Atomic structure: describe the charge, relative mass and location of protons, neutrons and electrons, and use atomic number and mass number to count the particles in an atom.

What this topic is asking

The Physical Setting/Chemistry Core Curriculum opens with the structure of the atom. You must know the three subatomic particles (proton, neutron, electron), state the charge, relative mass and location of each, and use the atomic number and mass number to count how many of each particle an atom contains. The Regents exam tests this in Part A as quick recall and in Part B-2 as short calculations, and every later topic, from bonding to nuclear chemistry, builds on it.

The three subatomic particles

The proton's positive charge and the electron's negative charge are equal in size and opposite in sign, so an atom with the same number of protons and electrons has no overall charge. The neutron adds mass without adding charge. Because the electron's mass is so small, almost all of an atom's mass is concentrated in the nucleus, even though the nucleus occupies only a minute fraction of the atom's volume.

Atomic number and mass number

For example, an atom with $A = 23$ and $Z = 11$ is sodium: it has $11$ protons, $11$ electrons (if neutral) and $23 - 11 = 12$ neutrons. The Periodic Table in the Reference Tables lists the atomic number and the atomic mass for every element, so you can identify an element from its proton count or look up its data once you know its symbol. Changing the number of protons changes the element; changing only the neutrons gives a different isotope of the same element.

Models of the atom

The modern picture grew out of a sequence of experiments. Rutherford's gold-foil experiment showed that the atom is mostly empty space with a small, dense, positively charged nucleus, overturning the earlier "plum pudding" idea. Bohr then proposed that electrons travel in fixed energy levels (orbits) around the nucleus, which explained the bright-line spectra of elements. The Regents exam uses this Bohr, energy-level view for electron configuration.

The atom is mostly empty space and neutral

Two ideas recur in Part A questions. First, the atom is mostly empty space: the nucleus is thousands of times smaller than the atom, so a particle fired at an atom usually passes straight through, which is exactly what Rutherford observed. Second, a neutral atom is electrically neutral because the positive charge of the protons is exactly balanced by the negative charge of an equal number of electrons. If the numbers are not equal, the particle is an ion, which is covered in the ions and nuclide notation page.

Try this

Q1. An atom has $12$ protons and $14$ neutrons. State its mass number. [1 point]

• Cue. Mass number $=$ protons $+$ neutrons $= 12 + 14 = 26$.

Q2. Explain why an atom has no overall electric charge even though it contains charged particles. [1 point]

• Cue. It contains equal numbers of protons ($1+$) and electrons ($1-$), so the positive and negative charges cancel.