I85PREP
⌘ K
Armenia Chemistry Unified Exam study materials
Armenia Chemistry Unified Exam study resources

Armenia Chemistry Unified Exam practice questions

One question at a time. Build your Armenia Chemistry Unified Exam understanding.

First 50 of 197 questions. Each one lists the choices, the correct answer, and an explanation. The checker at the top of this page saves progress in this browser when it is available.

Continue in the practice tool above to load more questions, 50 at a time.

Question 1Atomic Structure and the Periodic Law

How many protons does an atom of chlorine-37 contain?

  1. 20
  2. 37
  3. 17 Correct answer.
  4. 18

Explanation

The atomic number of chlorine is 17, and the atomic number equals the number of protons in the nucleus. The mass number 37 counts protons plus neutrons, so it does not change the proton count.

Question 2Atomic Structure and the Periodic Law

Which particle has no electric charge?

  1. Proton
  2. Electron
  3. Ion
  4. Neutron Correct answer.

Explanation

Neutrons are electrically neutral because they carry no net charge. Protons are positive and electrons are negative, so only the neutron is uncharged.

Question 3Atomic Structure and the Periodic Law

What is the electron configuration of a neutral magnesium atom (Z = 12)?

  1. 1s² 2s² 2p⁶ 3s¹
  2. 1s² 2s² 2p⁶ 3p²
  3. 1s² 2s² 2p⁸ 3s²
  4. 1s² 2s² 2p⁶ 3s² Correct answer.

Explanation

Filling orbitals in order of increasing energy gives 1s, 2s, 2p, then 3s, and twelve electrons fill these to give 1s² 2s² 2p⁶ 3s². The 2p subshell holds at most six electrons.

Question 4Atomic Structure and the Periodic Law

Which statement describes an isotope of an element?

  1. Atoms with the same mass number but different proton numbers
  2. Atoms with the same proton number but different neutron numbers Correct answer.
  3. Atoms with the same neutron number but different proton numbers
  4. Atoms with the same electron number but different proton numbers

Explanation

Isotopes are atoms of the same element, meaning identical proton number (atomic number), that differ in neutron number and therefore in mass number.

Question 5Atomic Structure and the Periodic Law

In the modern periodic table, elements are arranged primarily in order of increasing what?

  1. Neutron number
  2. Atomic number Correct answer.
  3. Density
  4. Atomic mass

Explanation

The modern periodic law orders elements by increasing nuclear charge, that is, atomic number. Atomic mass was used in early tables but fails to place some elements correctly.

Question 6Atomic Structure and the Periodic Law

Which group of the periodic table contains the noble gases?

  1. Group 1
  2. Group 17
  3. Group 18 Correct answer.
  4. Group 2

Explanation

The noble gases occupy Group 18, the far-right column, and have full outer electron shells. Their filled octet makes them very unreactive.

Question 7Atomic Structure and the Periodic Law

Which quantum number determines the shape of an atomic orbital?

  1. Angular momentum quantum number l Correct answer.
  2. Principal quantum number n
  3. Spin quantum number mₛ
  4. Magnetic quantum number mₗ

Explanation

The angular momentum quantum number l defines the subshell type (s, p, d, f) and hence the orbital shape. The principal quantum number sets size and energy level instead.

Question 8Atomic Structure and the Periodic Law

An atom of an element has the electron configuration 1s² 2s² 2p⁶ 3s² 3p⁴. In which period and group is the element found?

  1. Period 3, Group 14
  2. Period 2, Group 16
  3. Period 4, Group 6
  4. Period 3, Group 16 Correct answer.

Explanation

The highest occupied shell is n = 3, so the element is in period 3. The outer configuration 3s² 3p⁴ gives six valence electrons, placing it in Group 16.

Question 9Atomic Structure and the Periodic Law

How many unpaired electrons are present in a ground-state nitrogen atom (Z = 7)?

