Particles in Atoms

An atom is built from three subatomic particles: protons, neutrons and electrons. Protons and neutrons are in the nucleus. Electrons are outside the nucleus in the space around it.

The nucleus contains nearly all the mass of the atom because protons and neutrons have relative mass 1, while electrons have a very small mass. The proton number is especially important because it identifies the element. If a particle has 8 protons, it is oxygen; if it has 17 protons, it is chlorine.

A neutral atom has no overall charge. That means the positive charge from protons is exactly balanced by the negative charge from electrons.

Atomic Number and Mass Number

Atomic number counts protons. Mass number counts the heavy particles in the nucleus: protons plus neutrons.

The most useful relationship is:

An isotope can be written with its mass number and atomic number:

For example, $^{23}_{11}\mathrm{Na}$ represents sodium with mass number 23 and atomic number 11.

Worked example:

Determine the number of protons, neutrons and electrons in a neutral atom of $^{23}_{11}\mathrm{Na}$.

Step 1: atomic number $Z = 11$, so there are 11 protons.

Step 2: mass number $A = 23$, so:

Step 3: the atom is neutral, so electrons = protons = 11.

So $^{23}_{11}\mathrm{Na}$ has 11 protons, 12 neutrons and 11 electrons.

A common error is to use the larger number on a Periodic Table as if it were always the mass number. In isotope notation or a question stem, the mass number is a whole-number count for one atom. Relative atomic mass is a later idea and is not needed in this lesson.

Ions and Electrons

An ion is charged because it has a different number of electrons from protons. The nucleus is not changed when an ordinary ion forms, so the number of protons and neutrons stays the same.

A positive ion has lost electrons. A negative ion has gained electrons.

Worked example:

Determine the number of protons, neutrons and electrons in $^{55}_{25}\mathrm{Mn}^{2+}$.

Step 1: atomic number $Z = 25$, so there are 25 protons.

Step 2: mass number $A = 55$, so:

Step 3: the charge is $2+$. A $2+$ ion has lost two electrons compared with the neutral atom.

So $^{55}_{25}\mathrm{Mn}^{2+}$ has 25 protons, 30 neutrons and 23 electrons.

The sign of the charge is the trap. A $2+$ ion has fewer electrons, not more. A $2-$ ion has more electrons, not fewer.

Isotopes

Isotopes are atoms of the same element with different numbers of neutrons and different masses. They are the same element because they have the same number of protons.

Carbon gives a useful example:

All three are carbon because all three have 6 protons. They have different mass numbers because they have different numbers of neutrons.

Isotope and ion are different descriptions. "Isotope" tells you about the neutron number and mass. "Ion" tells you about the electron number and charge. For example, $^{37}\mathrm{Cl}^{-}$ is both an isotope specification and an ion specification: chlorine-37 tells you the mass number, while the $-$ charge tells you there is one extra electron compared with a neutral chlorine atom.

Models and Evidence

Atomic structure is taught through models. A model is not the atom itself; it is a simplified representation that helps explain evidence and make predictions.

The nuclear model is useful in this lesson because it separates the nucleus from the electrons. It explains why mass number depends on protons and neutrons, while ionic charge depends on electrons.

The Bohr model places electrons in energy levels around the nucleus. This model is useful for explaining some periodic properties, even though later models using orbitals are more detailed. The important How Science Works idea is that scientists accept, improve or replace models when evidence shows that a different model explains observations better.

For this lesson, use the model only for the counting it supports:

• protons define the element
• protons plus neutrons give the mass number
• electrons determine whether the species is neutral or charged
• different neutron numbers give isotopes

Do not turn the simple diagram of electrons around a nucleus into a literal solar-system picture. It is a useful model, not a full description of electron behaviour.