Structure of Atoms
What everything is made of, and why the electrons on the outside decide how a substance behaves.
What is an atom?
An atom is the smallest particle of an element that still behaves like that element. Split it further and you no longer have gold, or oxygen, or carbon. You just have the pieces every atom is made from.
Those pieces are called subatomic particles, and there are only three of them. Every one of the 118 known elements is built from the same three parts, arranged in different numbers.
That single idea is why chemistry works at all: change the number of parts, and you change the substance.
The three parts
Two of the three sit in the centre, packed into a tiny nucleus. The third moves around it.
- Proton: positive charge, mass 1, in the nucleus. The number of protons is the element.
- Neutron: no charge, mass 1, in the nucleus. Adds mass and keeps the nucleus stable.
- Electron: negative charge, almost no mass, orbiting in shells.
An atom has no overall charge because the protons and electrons balance exactly: number of protons = number of electrons
Atomic number and mass number
Two numbers describe any atom, and both appear on the Periodic Table.
Atomic number (Z) is the number of protons. It never changes for an element, every carbon atom has 6 protons, and anything with 6 protons is carbon.
Mass number (A) is protons plus neutrons.
number of neutrons = A − ZSo sodium, written 2311Na, has 11 protons, 11 electrons, and 23 − 11 = 12 neutrons.
Atoms of the same element with different neutron counts are called isotopes. Chlorine-35 and chlorine-37 are both chlorine, same chemistry, different mass.
Why electron shells matter most
Chemical reactions almost never touch the nucleus. They happen because of the electrons in the outermost shell, called the valence shell.
Shells fill in a fixed order: 2 electrons in the first, then 8, then 8 at this level. An atom is at its most stable when its outer shell is full.
Everything that follows in chemistry comes from atoms trying to reach that full outer shell: giving electrons away, taking them, or sharing them.
Sodium has one spare outer electron, so it gives it away easily, which is exactly why sodium is so reactive. Neon already has eight, so it reacts with almost nothing.
How we found the nucleus
Nobody could see an atom, so the structure had to be deduced. In 1911 Rutherford fired positively charged alpha particles at a sheet of gold foil only a few atoms thick.
The expectation, based on the older "plum pudding" model of evenly spread charge, was that everything would pass through with a slight deflection. Three things happened instead:
- Most particles went straight through, so the atom is mostly empty space.
- Some were deflected: so there is something positive in there.
- A very few bounced back: so that positive part is tiny and extremely dense.
Rutherford said it was "as if you fired a 15-inch shell at tissue paper and it came back and hit you". That single experiment gave us the nuclear atom.
Working out any atom, step by step
Given a symbol, you should be able to state every particle count. The method is always the same.
Question. How many protons, neutrons and electrons are in 4020Ca?
Step 1: protons. The bottom number is the atomic number: 20 protons.
Step 2: electrons. A neutral atom balances: 20 electrons.
Step 3: neutrons. Mass number minus atomic number:
40 − 20 = 20 neutronsNow an ion. For Ca2+, the 2+ means it has lost two electrons. Protons and neutrons never change, only the electrons do. So Ca2+ has 20 protons, 20 neutrons and 18 electrons.
Remember the direction: a positive ion has lost electrons, a negative ion has gained them.
Where this actually matters
Carbon dating. Living things absorb carbon-14 as well as ordinary carbon-12. When they die, absorption stops and the carbon-14 decays at a known rate, so the ratio of isotopes dates the remains. It is how archaeological finds in the Indus Valley are dated.
Medicine. Radioactive isotopes act as tracers. Iodine-131 concentrates in the thyroid, letting doctors image and treat it; cobalt-60 is used in radiotherapy.
Energy. Uranium-235 undergoes fission but uranium-238 does not. The two are chemically identical (same element, same reactions) so they can only be separated by mass. That single fact makes nuclear fuel enrichment difficult and expensive.
Every one of these depends on isotopes: same protons, different neutrons.
Where marks get lost
- "Mass number = number of neutrons." No. It is protons plus neutrons. Always subtract.
- Changing electrons to balance an ion's mass. Electrons have almost no mass; forming an ion changes charge, not mass number.
- "Isotopes react differently." They do not. Chemistry is decided by electrons, and isotopes have identical electron arrangements.
- Filling the third shell with 18. At this level it holds 8 before the fourth shell begins.
- Writing the charge backwards: it is Ca2+, not Ca+2.
Almost all of these come from muddling the nucleus with the electrons. Keep them separate: the nucleus sets identity and mass, the electrons set behaviour and charge.
What to remember
- Atoms are built from protons, neutrons and electrons.
- Atomic number = protons, and it defines the element.
- Mass number = protons + neutrons; neutrons = A − Z.
- Atoms are neutral because protons = electrons.
- Isotopes differ only in neutrons.
- Reactions are decided by the outer shell, not the nucleus.
Next: how those outer electrons are actually exchanged or shared, that is chemical bonding.
Watch one worked through
Here is one done slowly, with the reason for every move. Then one for you.
Chlorine has two isotopes, Cl-35 and Cl-37, in the ratio 3:1. Calculate its relative atomic mass.
- State what an isotope is, since the method depends on it.same protons, different neutronsIsotopes are the same element, so they behave identically chemically. Only the mass differs, which is why only mass is averaged.
- Turn the ratio into fractions out of the total parts.3 : 1 → 4 parts Cl-35 = 3/4, Cl-37 = 1/4A ratio is not a percentage. Converting to parts of the whole is where most errors happen.
- Multiply each mass by its share and add.(35 × 3/4) + (37 × 1/4) = 26.25 + 9.25This is a weighted mean, not a plain average. A plain average of 35 and 37 would wrongly give 36.
- Add to get the relative atomic mass.= 35.5It lands nearer 35 because that isotope is three times as common. Sanity-check that the answer sits nearer the commoner one.
Answer: 35.5
Two isotopes of mass 10 and 11 are in a 1:1 ratio. What is the relative atomic mass?
Show the answer
10.5
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