  1. 4
  2. 3 Correct answer.
  3. 1
  4. 2

Explanation

The configuration is 1s² 2s² 2p³. By Hund's rule the three 2p electrons occupy separate orbitals with parallel spins, so all three are unpaired.

Question 10Atomic Structure and the Periodic Law

Which series correctly shows the first ionisation energies in increasing order?

  1. Al < Na < Mg < Si
  2. Si < Al < Mg < Na
  3. Na < Al < Mg < Si Correct answer.
  4. Na < Mg < Al < Si

Explanation

Ionisation energy generally rises across a period, but magnesium is higher than aluminium because removing an electron from Mg's filled 3s orbital is harder than removing Al's single 3p electron. The correct increasing order is Na < Al < Mg < Si.

Question 11Atomic Structure and the Periodic Law

Which ion has the electron configuration 1s² 2s² 2p⁶?

  1. Cl⁻
  2. Mg²⁺
  3. Na⁺ Correct answer.
  4. O²⁻

Explanation

A sodium atom (Z = 11) loses one electron to form Na⁺, leaving ten electrons arranged as 1s² 2s² 2p⁶, the neon configuration.

Question 12Atomic Structure and the Periodic Law

Which property increases from left to right across a period?

  1. Metallic character
  2. Atomic radius
  3. Electronegativity Correct answer.
  4. Number of neutron shells

Explanation

Nuclear charge grows across a period while electrons enter the same shell, so the attraction for bonding electrons increases and electronegativity rises. Atomic radius and metallic character both decrease across a period.

Question 13Atomic Structure and the Periodic Law

Which element has the largest atomic radius?

  1. Na
  2. K
  3. Rb Correct answer.
  4. Li

Explanation

All four are Group 1 metals, and atomic radius increases down a group because each element adds a new electron shell. Rubidium lies lowest in the group, so it has the largest radius.

Question 14Atomic Structure and the Periodic Law

What is the maximum number of electrons that can occupy the 3d subshell?

  1. 6
  2. 2
  3. 10 Correct answer.
  4. 14

Explanation

A d subshell has five orbitals, and each orbital holds two electrons, giving a maximum of ten electrons.

Question 15Atomic Structure and the Periodic Law

Which is the correct ground-state electron configuration of a chromium atom (Z = 24)?

  1. [Ar] 4s¹ 3d⁵ Correct answer.
  2. [Ar] 4s⁰ 3d⁶
  3. [Ar] 4s² 3d⁴
  4. [Ar] 4s² 3d⁶

Explanation

Chromium is an exception to the aufbau order: a half-filled 3d subshell with a single 4s electron gives extra stability. The configuration is therefore [Ar] 4s¹ 3d⁵.

Question 16Atomic Structure and the Periodic Law

The first four ionisation energies of an element are 578, 1817, 2745 and 11575 kJ/mol. To which group does the element most likely belong?

  1. Group 13 Correct answer.
  2. Group 14
  3. Group 1
  4. Group 2

Explanation

The huge jump occurs between the third and fourth ionisation energies, meaning three electrons are removed relatively easily before a stable inner shell is breached. An atom with three valence electrons belongs to Group 13.

Question 17Atomic Structure and the Periodic Law

Which species is isoelectronic with the argon atom?

  1. Ca²⁺
  2. Na⁺
  3. K⁺
  4. Cl⁻ Correct answer.

Explanation

Argon has eighteen electrons. A chloride ion gains one electron to a chlorine atom (17), giving eighteen electrons, so it is isoelectronic with argon.

Question 18Atomic Structure and the Periodic Law

Which factor best explains why the first ionisation energy of fluorine is greater than that of chlorine?

  1. Fluorine has a higher mass number than chlorine
  2. Fluorine has more neutrons than chlorine
  3. Fluorine has a larger atomic radius than chlorine
  4. Fluorine's outer electron is in a shell closer to the nucleus Correct answer.

