Describe the structure of the atom in terms of protons, neutrons, and electrons, and explain how atomic number and mass number define an element and its isotopes (MA STE HS-PS1-1, atomic structure).

What this topic is asking

Standard HS-PS1-1 has you use the periodic table as a model, and that starts with knowing what an atom is made of. Massachusetts high school chemistry expects you to describe the atom in terms of its three subatomic particles, to use the atomic number and mass number to count those particles, and to understand isotopes and ions. This is the foundation for electron arrangement, periodic trends, and every reaction you will study, so getting the counting automatic pays off all year.

The three subatomic particles

Almost all of an atom's mass is in the nucleus, because protons and neutrons are about 1836 times heavier than electrons. Almost all of an atom's volume is the electron region, because the nucleus is thousands of times smaller than the atom. This is why the atom is described as mostly empty space with a dense center.

Atomic number and mass number

The atomic number (Z) is the number of protons in the nucleus. It is the atom's identity: every carbon atom has 6 protons, every oxygen atom has 8. Change the proton number and you change the element. On the periodic table, the atomic number is the whole number that orders the elements.

The mass number (A) is the total number of protons and neutrons:

where $N$ is the neutron count. To find neutrons, subtract: $N = A - Z$. Mass number is always a whole number because you are counting particles. Do not confuse it with the atomic mass on the periodic table, which is an average (covered in average atomic mass and the mole concept).

In a neutral atom, the positive protons and negative electrons balance, so the number of electrons equals the number of protons. This is why a neutral atom has no overall charge.

Isotopes

Carbon-12 and carbon-14 both have 6 protons (both are carbon), but carbon-12 has 6 neutrons and carbon-14 has 8. Isotopes of an element behave almost identically in chemical reactions, because chemistry is governed by electrons, and isotopes have the same electron arrangement. They differ in mass and in nuclear stability: some isotopes are radioactive, which connects to nuclear chemistry and radioactivity. Isotopes are written with the mass number, as in carbon-14 or $^{14}\text{C}$.

Ions

An atom that gains or loses electrons becomes an ion with a net charge. Losing electrons leaves more protons than electrons, giving a positive ion (a cation), such as $\text{Na}^+$. Gaining electrons gives a negative ion (an anion), such as $\text{Cl}^-$. Note that the number of protons does not change when an ion forms, so the element stays the same; only the electron count and therefore the charge change. This idea sets up ionic bonding in Module 2.

Try this

Q1. An atom has 11 protons and 12 neutrons. State its atomic number and mass number. [2]

• Cue. Atomic number 11 (it is sodium); mass number $11 + 12 = 23$.

Q2. Why do two isotopes of an element react the same way chemically? [2]

• Cue. They have the same number of protons and so the same number and arrangement of electrons, and chemistry is governed by electrons.