1.2Chemical Substances, Reactions and Essential Resources
Atomic Structure and The Periodic Table — WJEC GCSE Chemistry Revision Notes & Topical Questions
15 min read 31 parts Lesson 2 of 6
Lesson summary
Complete PDF with figures and original page layoutPage 1 WJEC GCSE Chemistry Your notes 1.2 Atomic Structure & The Periodic Table Contents Atomic Structure Atomic Number, Mass Number & Isotopes Relative Atomic Mass The Periodic Table Electroni…
Part 1 of 9Structure of the atom
Structure of the atom
Atoms
- Atoms all substances are made of atoms, the smallest part of an element that keeps its properties
Subatomic particles
- Subatomic particles protons, neutrons and electrons
Nucleus
- Nucleus protons and neutrons are at the centre, and the nucleus is positively charged
Electrons
- Electrons they move very fast around the nucleus in shells
Mass
- Mass the electron mass is negligible, so almost all of the mass of an atom is in the nucleus
Part 2 of 9Relative masses and charges
| Particle | Relative mass | Relative charge |
|---|---|---|
| Proton | 1 | +1 |
| Neutron | 1 | 0 |
| Electron | 1/2000 (almost zero) | -1 |
Why relative values
- Why relative values actual masses and charges are too small for grams or coulombs, so they are compared with each other
Neutral atoms
- Neutral atoms an atom has equal numbers of protons and electrons, so the charges cancel
Part 3 of 9Ions
Formation
- Formation an atom loses or gains electrons to achieve a full outer shell
Positive ion
- Positive ion an atom that loses electrons
Negative ion
- Negative ion an atom that gains electrons
Magnesium ion
- Mg has 12 protons and 12 electrons and loses two electrons, leaving 12 protons and 10 electrons, so the charge is 2+
Part 4 of 9Atomic number and mass number
Atomic number and mass number
Atomic (proton) number, Z
- Atomic (proton) number, Z the number of protons in the nucleus, which equals the number of electrons in a neutral atom
Uniqueness
- Uniqueness it is unique to each element and fixes the element’s position, and it does not change in chemical reactions
Mass (nucleon) number, A
- Mass (nucleon) number, A the total number of protons and neutrons in the nucleus
Neutrons
- Neutrons the number of neutrons is the mass number minus the atomic number
Periodic Table
- Periodic Table it shows both numbers, and the mass number is always the larger
Part 5 of 9Isotopes
Isotopes = atoms of the same element with the same number of protons and electrons but different numbers of neutrons
Notation
- Notation an isotope is written as the symbol or name then the mass number, such as C-14 or 14C, which has 6 protons and 8 neutrons
Chemistry
- Chemistry isotopes behave the same chemically because they have the same outer electrons, and neutrons add mass only
| Isotope | Protons | Neutrons |
|---|---|---|
| Hydrogen-1 | 1 | 0 |
| Hydrogen-2 | 1 | 1 |
| Hydrogen-3 | 1 | 2 |
- Condition
- isotopes must be of the same element, so C-13 and C-14 are isotopes but C-13 and H-2 are not
Part 6 of 9PEN numbers
Protons
- Protons equal to the atomic number
Electrons
- Electrons equal to the number of protons in a neutral atom
Neutrons
- Neutrons mass number minus atomic number
Example
- Example beryllium has atomic number 4 and mass number 9, so p = 4, e = 4 and n = 5
Part 7 of 9Relative atomic mass from isotopes (Higher tier)
Definition
- Ar is the mass of an atom compared with one twelfth of the mass of a carbon-12 atom, so it has no units
Comparison
- Comparison hydrogen has Ar 1, so 12 hydrogen atoms have the mass of 1 carbon atom
Equation
- Ar = Σ ÷ 100, extended to every isotope present
- (% abundance × mass number)
Example
- Example rubidium with 72% of mass 85 and 28% of mass 87 has Ar = ÷ 100 = 85.6
- (72 × 85 + 28 × 87)
Exam note
- Exam note the equation is provided in the exam
Part 8 of 9Arrangement of the Periodic Table
Arrangement of the Periodic Table
Order
- Order elements are arranged in order of increasing atomic number, so each has one more proton than the one before
Purpose
- Purpose elements with similar properties end up in the same column
Period
- Period a horizontal row, numbered 1 to 7
- The period number is the number of electron shells.
- Period 2 elements have two shells.
Group
- Group a vertical column, numbered 1 to 7 and then Group 0
- The group number is the number of outer electrons.
- Group 4 atoms have four outer electrons.
Part 9 of 9Metals and non-metals
Metals
- Metals they lose electrons to form positive ions and are on the left and centre of the table
Non-metals
- Non-metals they gain electrons to form negative ions, or form no positive ions as in Group 0, and are on the right
Metalloids
- Metalloids a few elements show properties of both types
Trends
- Metallic character decreases from left to right across a period.
- Metallic character increases down a group.
Stair line
- Stair line a stair-step line starting above aluminium separates metals from non-metals
Key terms
| Term | Meaning |
|---|---|
| Atomic number | The number of protons in the nucleus of an atom. |
| Mass number | The total number of protons and neutrons in the nucleus. |
| Isotopes | Atoms of the same element with different numbers of neutrons. |
| Relative atomic mass | The average mass of an element’s atoms compared with one twelfth of a carbon-12 atom. |
| Period | A horizontal row of the Periodic Table. |
| Group | A vertical column of the Periodic Table. |
Avoid these mistakes
1
Saying the number of protons changes in a reaction — only electrons are lost, gained or shared.
2
Confusing mass number with atomic number — the mass number is the larger.
3
Calling atoms of different elements isotopes — isotopes have the same atomic number.
4
Saying isotopes differ chemically — they have identical electron arrangements.
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