Explanation

Fluorine's valence electron is in the n = 2 shell, closer to the nucleus and less shielded than chlorine's n = 3 electron. The stronger effective nuclear attraction makes the electron harder to remove.

Question 19Atomic Structure and the Periodic Law

Which set of quantum numbers (n, l, mₗ, mₛ) is allowed for an electron in an atom?

  1. (1, 1, 0, +½)
  2. (2, −1, 1, +½)
  3. (2, 2, 0, +½)
  4. (3, 1, −1, −½) Correct answer.

Explanation

For n = 3, l may be 0, 1, or 2, and for l = 1 the magnetic quantum number may be −1, 0, or +1, so (3, 1, −1, −½) is permitted. The other sets violate the rule that l is less than n or that mₗ lies between −l and +l.

Question 20Atomic Structure and the Periodic Law

Which statement correctly describes the trend in atomic radius down Group 17 (F to I)?

  1. Radius increases because the number of protons decreases
  2. Radius stays constant because the valence configuration is unchanged
  3. Radius increases because new electron shells are added Correct answer.
  4. Radius decreases because nuclear charge increases

Explanation

Going down a group adds a principal shell each time, placing valence electrons farther from the nucleus. This increase in shell number outweighs the growing nuclear charge, so the radius increases.

Question 21Atomic Structure and the Periodic Law

Which element is expected to have the highest first ionisation energy?

  1. Na
  2. Ne Correct answer.
  3. Ar
  4. K

Explanation

Among these, neon has a small radius and a full n = 2 shell, so its electrons are tightly held and hard to remove. Argon is also a noble gas but its valence electrons lie in a larger, more shielded n = 3 shell, giving lower ionisation energy than neon.

Question 22Atomic Structure and the Periodic Law

An atom has 16 protons and 18 neutrons. What are its mass number and its electron configuration?

  1. 32 and 1s² 2s² 2p⁶ 3s² 3p²
  2. 34 and 1s² 2s² 2p⁶ 3s² 3p⁶
  3. 16 and 1s² 2s² 2p⁶ 3s² 3p⁴
  4. 34 and 1s² 2s² 2p⁶ 3s² 3p⁴ Correct answer.

Explanation

The mass number is the sum of protons and neutrons, 16 + 18 = 34. With 16 electrons the neutral atom fills to 1s² 2s² 2p⁶ 3s² 3p⁴.

Question 23Atomic Structure and the Periodic Law

Which of the following correctly lists the elements in order of increasing electronegativity?

  1. Cl < F < Br < I
  2. I < Br < Cl < F Correct answer.
  3. F < Cl < Br < I
  4. Br < I < F < Cl

Explanation

Electronegativity decreases down a group as the valence shell moves farther from the nucleus and shielding increases. Within Group 17 the order of increasing electronegativity is therefore I < Br < Cl < F.

Question 24Atomic Structure and the Periodic Law

Which of these atoms has the electron configuration ending in 4s¹ 3d¹⁰?

  1. Zn
  2. K
  3. Cu Correct answer.
  4. Ni

Explanation

Copper (Z = 29) is an exception to the aufbau order: a fully filled 3d subshell with a single 4s electron is more stable, giving [Ar] 4s¹ 3d¹⁰.

Question 25Chemical Bonding and Structure of Matter

Which type of bonding is present in solid potassium chloride?

  1. Hydrogen
  2. Covalent
  3. Metallic
  4. Ionic Correct answer.

Explanation

Potassium chloride is formed by electron transfer from a metal (K) to a nonmetal (Cl), producing oppositely charged ions held together by electrostatic attraction. This electrostatic attraction between ions is an ionic bond.

Question 26Chemical Bonding and Structure of Matter

What is the shape of a methane (CH4) molecule according to VSEPR theory?

  1. Trigonal planar
  2. Bent
  3. Square planar
  4. Tetrahedral Correct answer.

Explanation

The central carbon has four bonding pairs and no lone pairs, so the electron pairs arrange themselves as far apart as possible in a tetrahedral geometry. Four identical bond pairs give a tetrahedral molecular shape with bond angles of about 109.5 degrees.

