Atoms, Elements, and the Periodic Table
Core facts about atomic structure, electron shells, and reading groups and periods on the periodic table.
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Questions Covered in This Set
10 cards to master
What is an element?
A substance made of only one type of atom, defined by its number of protons (atomic number).
Give the charge and mass of protons, neutrons, and electrons.
Proton: charge +1, mass 1. Neutron: charge 0, mass 1. Electron: charge −1, mass ≈ 1/1836. Protons and neutrons sit in the nucleus; electrons occupy shells.
How do you calculate the number of neutrons in an atom?
Neutrons = mass number (A) − atomic number (Z). E.g. for ²³₁₁Na: 23 − 11 = 12 neutrons.
What are isotopes, and why is chlorine's relative atomic mass 35.5?
Isotopes are atoms of the same element (same Z) with different neutron numbers. Chlorine is ~75% Cl-35 and ~25% Cl-37: (0.75×35)+(0.25×37) = 35.5.
How do ions form?
Atoms lose electrons to form positive ions (cations, e.g. Na→Na⁺) or gain electrons to form negative ions (anions, e.g. Cl→Cl⁻). Proton number never changes.
What is the electron shell filling rule for the first 20 elements?
Shells fill from the inside out holding a maximum of 2, 8, 8 (then 2), e.g. calcium (Z=20) is 2,8,8,2.
How do you find group and period from an electronic configuration?
Number of outer-shell electrons = group number (main groups); number of shells = period number. Sodium 2,8,1 → Group 1, Period 3.
Describe Group 1 (alkali metals) reactivity and their reaction with water.
One outer electron, easily lost, so very reactive metals; reactivity increases down the group. 2Na + 2H₂O → 2NaOH + H₂.
Describe Group 7 (halogens) behaviour.
Seven outer electrons, gain one to form −1 ions; reactivity decreases down the group. A more reactive halogen displaces a less reactive one: Cl₂ + 2KBr → 2KCl + Br₂.
Name key properties of transition metals.
Central block (Fe, Cu, Zn): hard, dense, high melting points, often coloured compounds, useful catalysts, can form ions with more than one charge (Fe²⁺, Fe³⁺).
