Structure and bonding · GCSE Chemistry
Atomic structure
Protons, neutrons and electrons, atomic and mass numbers, isotopes, electronic structure and the evidence that replaced the plum pudding model.
Atomic number is protons. Mass number is protons plus neutrons. Electrons equal protons in a neutral atom, and they sit in shells: 2, then 8, then 8.
The important bits
What you need to know
- 1
Atoms contain a tiny positive nucleus of protons and neutrons, surrounded by electrons in shells. Most of the atom is empty space.
- 2
Proton: charge +1, relative mass 1. Neutron: charge 0, relative mass 1. Electron: charge −1, relative mass very small (1/1835).
- 3
Atomic number = number of protons. This defines the element. Mass number = protons + neutrons. Number of neutrons = mass number − atomic number.
- 4
In a neutral atom, electrons = protons. An ion has gained or lost electrons: a positive ion has lost electrons; a negative ion has gained them.
- 5
Isotopes are atoms of the same element (same protons) with different numbers of neutrons, so they have different mass numbers. They have the same chemical properties because they have the same electron arrangement.
- 6
Electronic structure is written as numbers in shells, for example sodium 2,8,1 and chlorine 2,8,7. The outer shell determines bonding and group number.
- 7
The plum pudding model had a ball of positive charge with electrons dotted through it. Rutherford’s alpha-particle scattering showed a small, dense, positive nucleus. Bohr placed electrons in energy levels (shells).
- 8
Most alpha particles went straight through gold foil (empty space). Some were deflected (positive nucleus). A few bounced back (nucleus is dense and contains most of the mass).
Go deeper
Count particles before you write anything else
A typical exam atom is given as ²³¹₁Na or as “sodium, atomic number 11, mass number 23”. Protons = 11. Electrons in the atom = 11. Neutrons = 23 − 11 = 12. If it becomes Na⁺, it has lost one electron, so 10 electrons, but protons stay 11 — that is why the ion is positive. Students lose marks by changing the proton number when they make an ion. The nucleus does not lose protons in ordinary chemistry; that would change the element. Isotopes of chlorine, ³⁵Cl and ³⁷Cl, both have 17 protons. Their neutron counts differ, which is why the relative atomic mass on the periodic table is not a whole number: it is a weighted mean of isotope abundances.
Go deeper
Electronic structure is the periodic table in code
Fill the first shell with two electrons, the second with eight, the third with eight at GCSE. Carbon is 2,4. Oxygen is 2,6. Calcium is 2,8,8,2. Group number for the main groups equals the number of outer electrons. Period number equals the number of shells. Once you can write the structure, ionic bonding is just “lose or gain to get a full outer shell” and covalent bonding is “share to get a full outer shell”. Do not invent a shell of 18 at this level unless a question explicitly goes beyond. Noble gases already have full outer shells, which is why they are unreactive.
Go deeper
The gold foil experiment is a three-observation story
Learn the observations with the conclusions attached, not as a history essay. Most alpha particles passed through: atoms are mostly empty space. Some were deflected: the nucleus is positive and repels the positive alpha particle. A tiny fraction rebounded: the nucleus is extremely dense and holds nearly all the mass. That destroyed the plum pudding model, which predicted only weak deflections. Later work placed electrons in shells to explain why atoms only emit certain frequencies of light. In the exam, match each observation to one conclusion. A paragraph that only says “Rutherford discovered the nucleus” is not enough.
See the idea in action
An aluminium ion is written Al³⁺. Aluminium has atomic number 13 and mass number 27. Protons = 13. Neutrons = 14. The atom would have 13 electrons (2,8,3). The 3+ ion has lost three outer electrons, so 10 electrons (2,8). Charge = +13 from protons and −10 from electrons, overall 3+. The proton number is still 13; it is still aluminium.
Exam technique
Turn knowledge into marks
Always state protons, neutrons and electrons as three separate numbers. For isotopes, stress same atomic number, different mass number. For the alpha-scattering experiment, pair each observation with its conclusion. Never say ions change their number of protons.
Common mistakes
Do not give these marks away
- 01
Swapping atomic number and mass number, or changing the proton count when an atom becomes an ion.
- 02
Giving electrons a relative mass of 1, or putting neutrons in shells.
- 03
Describing the plum pudding model as having a nucleus, or stating the gold foil conclusion without the observation that supports it.
An atom of ¹⁹F has 9 protons. How many neutrons and electrons does the neutral atom have?
A9 neutrons and 10 electrons
B10 neutrons and 9 electrons
C19 neutrons and 9 electrons
D9 neutrons and 9 electrons
Show the answer
10 neutrons and 9 electrons. Mass number 19 minus atomic number 9 gives 10 neutrons. A neutral atom has equal protons and electrons, so 9 electrons.
Quick questions
If this is the bit you searched
What is the difference between atomic number and mass number?
Atomic number is the number of protons and defines the element. Mass number is protons plus neutrons.
What are isotopes?
Atoms of the same element with the same number of protons but different numbers of neutrons, so they have different mass numbers.
What did the alpha-particle scattering experiment show?
Atoms are mostly empty space with a small, dense, positive nucleus that contains most of the mass.