Atomic Structure and Isotopes
AdvancedAtoms are made of protons, neutrons and electrons. The atomic number equals the proton number; the mass number is protons plus neutrons. Isotopes share proton number but differ in neutron number, with almost identical chemistry.
- Atoms consist of protons (+1, mass 1), neutrons (0, mass 1) and electrons (−1, mass 11836); protons and neutrons sit in the nucleus
- Z = proton number; A = Z + N; neutrons = A − Z; in a neutral atom electrons = protons
- Read symbols top-left A, bottom-left Z, e.g. ²³₁₁Na
文體小知識：科學知識說明文——以符號和數據講解次原子粒子、原子序與質量數的關係，以及同位素的概念與計算。
§Text Passage
1An element is the family of atoms sharing the same proton number, and each element has its own symbol: hydrogen is H, carbon C, sodium Na. By properties, elements divide into metals, non-metals and metalloids: metals conduct electricity and heat and shine; non-metals mostly do not conduct; metalloids such as silicon sit between the two and are vital for making chips.
2An atom is built from three subatomic particles: the proton, with relative charge +1; the neutron, uncharged; and the electron, with relative charge −1. Protons and neutrons each have relative mass about 1 and crowd into the central nucleus; the electron's relative mass is about 11836 — negligible — yet it races through the shells outside the nucleus and dominates the atom's chemical behaviour.
3The atomic number Z equals the number of protons and fixes an element's place in the periodic table. The mass number A is the protons plus the neutrons: A = Z + N. In a neutral atom the electron number equals the proton number. Take sodium: atomic number 11 and mass number 23, written ²³₁₁Na — so the neutron count is 23 − 11 = 12, and the electron count is also 11.
4Atoms of one element can share a proton number but differ in neutron number: these are isotopes. Carbon has three common isotopes — ¹²C, ¹³C and ¹⁴C. Isotopes have almost identical chemical properties because their electron numbers and arrangements match; only their masses differ. Relative atomic mass uses ¹²C = 12.00 as the standard and is the abundance-weighted average of the isotopes — chlorine's 35.5 is the famous example.
1Summary 重點摘要
Atoms are made of protons, neutrons and electrons. The atomic number equals the proton number; the mass number is protons plus neutrons. Isotopes share proton number but differ in neutron number, with almost identical chemistry.
2Vocabulary cards Vocabulary
The smaller particles that make up an atom: protons, neutrons and electrons.
A subatomic particle of relative charge +1 and relative mass about 1; its number fixes which element the atom is.
An uncharged particle of relative mass about 1, found in the nucleus.
A particle of relative charge −1 and relative mass about 11836, moving in shells outside the nucleus.
The central core of protons and neutrons, holding nearly all of the atom's mass.
The proton number Z of an element, fixing its position in the periodic table.
The sum of the proton and neutron numbers: A = Z + N.
Atoms of the same element with the same proton number but different neutron numbers, e.g. ¹²C and ¹³C.
The abundance-weighted average mass of an element's isotopes on the ¹²C = 12.00 scale.
An element with properties between metals and non-metals, such as silicon and boron.
3Step by step Steps
- Step 1: Elements, atoms and symbolsAn element is the family of atoms with the same proton number, and each has its own symbol: hydrogen H, carbon C, sodium Na, chlorine Cl. For example, a bottle of oxygen, an iron nail and a piece of graphite are made of oxygen atoms, iron atoms and carbon atoms respectively — one element each. By properties, elements are metals, non-metals or metalloids: metals conduct electricity and heat and shine; non-metals like oxygen and sulfur mostly do not; metalloids like silicon lie between the two and are key to chips and solar panels.
- Step 2: The three subatomic particlesAtoms are not indivisible: they are built from protons, neutrons and electrons. A proton has relative charge +1 and relative mass about 1; a neutron has no charge and relative mass about 1; an electron has relative charge −1 and relative mass about 11836, essentially negligible. Picture an atom as a stadium: protons and neutrons crowd into a tiny ball at the centre — the nucleus — while electrons swirl around the outer stands like midges. Tiny as they are, these electrons dominate the atom's chemistry.
- Step 3: The atomic model and electrical neutralityThe simple atomic model draws a central nucleus with shells of electrons outside: protons are positive, neutrons neutral, electrons negative. In a neutral atom the electron number exactly equals the proton number, so the charges cancel. For example, carbon's atomic number is 6, so a neutral carbon atom has 6 protons and 6 electrons. If the electron number differs from the proton number, the particle is charged — an ion. That is the hook for the next lesson on ionic bonding; memorise the neutrality rule now.
