What are the three subatomic particles that make up an atom?
Protons, neutrons, and electrons.
What is the charge and relative mass of a proton?
Proton: positive charge (+1), relative mass of 1 atomic mass unit.
What is the charge and relative mass of a neutron?
Neutron: no charge (0), relative mass of 1 atomic mass unit.
What is the charge and relative mass of an electron?
Electron: negative charge (-1), relative mass approximately 1/1836 of a proton (essentially negligible).
Where is most of an atom's mass located?
More than 99.94% of an atom's mass is in the nucleus.
What is the nucleus of an atom composed of?
The nucleus is composed of protons and neutrons (except hydrogen-1, which has only a proton).
What holds the protons and neutrons together in the nucleus?
The nuclear force, which is stronger than the electromagnetic repulsion between protons.
What determines the identity of an element?
The number of protons in the atom. For example, any atom with 11 protons is sodium.
What are isotopes?
Atoms of the same element (same number of protons) but with different numbers of neutrons.
What is an ion?
An atom that has gained or lost electrons, giving it a net electric charge.
What is a cation?
A positively charged ion, formed when an atom loses electrons (more protons than electrons).
What is an anion?
A negatively charged ion, formed when an atom gains electrons (more electrons than protons).
Why are atoms electrically neutral under normal conditions?
Because they have equal numbers of protons and electrons, balancing positive and negative charges.
What force binds electrons to the nucleus?
The electromagnetic force: opposite charges attract (positive nucleus attracts negative electrons).
What is the approximate size of an atom?
Around 100 picometers (1 × 10⁻¹⁰ m) in diameter.
Why can't atoms be seen with conventional microscopes?
Atoms are smaller than the shortest wavelength of visible light, so they cannot be resolved by light microscopes.
What is the origin of the word 'atom'?
From the ancient Greek word 'atomos', meaning 'uncuttable'.
How did John Dalton contribute to atomic theory?
He provided experimental evidence that matter is composed of discrete units (atoms) and formulated the law of multiple proportions.
State the law of multiple proportions as discovered by Dalton.
When two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in ratios of small whole numbers.
Give an example of the law of multiple proportions using tin oxides.
In grey tin oxide (SnO), 100 g of tin combines with about 13.5 g of oxygen; in white tin oxide (SnO₂), 100 g of tin combines with about 27 g of oxygen. The oxygen masses are in a 1:2 ratio.
What is the formula of iron(II) oxide and iron(III) oxide?
Iron(II) oxide is FeO; iron(III) oxide is Fe₂O₃.
What are the formulas for nitrous oxide, nitric oxide, and nitrogen dioxide?
Nitrous oxide: N₂O; nitric oxide: NO; nitrogen dioxide: NO₂.
What happens when the repelling electromagnetic force in the nucleus becomes stronger than the nuclear force?
The nucleus can split (nuclear decay), leaving behind different elements.