes that nuclear mass dominates atomic mass. Even though electrons determine whether an atom is neutral or ionized, electrons contribute only a very small fraction of the total mass. Removing or adding a few electrons changes charge clearly, but it changes mass only slightly.
For AP Physics 2, this means that when you compare the masses of atoms, you mainly compare their nuclei. Atoms with more neutrons generally have greater mass. A neutral atom and its ion have nearly the same mass because their nuclei are the same, while two isotopes of the same element can have noticeably different masses because their neutron numbers differ.
Keeping the Terms Separate
Element answers the question: how many protons are in the nucleus?
Isotope answers the question: how many neutrons are present compared with another atom of the same element?
Ion answers the question: is the atom charged because the electron number differs from the proton number?
Atomic mass depends mainly on the nucleus, so proton and neutron counts matter far more than electron count.
When identifying a particle, start with protons, then check neutrons, then check electrons.
FAQ
The value on the periodic table is usually an average, not the mass of just one atom.
It is a weighted average of the naturally occurring isotopes of that element. If an element exists in more than one isotopic form, the more abundant isotopes influence the listed value more strongly, so the result is often a decimal.
Mass number is for one specific atom or isotope. It is the whole-number count of protons plus neutrons.
Average atomic mass is for a sample of the element as it occurs in nature. It includes the contributions of different isotopes and their abundances, so it is often not a whole number.
Yes. These terms describe different features of the same atom.
Isotope refers to how many neutrons the atom has.
Ion refers to whether the atom has a net charge because of electron gain or loss.
So one atom can have a particular neutron count and also have a nonzero charge at the same time.
Yes. Two atoms can have the same total number of protons and neutrons but different numbers of protons.
Such atoms are different elements because element identity depends only on proton number. Even with the same mass number, they do not have the same nuclear charge, so they are not the same element.
Both forms describe the same physical idea: a net positive charge equal to two elementary charges.
In chemistry-style notation, the charge is often written as $2+$ or $3-$. In physics, $+2e$ or $-3e$ makes the unit of charge more explicit. The notation changes, but the meaning does not.
Practice Questions
A particle has 13 protons, 14 neutrons, and 10 electrons.
State whether the particle is neutral, positively charged, or negatively charged, and state its net charge in units of . [2 marks]
1 mark for identifying the particle as positively charged or a positive ion.
1 mark for stating the net charge is .
Three particles are described below.
Particle A: 17 protons, 18 neutrons, 17 electrons
Particle B: 17 protons, 20 neutrons, 16 electrons
Particle C: 18 protons, 20 neutrons, 18 electrons
(a) Which two particles are isotopes of the same element? Explain. [2 marks]
(b) Which particle is an ion? State whether it is positive or negative, and give its net charge in units of . [2 marks]
(c) Which particle has the greatest atomic mass? Explain briefly. [1 mark]
(a) 1 mark for identifying A and B.
(a) 1 mark for explaining that they have the same number of protons but different numbers of neutrons.
(b) 1 mark for identifying B.
(b) 1 mark for stating that B is positive with charge because it has one fewer electron than protons.
(c) 1 mark for C, with explanation that atomic mass is dominated by the nucleus and C has the largest total number of protons plus neutrons.
