Atomic Structure lessons

How the model of the atom changed

Nobody can see inside an atom directly. Scientists build models of it, then change them when an experiment disagrees.

Each model below fixed a problem with the one before. Watch how the picture changes.

The models of the atom in date order, from solid spheres to the electron cloud.

The gold foil experiment

In 1909, Hans Geiger and Ernest Marsden fired positive alpha particles at a very thin sheet of gold. The plum pudding model said they should all pass almost straight through.

Most did pass straight through. A few swerved, and a tiny number bounced back.

Alpha particles fired at gold atoms, first as the plum pudding model predicts, then as it really happened.

Ernest Rutherford explained this in 1911. The atom is mostly empty space, with its positive charge and almost all its mass in a tiny nucleus.

Practice: Read the evidence

Match each gold foil result to what it showed, then each scientist to their idea.

Protons, neutrons and electrons

Protons and neutrons have almost the same mass. We give each a relative mass of 1.

An electron is about 1836 times lighter than a proton, so its mass is very small. Almost all of an atom's mass is in its nucleus.

The relative mass, relative charge and place of the proton, neutron and electron.

A proton has a relative charge of +1 and an electron has -1. A neutron has no charge.

Inside a carbon atom

A carbon atom has 6 protons and 6 electrons. The charges cancel, so the atom is neutral.

This model is not to scale. A real nucleus is over 10,000 times smaller across than the whole atom.

A 3D carbon atom touring its nucleus, its protons and neutrons, and its two shells of electrons.

Practice: Sort the particles

Sort facts about mass, charge and place to the proton, neutron or electron.

Atomic number and mass number

The atomic number, Z, is the number of protons. Every atom of an element has the same atomic number.

The mass number, A, is the number of protons plus neutrons. Electrons are too light to count.

Counting neutrons: \text{neutrons} = A - Z

In nuclear notation, the mass number sits top left of the symbol and the atomic number bottom left. A neutral atom has as many electrons as protons.

Nuclear notation for several elements, with each number turned into a row of protons, neutrons and electrons.

Practice: Count the particles

Read nuclear notation and type the number of protons, neutrons and electrons.

Isotopes

Isotopes are atoms of the same element with different numbers of neutrons. They have the same atomic number but different mass numbers.

We name an isotope by its mass number. Carbon-12 has 6 neutrons and carbon-14 has 8.

The isotopes of hydrogen and of carbon side by side: same protons and electrons, different neutrons.

Isotopes have the same electrons, so they react in the same way. Some, like carbon-14, are unstable and radioactive.

Practice: Spot the isotope

Pick the atom that is an isotope of the one shown.

Relative atomic mass

Most elements are a mix of isotopes. The relative atomic mass, A_r, is the average mass of their atoms.

It is a weighted average: common isotopes count for more. Chlorine is about three quarters chlorine-35 and one quarter chlorine-37.

1. A_r = \frac{(35 \times 75) + (37 \times 25)}{100} (each mass times its percentage)
2. = \frac{2625 + 925}{100} (multiply)
3. = \frac{3550}{100} = \textcolor{#0d9488}{35.5} (add, then divide by 100)

A sample of 100 chlorine atoms: change the share of chlorine-35 and watch the average mass move.

That is why the periodic table gives chlorine as 35.5. No single chlorine atom has that mass.

Practice: Calculate relative atomic mass

Work out the relative atomic mass of an element from its isotopes and their abundances.

Electron shells

Electrons sit in shells around the nucleus. Each shell is an energy level, and the shells nearest the nucleus are lowest in energy.

Each shell holds a limited number of electrons. For the first 20 elements, the first shell holds 2 and the next two hold 8 each.

A 3D oxygen atom touring its nucleus, its inner shell of 2 electrons and its outer shell of 6.

Oxygen has 8 electrons: 2 in the first shell and 6 in the second. We write this electron configuration as 2, 6.

Filling the shells

Electrons fill the lowest shell with space first. A new shell only starts once the one inside it is full.

Hydrogen to calcium, one electron at a time, with the configuration under the shells.

Sodium has 11 electrons, so its configuration is 2, 8, 1. Calcium has 20, giving 2, 8, 8, 2.

Practice: Fill the shells

Set how many electrons go in each shell for a given element.

Inside the shells: s, p and d

For the curious: shells are split into subshells called s, p and d. An s subshell holds 2 electrons, a p holds 6 and a d holds 10.

The second shell is 2s and 2p, so it holds 2 + 6 = 8. The 4s subshell is lower in energy than 3d, so 4s fills first.

Subshells stacked by energy, filling in order for oxygen, sodium, argon, potassium and calcium.

That gives the 2, 8, 8 pattern. After argon, the next electron goes into 4s, so potassium starts a fourth shell.

Shells and the periodic table

The periodic table lists the elements in order of atomic number. Its columns are groups and its rows are periods.

The group number is the number of outer electrons. The period number is the number of shells in use.

The first 20 elements in their groups and periods, with each one's electron configuration.

Group 0 holds the noble gases, whose outer shells are full. Helium belongs there too, because its only shell is full with 2 electrons.

Practice: Find the element

Tap the element on the periodic table that has the electron configuration shown.

Ions

An ion is an atom that has lost or gained electrons. Its protons stay the same, so the charges no longer cancel.

Metals lose their outer electrons and become positive ions. Non-metals gain electrons to fill their outer shell and become negative ions.

1. \text{Mg}:\ 2,\ 8,\ 2 (12 protons, 12 electrons)
2. \text{Mg} \to \text{Mg}^{2+} + 2e^- (loses its 2 outer electrons)
3. \text{Mg}^{2+}:\ 2,\ 8 (12 protons, 10 electrons, so the charge is +2)

Sodium and magnesium lose electrons, chlorine and oxygen gain them, and the charge updates.

Practice: Make the ion

Remove or add outer electrons until the atom has a full outer shell, then read its charge.

Why the outer shell matters

When atoms react, only their outer electrons take part. So atoms with the same number of outer electrons react in similar ways.

Group 1 metals each have one outer electron to lose, so they all react with water in a similar way. Noble gases already have full outer shells and barely react at all.

Groups 1, 7 and 0 side by side: each atom in a group has the same outer shell and does the same thing with it.

Atoms react to reach a full outer shell. The Chemical Bonding course shows how they swap or share electrons to get there.

Practice: Atomic structure review

Mixed questions from the whole course.