# Atomic Models and Isotopes

Chemistry I · Atoms, Bonds and Reaction Rates · https://tryals.app/learn/chemistry-i/atomic-models-and-isotopes

## Building the Atom from Evidence

Every atomic model was killed by an experiment it could not explain. **Dalton** (1803) treated atoms as indivisible spheres, which failed the moment **Thomson** (1897) pulled electrons out of them with cathode rays, measuring a charge-to-mass ratio nearly 2000 times larger than any ion. Thomson's "plum pudding" spread positive charge thinly through the whole atom.

That model died in **Rutherford's** gold-foil experiment (1911). Firing alpha particles at foil, almost all passed straight through, but roughly 1 in 8000 bounced back. Diffuse charge cannot repel a fast alpha particle; only a dense, tiny, positive **nucleus** can. Rutherford put essentially all the mass into a volume about $10^{-5}$ times the atom's diameter. **Chadwick** (1932) completed the picture with the neutron.

| Particle | Charge | Mass (u) | Location |
|---|---|---|---|
| Proton | $+1$ | $1.007$ | Nucleus |
| Neutron | $0$ | $1.009$ | Nucleus |
| Electron | $-1$ | $0.00055$ | Orbitals |

Two numbers identify a nucleus: the **atomic number** $Z$ (protons, this alone fixes the element) and the **mass number** $A$ (protons plus neutrons). **Isotopes** are atoms of one element with different $A$, so they differ in neutron count and mass but share $Z$ and therefore essentially all chemistry.

The atomic mass on the periodic table is an **abundance-weighted average**, not any one isotope's mass:

$$M = \sum f_i m_i$$

Chlorine is 75.77 % $^{35}$Cl and 24.23 % $^{37}$Cl, giving $0.7577 \times 35 + 0.2423 \times 37 = 35.48$ u, which is why no chlorine atom weighs what the table says.

> **Common pitfall:** reading the periodic table's mass as the mass of one atom. It is a population average; an individual atom always has a near-integer mass number, and the fractional table value tells you about isotope *abundances*, not about any single atom.

## Practice questions

9 of this lesson's 13 practice questions, with answers. The full set is in the app.

### 1. Almost all alpha particles passed straight through the gold foil, but about 1 in 8000 rebounded. What does the rebounding minority establish?

A. Atoms are completely solid and indivisible throughout their volume
B. Positive charge is thinly and evenly spread throughout the atom
C. Electrons carry most of the atomic mass in diffuse outer shells
D. Positive charge and mass are concentrated in a tiny nucleus

**Answer:** D. Positive charge and mass are concentrated in a tiny nucleus

**Why:** A thinly spread positive charge exerts a weak force everywhere and could never reverse a fast alpha particle. Only a small, dense, highly charged nucleus can, and the rarity of the rebound shows how tiny that target is.

Page: https://tryals.app/practice/chemistry-i/atomic-models-and-isotopes/almost-all-alpha-particles-passed-straight-through-the-gold-foil-but

### 2. An isotope has mass number $A = 56$ and atomic number $Z = 26$. How many neutrons does its nucleus contain?

**Answer:** 30

**Why:** Neutrons $= A - Z = 56 - 26 = 30$. This is iron-56, the most tightly bound nucleus per nucleon.

Page: https://tryals.app/practice/chemistry-i/atomic-models-and-isotopes/an-isotope-has-mass-number-a-56-and-atomic-number-z-26-how-many

### 3. Two isotopes of the same element show almost identical chemical behaviour.

**Answer:** True

**Why:** True, chemical behaviour is set by the electron arrangement, which depends on $Z$. Isotopes share $Z$, so they differ measurably in mass and nuclear stability but barely at all in chemistry.

Page: https://tryals.app/practice/chemistry-i/atomic-models-and-isotopes/two-isotopes-of-the-same-element-show-almost-identical-chemical

### 4. Chlorine is 75.77 % chlorine-35 (mass 35 u) and 24.23 % chlorine-37 (mass 37 u). Compute its average atomic mass in u, to two decimal places.