Question 27Chemical Bonding and Structure of Matter

Which intermolecular force is primarily responsible for the relatively high boiling point of water compared with hydrogen sulfide?

  1. Hydrogen bonding Correct answer.
  2. London dispersion forces
  3. Dipole-dipole attraction
  4. Ion-dipole attraction

Explanation

Water molecules form hydrogen bonds because hydrogen is bonded to a highly electronegative oxygen atom with lone pairs. These strong intermolecular hydrogen bonds require more energy to overcome, giving water a higher boiling point than H2S, which lacks hydrogen bonding.

Question 28Chemical Bonding and Structure of Matter

An atom of element X has the electron configuration 1s2 2s2 2p6 3s2 3p6 4s2. Which ion is it most likely to form?

  1. X2+ Correct answer.
  2. X2-
  3. X-
  4. X+

Explanation

The atom has two electrons in its outermost 4s subshell, which it tends to lose to achieve a stable noble-gas configuration. Losing two electrons produces a doubly positive ion, X2+.

Question 29Chemical Bonding and Structure of Matter

Which of the following molecules is nonpolar despite containing polar bonds?

  1. Carbon dioxide Correct answer.
  2. Ammonia
  3. Hydrogen chloride
  4. Water

Explanation

In CO2 the two polar C=O bonds are arranged linearly and point in opposite directions, so their dipole moments cancel. A molecule with polar bonds can be nonpolar if its geometry makes the bond dipoles sum to zero.

Question 30Chemical Bonding and Structure of Matter

The bond in a hydrogen molecule (H2) is best described as:

  1. A single covalent bond Correct answer.
  2. A coordinate covalent bond
  3. An ionic bond
  4. A double covalent bond

Explanation

Each hydrogen atom contributes one electron to form one shared electron pair, giving a single covalent bond. The shared pair is attracted to both nuclei, holding the atoms together.

Question 31Chemical Bonding and Structure of Matter

Which property is characteristic of substances with a giant covalent (network) structure such as diamond?

  1. Very high melting point and hardness Correct answer.
  2. Formation of ions in aqueous solution
  3. Good electrical conductivity as a solid
  4. Low melting point and softness

Explanation

In a giant covalent structure, atoms are joined by a continuous network of strong covalent bonds throughout the crystal. Breaking the solid requires breaking many strong covalent bonds, giving very high melting points and great hardness.

Question 32Chemical Bonding and Structure of Matter

How many lone pairs of electrons are on the central atom of a water molecule, and what is the resulting shape?

  1. One lone pair; bent
  2. Two lone pairs; bent Correct answer.
  3. Two lone pairs; linear
  4. No lone pairs; tetrahedral

Explanation

Oxygen in water has six valence electrons, two of which form bonds with hydrogen, leaving two lone pairs. Four electron domains (two bonds and two lone pairs) give a tetrahedral electron arrangement, but the molecular shape is bent.

Question 33Chemical Bonding and Structure of Matter

Which of the following correctly orders the bond types by increasing polarity between the given atoms?

  1. H-F < H-Cl < C-H
  2. C-H < H-Cl < H-F Correct answer.
  3. C-H < H-F < H-Cl
  4. H-Cl < C-H < H-F

Explanation

Bond polarity increases with the difference in electronegativity between the bonded atoms. The electronegativity difference is smallest for C-H, intermediate for H-Cl, and largest for H-F.

Question 34Chemical Bonding and Structure of Matter

Why does magnesium have a higher melting point than sodium?

  1. Magnesium has a larger atomic radius than sodium
  2. Magnesium has weaker London dispersion forces
  3. Magnesium forms covalent bonds between atoms
  4. Magnesium atoms donate two delocalised electrons per atom, giving stronger metallic bonding Correct answer.

