Atomic Structure And The Periodic Table Flashcards
(17 cards)
Describe how the model of the atom has changed over time.
• John Dalton (early 1800s): Atoms are tiny, indivisible solid spheres.
• J.J. Thomson (1897): Discovered electrons → Plum pudding model (a sphere of positive charge with embedded negative electrons).
• Ernest Rutherford (1909): Gold foil experiment → Most alpha particles passed through, but some deflected → Atom is mostly empty space, dense positive nucleus, electrons orbit nucleus.
• Niels Bohr (1913): Proposed electrons orbit nucleus in fixed energy levels (shells).
• James Chadwick (1932): Discovered the neutron → Explained isotopes (same element, different masses).
• Current accepted model: Nucleus contains protons and neutrons, electrons occupy shells.
What did Dalton, Thomson, Rutherford, Bohr and Chadwick each contribute to atomic theory?
• Dalton → Solid, indivisible spheres
• Thomson → Discovered electrons, Plum pudding model
• Rutherford → Nuclear model, dense nucleus, electrons orbit
• Bohr → Electrons in shells/energy levels
• Chadwick → Discovered neutron
What are the relative charges and masses of protons, neutrons and electrons?
• Proton: Charge +1, Mass 1
• Neutron: Charge 0, Mass 1
• Electron: Charge –1, Mass ~0 (1/1836)
Why is an atom electrically neutral?
Because it has equal numbers of protons (+1) and electrons (–1), so the overall charge cancels out.
Define atomic number and mass number.
• Atomic number = Number of protons
• Mass number = Number of protons + neutrons
How do isotopes of an element differ?
Isotopes have the same number of protons (same atomic number) but different numbers of neutrons (different mass numbers).
Define relative atomic mass (Ar).
The weighted average mass of the isotopes of an element, relative to 1/12th the mass of carbon-12.
How is relative atomic mass (Ar) calculated?
• Multiply each isotope’s mass number by its relative abundance.
• Add the results together.
• Divide by the sum of the abundances.
Formula:
Ar = (Isotope₁ mass × % abundance) + (Isotope₂ mass × % abundance) / 100
What are the rules for writing electronic configurations?
• Electrons occupy the lowest available energy level first.
• The maximum electrons per shell: 2 (1st), 8 (2nd), 8 (3rd).
• Write configuration as numbers separated by commas (e.g., 2,8,1).
How can you work out the electronic configuration of an element from its position in the periodic table?
• Period (row) = Number of occupied shells.
• Group (column) = Number of electrons in the outer shell (for Groups 1–7).
• Elements in Group 0 (Noble Gases) have full outer shells.
What are groups and periods in the periodic table?
• Groups = Vertical columns → Elements with similar chemical properties and same number of outer electrons.
• Periods = Horizontal rows → Elements with the same number of electron shells.
Describe Mendeleev’s contribution to the periodic table.
• Arranged elements in order of increasing atomic mass.
• Left gaps for undiscovered elements and predicted their properties.
• Re-ordered some elements so that those with similar properties stayed in the same group (ignoring strict atomic mass order).
• His predictions were later confirmed (e.g., gallium, germanium).
• His table became widely accepted because it could explain relationships between elements and predicted new elements successfully.
How is the modern periodic table arranged?
• By increasing atomic number (number of protons).
• Elements with similar properties in vertical groups.
• Metals on the left and non-metals on the right.
• Noble gases in Group 0 (far right).
Why was the discovery of isotopes important for the development of the periodic table?
It explained why some elements appeared to have the same atomic mass but different properties — isotopes have different masses but identical chemical properties.
Why is the modern periodic table more accurate than Mendeleev’s?
Because it is based on atomic number (proton count), not atomic mass, which removes inconsistencies caused by isotopes.
How do the properties of isotopes compare?
• Same chemical properties (because same number of electrons).
• Different physical properties (e.g., density, melting point).
How is the periodic table divided into sections
Groups show outer electron count and metals are on the left