**Answer:** 35.48 (within ±0.03)

**Why:** $0.7577 \times 35 + 0.2423 \times 37 = 26.52 + 8.97 = 35.48$ u. The value sits near 35 because the lighter isotope dominates the population.

Page: https://tryals.app/practice/chemistry-i/atomic-models-and-isotopes/chlorine-is-75-77-chlorine-35-mass-35-u-and-24-23-chlorine-37

### 5. Match each subatomic particle to its defining property.

**Answer:**

- Proton → Positive, in the nucleus, fixes the element
- Neutron → Neutral, in the nucleus, changes the isotope
- Electron → Negative, outside the nucleus, drives chemistry
- Alpha particle → A helium nucleus used as a probe

**Why:** The proton count $Z$ names the element; changing neutrons changes the isotope; electrons determine bonding; the alpha particle is a helium nucleus, used by Rutherford as a probe.

Page: https://tryals.app/practice/chemistry-i/atomic-models-and-isotopes/match-each-subatomic-particle-to-its-defining-property

### 6. Which statements about isotopes are correct?

A. They have different numbers of electrons when neutral
B. They have the same number of protons
C. They have different numbers of neutrons
D. They occupy the same position on the periodic table

**Answer:** B. They have the same number of protons; C. They have different numbers of neutrons; D. They occupy the same position on the periodic table

**Why:** Isotopes share $Z$, so they share proton count, position on the table, and, when neutral, electron count too. Only the neutron count, and therefore the mass, differs.

Page: https://tryals.app/practice/chemistry-i/atomic-models-and-isotopes/which-statements-about-isotopes-are-correct

### 7. The periodic table lists the atomic mass of chlorine as 35.45 u, yet no single chlorine atom possesses this mass. What does this distinction imply when calculating macroscopic versus microscopic quantities of matter?

A. The tabular value applies only to bulk samples containing isotopic mixtures
B. Individual atoms possess non-integer masses due to nuclear binding energy
C. Stoichiometry on real samples requires identifying specific individual isotopes
D. The listed atomic mass represents an unweighted arithmetic mean across isotopes

**Answer:** A. The tabular value applies only to bulk samples containing isotopic mixtures

**Why:** Non-integer masses in bulk data arise strictly from mixed populations of discrete isotopes rather than fractional nucleons. Macroscopic measurements reflect these statistical blends, whereas single-atom interactions depend entirely on specific, integer-valued isotope masses.

Page: https://tryals.app/practice/chemistry-i/atomic-models-and-isotopes/the-periodic-table-lists-the-atomic-mass-of-chlorine-as-35-45-u-yet

### 8. Why could Thomson's plum-pudding model not survive the gold-foil result?

A. It relied on neutral particles that had not yet been identified experimentally
B. It predicted that neutral atoms would inevitably carry a net negative charge
C. It treated the entire atom as an indivisible sphere with no internal electrons
D. It spread positive charge too thinly to deflect an alpha particle backwards

**Answer:** D. It spread positive charge too thinly to deflect an alpha particle backwards

**Why:** Plum pudding did contain both charges; its failure was the *distribution*. Charge smeared over the whole atom produces only gentle deflections, never the large-angle rebounds Rutherford measured.

Page: https://tryals.app/practice/chemistry-i/atomic-models-and-isotopes/why-could-thomsons-plum-pudding-model-not-survive-the-gold-foil

### 9. Place these atomic models and discoveries in chronological order.

**Answer:**

1. Dalton's indivisible atom
2. Thomson's discovery of the electron
3. Rutherford's nuclear atom
4. Chadwick's discovery of the neutron

**Why:** Dalton (1803) preceded any subatomic particle; Thomson found the electron (1897); Rutherford inferred the nucleus (1911); Chadwick identified the neutron (1932).

Page: https://tryals.app/practice/chemistry-i/atomic-models-and-isotopes/place-these-atomic-models-and-discoveries-in-chronological-order