C. Chemistry: The Nature of Matter · Atoms, Elements and Compounds

Inside the Atom: Protons, Neutrons and Electrons

C2.3About 65 minutes

Learning goals

I can state the location, relative mass and charge of protons, neutrons and electrons.
I can use the atomic number and mass number to find the number of protons, neutrons and electrons in an atom.
I can read and write standard atomic notation.
I can explain what isotopes are and why atomic masses on the periodic table are usually not whole numbers.

Words to know

subatomic particle
A particle smaller than an atom: a proton, neutron or electron.

atomic number (Z)
The number of protons in the nucleus of an atom. It identifies the element.

mass number (A)
The total number of protons and neutrons in the nucleus of an atom.

isotopes
Atoms of the same element (same number of protons) with different numbers of neutrons, so different mass numbers.

atomic mass
The average mass of an element's atoms, in atomic mass units (u), taking into account its isotopes and how common each one is.

neutral atom
An atom with equal numbers of protons and electrons, so its overall charge is zero.

Three particles build everything

Your phone, a maple leaf, the air, the ice on the rink: every bit of ordinary matter you have ever touched is built from just three kinds of particles. The only difference between gold and oxygen is how many of each particle their atoms have. Change the number of protons, and you change the element.

In the last two lessons you saw how scientists discovered these particles. Now you will learn to count them.

Particle	Symbol	Charge	Relative mass	Location
Proton	p⁺	+1	1	In the nucleus
Neutron	n⁰	0 (neutral)	1	In the nucleus
Electron	e⁻	−1	1/1836 (almost 0)	In energy levels (shells) around the nucleus

The three subatomic particles. Masses are compared with a proton; a neutron is just slightly heavier than a proton, so both are counted as 1.

A lithium atom drawn with the Bohr-Rutherford model. The nucleus is drawn large so we can label it; really it is about 100 000 times smaller across than the atom.

Atomic number and mass number

Two numbers tell you everything about the particles in an atom.

The atomic number (Z) is the number of protons. Every atom of an element has the same number of protons: every carbon atom has 6, every oxygen atom has 8. On the periodic table, the elements are listed in order of atomic number.
In a neutral atom, the number of electrons = the number of protons, so the positive and negative charges cancel.
The mass number (A) is the number of protons + neutrons. (Electrons are so light that we don't count them.)

So: number of neutrons = mass number − atomic number, or n = A − Z.

Standard atomic notation puts the mass number at the top left of the symbol and the atomic number at the bottom left. You can also write the same atom as sodium-23 or Na-23.

Worked example

Example 1: Count the particles

How many protons, neutrons and electrons are in a neutral atom of aluminium-27? (Aluminium's atomic number is 13.)

Protons = atomic number = 13.
Electrons = protons (neutral atom) = 13.
Neutrons = mass number − atomic number = 27 − 13 = 14.
Check: 13 protons + 14 neutrons = 27 = mass number. ✔

13 protons, 14 neutrons, 13 electrons.

Worked example

Example 2: Work backwards

An atom has 26 protons and 30 neutrons. Identify the element and write its standard atomic notation and its name with mass number.

The atomic number is 26. On the periodic table, element 26 is iron (Fe).
Mass number = 26 + 30 = 56.
Notation: mass number 56 at the top left, atomic number 26 at the bottom left of Fe.

It is iron-56: ⁵⁶₂₆Fe (26 protons, 30 neutrons and, if neutral, 26 electrons).

Isotopes

All carbon atoms have 6 protons. But not all carbon atoms have the same number of neutrons. Most have 6, some have 7 and a very few have 8. These are isotopes of carbon: carbon-12, carbon-13 and carbon-14.

Isotopes of an element have the same number of protons and electrons, so they behave the same way in chemical reactions.
They have different numbers of neutrons, so they have different masses.
Some isotopes are radioactive: their nuclei are unstable and break down over time. Carbon-14 is radioactive, which lets scientists estimate the age of materials that were once alive (carbon dating). In Ontario, CANDU nuclear reactors also produce the radioactive isotope cobalt-60, which is used to sterilize medical equipment and to treat some cancers.

Three isotopes of carbon. They all have 6 protons and 6 electrons (2 in the first level, 4 in the second), so they are all carbon. Only the number of neutrons changes.

