How many elements does the modern periodic table contain?
118 elements.
How are elements arranged in the modern periodic table?
In increasing atomic number.
Who grouped elements into triads in 1829?
Döbereiner.
What was the key property of Döbereiner's triads?
The middle element's atomic weight was roughly the average of the other two.
Who arranged 62 elements by increasing atomic mass in 1864?
John Newlands.
What pattern did John Newlands observe in 1864?
Every eighth element resembled the first in properties.
Who predicted undiscovered elements by leaving gaps in 1869?
Dmitri Mendeleev.
What method did Moseley use in 1913 to determine atomic numbers?
X-ray emission.
How many periods does the modern periodic table have?
Seven horizontal rows called periods.
How many groups are in the modern periodic table?
Eighteen vertical columns called groups.
Why do elements in the same group have similar chemical properties?
Because they have the same number of valence electrons.
How do physical properties change within a group from top to bottom?
They change gradually from top to bottom.
How do element properties change within a period?
They show a gradual change from left to right.
What ion do metals tend to form?
Positive ions by losing electrons.
What ion do nonmetals tend to form?
Negative ions by gaining electrons.
What properties do metalloids display?
Properties of both metals and nonmetals.
Which block contains elements with valence electrons in s subshells?
The s-block.
Which block contains transition elements?
The d-block.
Which groups does the p-block contain?
Groups 13 to 18.
Which elements are alkali metals?
Group 1 elements Li, Na, K, Rb, Cs, and Fr.
How many valence electrons do alkali metals have?
One valence electron.
Which group contains alkaline earth metals?
Group 2.
How reactive are alkaline earth metals compared to alkali metals?
They are less reactive than alkali metals.
Why are noble gases almost unreactive under normal conditions?
They have complete outer shells.
What does the period number indicate in the periodic table?
The number of electron shells.
What does the group number indicate for main-group elements?
The number of valence electrons.
Which element is in Period 3 and Group 2 with a 3s² valence configuration?
Magnesium.
How many electron shells does magnesium have?
Three shells.
How many valence electrons does magnesium have?
Two valence electrons.
How many valence electrons does an element in Group 13 have?
Three valence electrons.
Which shell contains the valence electrons for a Group 13, Period 3 element?
The third shell.
What is the electron configuration of a Group 13, Period 3 element?
1s² 2s² 2p⁶ 3s² 3p¹.
What is atomic radius defined as?
Half the distance between two identical bonded atoms.
In what units is atomic radius commonly measured?
Picometres or angstroms.
Why does atomic radius decrease from left to right across a period?
Increasing nuclear charge pulls the electron cloud closer.
Why does atomic radius increase down a group?
Additional shells increase shielding and enlarge the atom.
Why is a cation generally smaller than its neutral atom?
Electron loss removes a shell and strengthens nuclear attraction.
Why is an anion generally larger than its neutral atom?
Added electrons increase repulsion.
Why does ionization energy decrease down a group?
Because increasing atomic size and shielding make valence electrons easier to remove.
Why does ionization energy increase from left to right across a period?
Because effective nuclear charge increases.
What is the order of first ionization energy in Group 1 elements?
Li > Na > K > Rb > Cs.
How does spin-pair repulsion affect ionization energy?
It slightly lowers ionization energy by making paired electrons easier to remove.
What causes spin-pair repulsion to lower ionization energy?
Paired electrons repel one another.
What is the electron configuration of chromium?
[Ar] 3d⁵4s¹.
What is the electron configuration of manganese?
[Ar] 3d⁵4s².
Why is removing a paired 4s electron from manganese easier?
Because paired electrons experience repulsion, lowering ionization energy.
What is first electron affinity?
The enthalpy change when gaseous atoms gain electrons to form uninegative ions.
How is chlorine's first electron affinity reaction represented?
Cl(g) + e⁻ → Cl⁻(g) with ΔH°ea¹ = -348.8 kJ/mol.
What is oxygen's first electron affinity value?
ΔH°ea¹ = -142 kJ/mol.
Why is oxygen's second electron affinity positive?
Because the negative ion repels the incoming electron.
How does electron affinity change across a period?
It generally becomes more negative from left to right.
How does electron affinity change down a group?
It decreases as atomic size and nucleus distance increase.
What is electronegativity?
The power of an atom to attract shared electrons in a molecule.
What values does the Pauling scale assign to fluorine and alkali metals?
Fluorine has 4.0 and alkali metals have 0.8.
What is the reaction formula for sodium with water?
2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
What product forms when sodium burns in oxygen?
Sodium peroxide (Na2O2) forms.
What can limited oxygen or high temperature produce when sodium burns?
Sodium oxide can be produced.
What is the formula for sodium reacting with chlorine?
2Na(s) + Cl2(g) → 2NaCl(s)
What is the formula for magnesium reacting with chlorine?
Mg(s) + Cl2(g) → MgCl2(s)
How do oxides and chlorides of Groups 1–3 differ from those of Groups 4–7?
Groups 1–3 compounds are predominantly ionic, Groups 4–7 are more covalent.
Why are oxides and chlorides of Groups 4–7 more covalent?
Because electronegativity increases across the period.
What does a basic oxide produce when reacting with water?
An alkali.
What type of element oxides are usually basic oxides?
Group 1 and Group 2 metal oxides.
What does an acidic oxide produce when reacting with water?
An acid.
What type of oxides are generally acidic oxides?
Covalent nonmetal oxides.
What is an amphoteric oxide?
An oxide that reacts with both acids and bases.
How do Group 1 and Group 2 chlorides behave in water compared to chlorides from aluminium to sulfur?
Group 1 and 2 chlorides are neutral; aluminium to sulfur chlorides produce acidic solutions.
Why is the oxidation number of a Period 3 element in oxides or chlorides positive?
Because oxygen and chlorine are more electronegative than Period 3 elements.
How does the oxidation number change across Period 3 oxides and chlorides?
It increases from +1 in sodium to +6 in sulfur for oxides, and to +5 in phosphorus for chlorides.
Test your knowledge with 28 questions on Periodic Table and Periodic Trends.
1. Which principle determines the order of elements in the modern periodic table?
2. What significant contribution did Dmitri Mendeleev make in 1869?
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