History of Atomic Theory
Who was the first to propose that matter is made of tiny, indivisible particles called “atomos”?
Democritus
What are the three main subatomic particles?
Protons, neutrons, and electrons.
Define “isotope” in your own words.
Atoms of the same element with the same number of protons but different numbers of neutrons.
What type of ion forms when an atom loses electrons?
A cation (positive ion).
What is one mole equal to? (Number of particles)
6.022 × 10²³ particles (Avogadro’s number).
Which scientist proposed that atoms of the same element are identical and combine in whole-number ratios?
John Dalton
Which two subatomic particles are found in the nucleus?
Protons and neutrons
What stays the same between two isotopes of the same element?
Number of protons (and atomic number)
What is an ion?
An atom or group of atoms that has gained or lost electrons and now has a charge.
How many moles are in 36 g of H₂O? (Molar mass = 18 g/mol)
2.0 mol (36 ÷ 18).
What did Thomson’s cathode ray experiment help him discover about the atom?
The electron — a small, negatively charged particle.
If an atom has 11 protons, 11 electrons, and 12 neutrons, what is its mass number?
23 (11 + 12)
Chlorine has two isotopes: Cl-35 (75%) and Cl-37 (25%). Calculate its average atomic mass.
(35 × 0.75) + (37 × 0.25) = 35.5 amu.
Write the formula for magnesium chloride.
MgCl₂
Define “empirical formula.”
The simplest whole-number ratio of elements in a compound.
What did Rutherford’s gold foil experiment reveal about the structure of the atom?
Atoms are mostly empty space with a small, dense, positively charged nucleus.
A neutral sulfur atom has 16 protons and 16 electrons. It gains two electrons. What ion does it form, and what’s its new charge?
S²⁻ (sulfide ion).
Why is the atomic mass on the periodic table usually not a whole number?
It’s a weighted average of all the isotopes of that element.
What is the name of the compound Fe₂O₃?
Iron(III) oxide
Describe how atomic radius changes across a period and down a group.
Decreases across a period (more protons); increases down a group (more energy levels).
Explain how Bohr’s model improved on Rutherford’s atomic model.
Bohr proposed that electrons move in specific energy levels or orbits around the nucleus.
What happens to the size of an atom when it becomes a cation? Why?
It becomes smaller because it loses electrons, increasing nuclear pull on the remaining electrons.
A sample of element X has 20% X-10 and 80% X-11. Which isotope is more abundant and why?
X-11, because it has the higher percent abundance (80%).
Write the correct chemical formula for a compound made of Al³⁺ and SO₄²⁻. Show how you cross charges.
Al₂(SO₄)₃ — aluminum sulfate (cross: Al³⁺ → subscript 3, SO₄²⁻ → subscript 2).
Use Coulomb’s Law to explain why fluorine has a higher ionization energy than lithium.
Fluorine has more protons and a smaller radius → stronger nuclear attraction → harder to remove an electron.