The nuclear atom
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Where are protons and neutrons found?
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All Flashcards in Topic 1.2
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1.2.112 cards
Where are protons and neutrons found?
Together in the tiny, dense central **nucleus** of the atom.
Where are electrons found?
Moving around the nucleus in **shells** (energy levels); this region is mostly empty space.
Relative mass and charge of a proton?
Relative mass **1**, relative charge **+1**.
Relative mass and charge of a neutron?
Relative mass **1**, relative charge **0** (neutral).
Relative mass and charge of an electron?
Relative mass ≈ **1/1836** (negligible), relative charge **−1**.
What is the atomic number, Z?
The number of **protons** in the nucleus; it defines the element.
What is the mass number, A?
The number of **protons + neutrons** (nucleons) in the nucleus.
How do you find the number of neutrons?
**neutrons = A − Z** (mass number − atomic number).
Electrons in a neutral atom?
**electrons = protons = Z** — the + and − charges balance.
How do you find electrons in an ion?
Adjust electrons by the charge: **electrons = Z − charge** (lose e⁻ for +, gain e⁻ for −).
Read the symbol $^{A}_{Z}\text{X}$.
Top = **mass number A**, bottom = **atomic number Z**, X = element symbol.
What changes when an atom becomes an ion?
Only the **electron** count; the protons and neutrons stay the same.
1.2.28 cards
What is an isotope?
Atoms of the **same element** with the **same number of protons** but **different numbers of neutrons**.
Isotopes have the same Z but different what?
The same atomic number Z, but a **different mass number A** (because they have different numbers of neutrons).
Why do isotopes have identical chemical properties?
They have the **same number of electrons** and the **same electron arrangement**, and chemistry is controlled by the electrons.
Which properties of isotopes differ?
**Physical** properties that depend on mass — e.g. **density** and rate of **diffusion** — because of the different number of neutrons.
Neutrons in chlorine-37? (Z = 17)
37 − 17 = **20 neutrons**.
What is a radioisotope?
An isotope with an **unstable nucleus** that decays and gives out radiation; chemically it behaves like the stable isotope.
Give a use of a radioisotope.
**Carbon-14** for dating, **cobalt-60** for radiotherapy/sterilising, or **iodine-131** for treating the thyroid.
Do extra neutrons change how an atom bonds?
No — neutrons have **no charge** and do not affect the electrons, so **bonding and reactions are unchanged**.
1.2.311 cards
What is relative atomic mass, A_{r}?
The **weighted average** mass of an element's isotopes, relative to one-twelfth of a carbon-12 atom. It has **no units**.
Why is A_{r} usually not a whole number?
Because it averages **isotopes of different masses**, weighted by their **abundance** (e.g. Cl = 35.5).
What does a mass spectrometer do?
It separates the atoms/ions of a sample by **mass**, producing a **mass spectrum**.
What is on the axes of a mass spectrum?
**m/z** (mass-to-charge ratio) on the x-axis; **relative abundance** on the y-axis.
What does the m/z of a peak tell you?
For singly-charged ions, the **mass of that isotope**.
What does the height of a peak tell you?
The **relative abundance** of that isotope — the taller the peak, the more common the isotope.
How do you calculate A_{r} from a spectrum?
$A_{r} = \dfrac{\sum(\text{mass} \times \%\,\text{abundance})}{100}$ — weight each isotope mass by its abundance, sum, divide by 100.
How many peaks for an element with 3 isotopes?
**Three** peaks — one peak per isotope.
What if abundances are given as a ratio, not %?
Divide the weighted sum by the **total of the abundances** instead of by 100.
Sanity check on a calculated A_{r}?
It must lie **between** the lightest and heaviest isotope masses, closest to the **most abundant** one.
Is A_{r} = 35.5 a real chlorine atom's mass?
No — chlorine atoms are ³⁵Cl or ³⁷Cl; 35.5 is the **weighted average** (75% ³⁵Cl, 25% ³⁷Cl).
Topic 1.2 study notes
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