- Step 4: Atomic number Z and mass number AThe atomic number Z is the proton number and decides which element the atom is; the mass number A is protons plus neutrons, A = Z + N, reflecting the atom's mass scale. Rearranged, the neutron number N = A − Z. For example, sodium has Z = 11 and A = 23, so it has 23 − 11 = 12 neutrons. Exams often give A and Z and ask for the neutron count, or the reverse; the formula must be second nature. The unit is simply 'particles' — never grams — and do not confuse A with the decimal relative atomic mass.
- Step 5: Reading symbols like ²³₁₁NaIn such symbols the mass number A sits at the top left and the atomic number Z at the bottom left. Read ²³₁₁Na: top left 23 is the mass number, bottom left 11 the atomic number — so a sodium atom has 11 protons, 12 neutrons and, when neutral, 11 electrons. For example, seeing ⁵⁶₂₆Fe you should instantly read 26 protons, mass number 56, neutrons 56 − 26 = 30, electrons 26. Never swap the positions of the two numbers; misplacing them is a classic mark-loser. Read positions first, then apply the formula.
- Step 6: Isotopes — same Z, different NIsotopes are atoms of the same element with the same proton number but different neutron numbers. Carbon has three common isotopes: ¹²C with 6 protons and 6 neutrons, ¹³C with 6 protons and 7 neutrons, and ¹⁴C with 6 protons and 8 neutrons. Think of three brothers with the same surname — all carbon, almost identical in chemistry — differing only in body weight (mass number). Isotopes share chemical properties because their electron numbers and arrangements are identical; only physical properties like mass differ. That contrast is the standard answer to analysis questions.
- Step 7: Isotopic masses and relative atomic massRelative atomic mass is measured on the ¹²C = 12.00 scale and is the abundance-weighted average of an element's isotopes. For example, chlorine consists of ³⁵Cl (about 75%) and ³⁷Cl (about 25%): relative atomic mass = (35 × 75 + 37 × 25) ÷ 100 = 35.5, which is why the periodic table lists chlorine as 35.5 rather than a whole number. Calculations give you isotopic masses and abundances and ask for the weighted average — multiply each mass by its percentage, add, and divide by 100. A non-integer answer is perfectly normal.
4Diagrams Diagrams
5Worked example Example
Isotope calculation: carbon's atomic number is 6, and nature holds three common isotopes — ¹²C, ¹³C and ¹⁴C. State the proton, neutron and electron numbers of each, and explain why the three share almost identical chemical properties.
- Read the question: every isotope of carbon has the proton number 6; only the neutron number — hence the mass number — differs
- Compute neutrons N = A − Z one by one: ¹²C gives 12 − 6 = 6; ¹³C gives 13 − 6 = 7; ¹⁴C gives 14 − 6 = 8
- Electrons: a neutral atom's electron number equals its proton number — 6 for all three
- Explain: chemical properties depend on the electron arrangement; all three share the same electron number and arrangement, so their chemistry is almost identical — only their masses differ
- Answer: The proton number of ¹²C, ¹³C and ¹⁴C is 6 in each case (carbon's atomic number), and each neutral atom carries 6 electrons. Neutron number = mass number − proton number: ¹²C has 12 − 6 = 6 neutrons; ¹³C has 13 − 6 = 7; ¹⁴C has 14 − 6 = 8. The three share almost identical chemical properties because chemical behaviour is governed by the electron arrangement, and all three isotopes have exactly the same electron number and arrangement. They differ only in neutron number and mass number — true isotopes of one element.
6Common mistakes Mistakes
7Quick check Quick check
Q1State the relative charge and relative mass of the three subatomic particles.
Q2How many protons, neutrons and electrons are in ²³₁₁Na?
Q3What is an isotope?
Q4Iron has atomic number 26 and mass number 56 — how many neutrons?
Q5Chlorine is 75% ³⁵Cl and 25% ³⁷Cl — what is its relative atomic mass?
8Recap Recap
- Atoms consist of protons (+1, mass 1), neutrons (0, mass 1) and electrons (−1, mass 11836); protons and neutrons sit in the nucleus
- Z = proton number; A = Z + N; neutrons = A − Z; in a neutral atom electrons = protons
- Read symbols top-left A, bottom-left Z, e.g. ²³₁₁Na
- Isotopes share Z but differ in N — almost identical chemistry, different masses
- Relative atomic mass uses the ¹²C = 12.00 scale and is abundance-weighted (chlorine 35.5)