What atoms are made of, how we count their particles, and how they become ions.
Atom: the smallest unit of an element that keeps the element's properties.
Not to scale: if the atom were a stadium, the nucleus would be a marble at the centre.
| Particle | Charge | Mass | Location |
|---|---|---|---|
| Proton | +1 | 1 amu (atomic mass unit) | Nucleus |
| Neutron | 0 (neutral) | 1 amu | Nucleus |
| Electron | −1 | ~0.0005 amu (negligible) | Electron cloud / orbitals |
Notation: \( {}^{A}_{Z}\mathrm{X} \), e.g. \( {}^{12}_{\;6}\mathrm{C} \) for carbon-12
Neutral atom: number of protons = number of electrons.
Ion: an atom that has gained or lost electrons.
Answer each question before revealing it. The atom builder can check many of them.
What is an atom?
The smallest unit of an element that retains its properties
What are the three main subatomic particles?
Protons, neutrons, electrons
Where are protons located?
In the nucleus
What is the charge of a neutron?
Zero (neutral)
What is atomic number?
The number of protons in the nucleus; it defines the element
What is mass number?
Total protons plus neutrons, A = Z + N
Carbon has 6 protons and 6 neutrons. What is its mass number?
A = 6 + 6 = 12 (carbon-12)
Does atomic number change in chemical reactions?
No: atomic number defines the element's identity
What makes an atom neutral?
Number of protons equals number of electrons
Sodium has 11 protons. How many electrons does neutral sodium have?
11 electrons
What is a cation?
A positive ion: an atom that lost electrons
What is an anion?
A negative ion: an atom that gained electrons
An oxygen atom gains 2 electrons. What is the ion?
O²⁻ (oxide ion, an anion)
Magnesium loses 2 electrons. What is the ion?
Mg²⁺ (magnesium ion, a cation)
An ion has 12 protons and 10 electrons. What is its charge?
+2 (12 − 10 = +2)
Which particle determines element identity?
The proton (atomic number)
Aluminium has atomic number 13 and mass number 27. How many neutrons?
N = A − Z = 27 − 13 = 14 neutrons
An atom has 8 protons, 8 neutrons and 8 electrons. What element is it?
Oxygen (atomic number 8)
What is its mass number?
A = 8 + 8 = 16 (oxygen-16)
Is this atom neutral or charged?
Neutral (protons = electrons)
Why is atomic mass not exactly a whole number?
It is a weighted average over the element's isotopes, so it includes fractional contributions
Which particle has negligible mass?
The electron (about 1/2000 of a proton's mass)
Iron loses 3 electrons. Write the ion symbol.
Fe³⁺ (iron(III) cation)
Chlorine gains 1 electron. Write the ion symbol.
Cl⁻ (chloride anion)
What is mostly empty in an atom?
The space between the nucleus and the electron cloud
What is a typical atomic diameter?
About 10⁻¹⁰ m (1 ångström)
What holds electrons around the nucleus?
Electrostatic attraction (opposite charges attract)
Can the number of neutrons change without changing the element?
Yes: that creates isotopes (same element, different mass)