Explanation

Both are metals, but magnesium contributes two electrons per atom to the delocalised electron sea, while sodium contributes one. More delocalised electrons give stronger electrostatic attraction between the cations and the electron sea, raising the melting point.

Question 35Chemical Bonding and Structure of Matter

Which of the following species has a trigonal planar shape?

  1. NH3
  2. BF3 Correct answer.
  3. CH4
  4. H2O

Explanation

Boron in BF3 has three bonding pairs and no lone pairs, so the three electron domains spread out in a plane at 120 degrees. This gives a trigonal planar shape.

Question 36Chemical Bonding and Structure of Matter

A substance conducts electricity when molten but not when solid, and has a high melting point. Which structure is most consistent with these observations?

  1. An ionic lattice Correct answer.
  2. A simple molecular solid
  3. A giant covalent network
  4. A molecular solid with hydrogen bonding

Explanation

Ionic solids have high melting points because of strong electrostatic forces in the lattice, and they conduct only when molten because the ions become free to move. In the solid state the ions are fixed and cannot carry current.

Question 37Chemical Bonding and Structure of Matter

Which statement correctly describes the number of sigma and pi bonds in a molecule of ethene (C2H4)?

  1. Six sigma bonds and no pi bonds
  2. Five sigma bonds and one pi bond Correct answer.
  3. Five sigma bonds and two pi bonds
  4. Four sigma bonds and two pi bonds

Explanation

Ethene has four C-H single bonds and one C-C sigma bond, giving five sigma bonds. The C=C double bond consists of one sigma and one pi bond, so there is exactly one pi bond.

Question 38Chemical Bonding and Structure of Matter

Which of the following best explains why the boiling point of ethanol is higher than that of dimethyl ether, despite both having the formula C2H6O?

  1. Ethanol molecules can form hydrogen bonds with each other, whereas dimethyl ether cannot Correct answer.
  2. Dimethyl ether has stronger London dispersion forces
  3. Ethanol has a larger molar mass than dimethyl ether
  4. Ethanol is ionic while dimethyl ether is covalent

Explanation

Ethanol contains an O-H group, so its molecules can hydrogen bond to one another, requiring more energy to separate. Dimethyl ether has no O-H bond, so it cannot form hydrogen bonds between its molecules and boils at a lower temperature.

Question 39Chemical Bonding and Structure of Matter

Which of the following pairs of ions is isoelectronic with the argon atom?

  1. Na+ and Cl-
  2. K+ and Cl- Correct answer.
  3. Ca2+ and F-
  4. Mg2+ and O2-

Explanation

Argon has 18 electrons. K+ has 19 - 1 = 18 electrons and Cl- has 17 + 1 = 18 electrons, so both are isoelectronic with argon.

Question 40Chemical Bonding and Structure of Matter

Which of the following molecules has a net dipole moment of zero?

  1. Sulfur dioxide
  2. Chloroform
  3. Water
  4. Carbon tetrachloride Correct answer.

Explanation

Carbon tetrachloride is tetrahedral with four identical polar C-Cl bonds arranged symmetrically, so the individual bond dipoles cancel. The result is a molecule with no net dipole moment.

Question 41Chemical Bonding and Structure of Matter

Which of the following correctly describes the hybridisation of carbon in ethyne (C2H2) and the geometry around each carbon atom?

  1. sp2 hybridisation with trigonal planar geometry
  2. sp2 hybridisation with linear geometry
  3. sp3 hybridisation with tetrahedral geometry
  4. sp hybridisation with linear geometry Correct answer.

Explanation

Each carbon in ethyne forms a triple bond using two pi bonds and one sigma bond, so it uses one s and one p orbital to make two sp hybrid orbitals. The two sp orbitals point in opposite directions, giving linear geometry with 180 degree bond angles.

Question 42Chemical Bonding and Structure of Matter

In the carbonate ion (CO3 2-), all three C-O bonds are found to be identical and intermediate in length between a single and a double bond. Which explanation accounts for this observation?