Why atomic masses have decimals

On the periodic table, chlorine's atomic mass is 35.45, not a whole number. That is because natural chlorine is a mix of two isotopes: about three-quarters chlorine-35 and about one-quarter chlorine-37. The atomic mass is a weighted average:

0.75 × 35 + 0.25 × 37 = 26.25 + 9.25 = 35.5, close to 35.45.

To find the mass number of the most common isotope of an element, you can usually round the atomic mass to the nearest whole number (carbon 12.01 → 12; sodium 22.99 → 23). This works for all of the first 20 elements, but not for every element, so when a question gives you the mass number, use it.

Hands-on activity

Build a scale model of an atom in your neighbourhood

You need: a small bead, pea or peppercorn (about 1 cm or less), a map of your neighbourhood (paper, or a map app with a grown-up), a ruler, and a 1 kg bag of flour or sugar plus a paper clip.

Part A: size. The nucleus is roughly 100 000 times smaller across than the atom.

Let your bead (about 1 cm) be a nucleus. Multiply: 1 cm × 100 000 = 100 000 cm = 1 km. That is the width of the whole atom!
On the map, put the bead at your home and find places about 500 m away in different directions (the edge of the atom). Name them: "The outer electrons are near the school, the arena and the park."
Everything in between is almost empty space.

Part B: mass. Hold the 1 kg bag: pretend it is a proton. An electron would then have a mass of 1000 g ÷ 1836 ≈ 0.54 g, about half the mass of a typical paper clip. Hold both. That is why electrons are left out of the mass number.

What to look for: students should see that the nucleus is tiny but holds almost all of the mass, and that atoms are mostly empty space, just as the gold foil experiment showed. (The 100 000 figure is approximate; real ratios vary between about 10 000 and 100 000 depending on the atom.)

Just for fun

If you removed all the empty space from the atoms in every person on Earth, scientists estimate all of humanity would squeeze into something about the size of a sugar cube. It would still have the same mass, though, so good luck lifting it.

Your turn

Which subatomic particle has a negative charge, and where is it found?

Which two particles have about the same mass?

How many protons, neutrons and electrons are in a neutral atom of fluorine-19? (Z = 9)

What element has atomic number 20? How many electrons does a neutral atom of it have?

Write the name and mass number of an atom with 17 protons and 20 neutrons.

What is the difference between atomic number and mass number?

Are carbon-12 and carbon-14 the same element? Explain.

More practice: warm-up

What is the charge of a neutron?

Where is almost all of an atom's mass?

How many protons does every oxygen atom have?

A neutral atom has 12 protons. How many electrons does it have?

What do we call atoms of the same element with different numbers of neutrons?

More practice: on your own

Find the protons, neutrons and electrons in a neutral atom of potassium-39 (Z = 19).

An atom has 15 protons, 16 neutrons and 15 electrons. Identify it and give its mass number.

Uranium-238 has atomic number 92. How many neutrons does it have?

Why don't electrons count in the mass number?

Boron has two natural isotopes: boron-10 (about 20%) and boron-11 (about 80%). Estimate boron's average atomic mass.

Challenge

An atom has a mass number of 40 and 22 neutrons. Identify it and say how it compares with an atom of calcium-40.

A student says, "Adding a neutron to an atom changes it into a new element." Is she right? What would change the element?

Copper's atomic mass is 63.55. Natural copper is made of copper-63 and copper-65. Which isotope is more common? Explain without calculating exactly.

Check your understanding

Quiz

Which particle is found in the nucleus and has no charge?

The atomic number of an element tells you the number of...

How many neutrons are in magnesium-26 (Z = 12)?

Which pair are isotopes of the same element?

A neutral atom has 16 protons and 16 neutrons. What is it?

Compared with a proton, an electron's mass is...

For parents and teachers

Big idea: C2.3 asks students to identify the location, relative mass and charge of protons, neutrons and electrons using the Bohr-Rutherford model. Counting particles from the atomic number and mass number is a core skill they will use for the rest of the unit and in Grade 10 chemistry.

Common difficulties:

Using the atomic mass (with decimals) as the number of neutrons. Subtract the atomic number from the mass number.
Thinking a different number of neutrons makes a different element. Only protons decide the element.
Forgetting that electrons = protons only in a neutral atom. (Ions, coming later in this unit, have unequal numbers.)

How to help: quiz your teen with quick questions using a periodic table (many free ones are online): "How many protons in neon? How many neutrons in neon-20?" The neighbourhood scale model is a memorable way to feel how empty atoms are.