The Periodic Table and Electron Arrangement
FoundationThe periodic table orders elements by atomic number. Electrons fill shells 2, 8, 8; shell count gives the period and outermost electrons the group. Group I alkali metals are reactive, Group VII halogens, Group 0 noble gases stable.
- The table is ordered by atomic number; rows are periods, columns are groups
- Arrangements up to Z = 20 fill 2, 8, 8 — inner shells first
- Period = shell count; group = outermost electron count (Groups I, II, VII, 0)
文體小知識：科學知識說明文——說明週期表的編排原則與電子排布規則，並示範由位置推測元素性質的方法。
§Text Passage
1The periodic table is the chemist's map: elements are arranged in order of atomic number, in horizontal rows called periods and vertical columns called groups. Metals occupy the lower left, non-metals the upper right, and metalloids such as silicon lie along the staircase between. An element's position is not arbitrary — it is fixed by atomic structure, and reading the table lets you predict properties.
2Electrons arrange themselves in shells outside the nucleus: the innermost shell holds at most 2, the second at most 8, and for elements up to Z = 20 the third also holds at most 8. Take sodium (Z = 11): its 11 electrons fill the first shell with 2, the second with 8, and the last 1 enters the third shell — the arrangement is written 2,8,1. Calcium (Z = 20) gives 2,8,8,2.
3The period number equals the number of occupied shells, and the group number relates directly to the outermost electrons: Group I has 1, Group II has 2, Group VII has 7, Group 0 has 8 (helium has 2). Sodium's 2,8,1 therefore places it in Period 3 and Group I. Elements in the same group share the same outermost electron count and so have similar chemical properties.
4Each family has its own name and character: Group I is the alkali metals — soft metals that react vigorously with water to release hydrogen, reactivity rising down the group; Group VII is the halogens, existing as diatomic molecules, reactivity falling down the group; Group 0 is the noble gases with full outermost shells and very low reactivity. A full outer shell of 8 electrons is the octet — the key to chemical bonding.
1Summary 重點摘要
The periodic table orders elements by atomic number. Electrons fill shells 2, 8, 8; shell count gives the period and outermost electrons the group. Group I alkali metals are reactive, Group VII halogens, Group 0 noble gases stable.
2Vocabulary cards Vocabulary
A horizontal row of the periodic table; the period number equals the number of occupied shells.
A vertical column; members share the same outermost electron number and similar chemistry.
The regions around the nucleus where electrons reside, holding up to 2, 8 and 8 (up to Z = 20).
How electrons are written shell by shell, e.g. 2,8,1 for sodium.
A Group I metal — soft, reactive, and reacts with water to give hydrogen.
A Group VII non-metal, existing as diatomic molecules, reactivity decreasing down the group.
A Group 0 element with a full outermost shell; very unreactive, e.g. helium and argon.
The rule that a full outermost shell of 8 electrons is stable, explaining bonding tendencies.
A molecule of two atoms, such as H₂, O₂ and Cl₂.
3Step by step Steps
- Step 1: How the table is orderedThe periodic table lists all elements by atomic number, smallest first: hydrogen first (Z = 1), helium second (Z = 2), and so on. Rows are periods — seven of them — and columns are groups, with main groups labelled in Roman numerals. For example, sodium has Z = 11: counting up the table to 11 locates it exactly. The atomic number is an element's seat number in the table — the first step in reading it. Metals sit lower left, non-metals upper right, metalloids along the staircase.
- Step 2: Filling the electron shellsElectrons fill shells from the inside out: at most 2 in the first shell, at most 8 in the second, and for elements up to Z = 20 at most 8 in the third. For example, chlorine (Z = 17) takes 2, then 8, leaving 7 for the third shell — written 2,8,7. Calcium, Z = 20, gives 2,8,8,2. Remember that the third shell's limit of 8 applies up to Z = 20: writing potassium as 2,8,9 is wrong — the correct arrangement is 2,8,8,1.
