What is an Atom?

Atom: Smallest unit of an element that retains properties

Structure:

• Nucleus: Center, contains protons and neutrons
• Electron cloud: Electrons orbit nucleus
• Mostly empty space

Size: Approximately 10⁻¹⁰ meters (1 Angstrom)

Subatomic Particles

Proton:

• Charge: +1
• Mass: 1 amu (atomic mass unit)
• Location: Nucleus

Neutron:

• Charge: 0 (neutral)
• Mass: 1 amu
• Location: Nucleus

Electron:

• Charge: -1
• Mass: ~0.0005 amu (negligible)
• Location: Electron cloud/orbitals

Atomic Number and Mass Number

Atomic Number (Z):

• Number of protons
• Defines the element
• Never changes for an element

Mass Number (A):

• Protons + Neutrons
• A = Z + N (where N = neutrons)

Notation: ᴬ𝑍X (e.g., ¹²₆C for Carbon-12)

Neutral Atoms

Neutral atom: Number of protons = Number of electrons

Charge balance:

• Total positive charge = Total negative charge
• (+Z) + (-Z) = 0

Example: Carbon has 6 protons, 6 electrons in neutral state

Ions

Ion: Atom that gained or lost electrons

Cation: Positive ion

• Lost electrons
• More protons than electrons
• Example: Na⁺ (11 protons, 10 electrons)

Anion: Negative ion

• Gained electrons
• More electrons than protons
• Example: Cl⁻ (17 protons, 18 electrons)

Exercises - Part 1

• What is an atom?
• ans: Smallest unit of element retaining its properties
• What are the three main subatomic particles?
• ans: Protons, neutrons, electrons
• Where are protons located?
• ans: In the nucleus
• What is the charge of a neutron?
• ans: Zero (neutral)

Exercises - Part 2

• What is atomic number?
• ans: Number of protons in nucleus, defines the element
• What is mass number?
• ans: Total protons plus neutrons, A = Z + N
• Carbon has 6 protons and 6 neutrons. What is its mass number?
• ans: A = 6 + 6 = 12 (Carbon-12)
• Does atomic number change in chemical reactions?
• ans: No, atomic number defines the element identity

Exercises - Part 3

• What makes an atom neutral?
• ans: Number of protons equals number of electrons
• Sodium has 11 protons. How many electrons in neutral sodium?
• ans: 11 electrons
• What is a cation?
• ans: Positive ion, atom that lost electrons
• What is an anion?
• ans: Negative ion, atom that gained electrons

Exercises - Part 4

• Oxygen atom gains 2 electrons. What is the ion?
• ans: O²⁻ (oxide ion, anion)
• Magnesium loses 2 electrons. What is the ion?
• ans: Mg²⁺ (magnesium ion, cation)
• An ion has 12 protons and 10 electrons. What is its charge?
• ans: +2 (12 - 10 = +2)
• Which particle determines element identity?
• ans: Proton (atomic number)

Exercises - Part 5

• Aluminum has atomic number 13 and mass number 27. How many neutrons?
• ans: N = A - Z = 27 - 13 = 14 neutrons
• An atom has 8 protons, 8 neutrons, 8 electrons. What element?
• ans: Oxygen (atomic number 8)
• What is its mass number?
• ans: A = 8 + 8 = 16 (Oxygen-16)
• Is this atom neutral or charged?
• ans: Neutral (protons = electrons)

Exercises - Part 6

• Why is atomic mass not exactly a whole number?
• ans: Weighted average of isotopes, includes fractional contributions
• Which particle has negligible mass?
• ans: Electron (~1/2000 of proton mass)
• Iron loses 3 electrons. Write the ion symbol
• ans: Fe³⁺ (iron(III) cation)
• Chlorine gains 1 electron. Write the ion symbol
• ans: Cl⁻ (chloride anion)

Exercises - Part 7

• What is mostly empty in an atom?
• ans: Space between nucleus and electron cloud
• Typical atomic diameter?
• ans: About 10⁻¹⁰ m (1 Angstrom)
• What holds electrons in orbit around nucleus?
• ans: Electrostatic attraction (opposite charges attract)
• Can neutrons change without changing element?
• ans: Yes, creates isotopes (same element, different mass)