- Step 3: Period number = shell countAn element's period number equals its number of occupied shells. For example, sodium's 2,8,1 spans three shells, so sodium is in Period 3; calcium's 2,8,8,2 spans four shells, placing it in Period 4. The converse holds too: an element in Period 3 must have 3 occupied shells. This rule — shell count equals period number — is the key step from arrangement to position. In answers, write the arrangement, the shell count and the period number in order, so the reasoning is complete.
- Step 4: Group number and outermost electronsFor Groups I, II, VII and 0, the group number maps directly onto the outermost electrons: 1 outer electron means Group I, 2 means Group II, 7 means Group VII and 8 means Group 0 — except helium, which is full with only 2. For example, chlorine's 2,8,7 has 7 outer electrons, so it belongs to Group VII; argon's 2,8,8 is full, so Group 0. For main-group elements the group number and electron arrangement lock together — the core skill of reading the table and the basis of predicting properties.
- Step 5: Group I, the alkali metalsGroup I holds lithium, sodium, potassium and the rest — the alkali metals: soft, low in density, with modest melting points. With only 1 outer electron, easily lost, they are very reactive and react with water to form hydroxides and hydrogen. For example, a pea-sized piece of sodium skims the water surface, hissing as it reacts; potassium reacts even more fiercely, igniting with a lilac flame. Reactivity therefore rises down the group, because the outer electron sits farther from the nucleus and is lost more easily.
- Step 6: The halogens and the noble gasesGroup VII halogens — fluorine, chlorine, bromine, iodine — exist as diatomic molecules such as Cl₂. They are reactive non-metals: with 7 outer electrons they tend to gain 1 electron to complete the octet, and reactivity falls down the group. Group 0 noble gases — helium, neon, argon — have full outermost shells and are extremely unreactive. For example, argon fills light bulbs and shields welding precisely because it is 'lazy', refusing to react even with hot metals; the halogens are the opposite — chlorine bleaches and disinfects.
- Step 7: From arrangement to properties — sodium closes the loopString the skills together: sodium's 2,8,1 has three shells — Period 3; 1 outer electron — Group I, an alkali metal; it readily loses that 1 electron to reach the stable 2,8 arrangement, so it is reactive and resembles its group-mates such as potassium. For example, if a question asks about potassium (Z = 19), the same method gives 2,8,8,1, Period 4, Group I — so predict a violent reaction with water, fiercer than sodium's. Drill the chain — arrangement, position, property — until it is automatic.
4Diagrams Diagrams
5Worked example Example
Electron arrangement deduction: sodium's atomic number is 11. Write the electron arrangement of a sodium atom, deduce its period and group in the periodic table, and use its position to describe sodium's chemical properties.
- Arrangement: the 11 electrons fill the first shell with 2 and the second with 8, leaving 1 for the third — 2,8,1
- Period: three occupied shells, so sodium is in Period 3
- Group: 1 electron in the outermost shell, so sodium is in Group I (an alkali metal)
- Properties: the single outer electron is easily lost, giving the stable 2,8 arrangement — sodium is a reactive metal, reacts vigorously with water, and resembles potassium
- Answer: A sodium atom's electron arrangement is 2,8,1. With three occupied shells, sodium sits in Period 3; with 1 outermost electron, it belongs to Group I. Group I elements are alkali metals: the lone outer electron is easily lost, letting sodium reach the stable 2,8 arrangement. Sodium is therefore a reactive metal that reacts vigorously with water to form sodium hydroxide and hydrogen, and its chemistry closely resembles lithium and potassium in the same group.
6Common mistakes Mistakes
7Quick check Quick check
Q1In what order are elements arranged in the periodic table?
Q2What is sodium's (Z = 11) electron arrangement, and where does it sit?
Q3What is chlorine's (Z = 17) electron arrangement and group?
Q4Why are Group 0 elements so unreactive?
Q5What is the electron arrangement of calcium (Z = 20)?
8Recap Recap
- The table is ordered by atomic number; rows are periods, columns are groups
- Arrangements up to Z = 20 fill 2, 8, 8 — inner shells first
- Period = shell count; group = outermost electron count (Groups I, II, VII, 0)
- Alkali metals grow more reactive down the group, halogens less; noble gases are full and unreactive
- The octet — 8 outer electrons — is the stable arrangement explaining bonding tendencies