  1. The pi electrons are delocalised over the three C-O bonds through resonance Correct answer.
  2. The carbon atom is sp3 hybridised, making all bonds identical
  3. Two of the oxygen atoms form ionic bonds with carbon
  4. One C=O bond rotates rapidly between the three oxygen atoms

Explanation

The carbonate ion is described by a set of resonance structures in which the double bond is distributed over the three C-O positions. The actual structure is a resonance hybrid with delocalised pi electrons, making all three bonds equivalent and intermediate in length.

Question 43Chemical Bonding and Structure of Matter

Which of the following best explains why the first ionisation energy of nitrogen is greater than that of oxygen?

  1. Oxygen has a larger atomic radius than nitrogen
  2. Oxygen's outermost electron is in a lower principal energy level
  3. Nitrogen has a half-filled 2p subshell, which gives extra stability Correct answer.
  4. Nitrogen has fewer protons in its nucleus than oxygen

Explanation

Nitrogen has the configuration 2p3, a half-filled subshell with extra exchange stability, so removing an electron requires more energy than expected. Oxygen has 2p4, and removing the fourth electron relieves electron-electron repulsion in a doubly occupied p orbital, lowering its ionisation energy.

Question 44Chemical Bonding and Structure of Matter

A molecule of the type AB4 is found to be nonpolar even though the A-B bonds are polar. Which conclusion about its geometry is most reliable?

  1. The molecule must contain a lone pair on A
  2. The molecule must have a square planar shape with bond angles of 90 degrees
  3. The four B atoms are arranged symmetrically around A, so the bond dipoles cancel Correct answer.
  4. The A-B bonds must be nonpolar in reality

Explanation

For a molecule with polar bonds to have no net dipole, the bond dipoles must cancel by symmetry. A symmetric arrangement of four identical B atoms around A, such as tetrahedral, achieves this cancellation.

Question 45Inorganic Chemistry: Periodic Trends and Main-Group Elements

Across a period from left to right, the first ionisation energy of the elements generally:

  1. increases Correct answer.
  2. remains unchanged
  3. decreases
  4. decreases then increases sharply

Explanation

Nuclear charge rises across a period while electrons enter the same shell, so the outer electron is held more tightly. The general trend is therefore an increase in first ionisation energy.

Question 46Inorganic Chemistry: Periodic Trends and Main-Group Elements

Which halogen is the most powerful oxidising agent?

  1. F2 Correct answer.
  2. Br2
  3. I2
  4. Cl2

Explanation

Oxidising strength of the halogens falls down the group as the X–X bond weakens and electron affinity decreases. Fluorine, at the top of Group 17, is therefore the strongest oxidiser.

Question 47Inorganic Chemistry: Periodic Trends and Main-Group Elements

Which oxide is amphoteric?

  1. MgO
  2. Al2O3 Correct answer.
  3. Na2O
  4. SO3

Explanation

Aluminium oxide reacts with both acids and bases, forming salts in each case, which is the definition of an amphoteric oxide. Oxides of typical metals are basic and those of non-metals are acidic.

Question 48Inorganic Chemistry: Periodic Trends and Main-Group Elements

Which property increases down Group 2 from magnesium to barium?

  1. First ionisation energy
  2. Atomic radius Correct answer.
  3. Reactivity with water decreases
  4. Electronegativity

Explanation

Each step down Group 2 adds a shell, so atomic radius grows. Ionisation energy and electronegativity fall and reactivity with water rises as the atoms become larger and more easily oxidised.

Question 49Inorganic Chemistry: Periodic Trends and Main-Group Elements

Which species has the electron configuration 1s² 2s² 2p⁶?

  1. Mg²⁺
  2. Na⁺ Correct answer.
  3. F⁻
  4. Ne

Explanation

Sodium has 11 electrons, so losing one electron gives Na⁺ with 10 electrons, matching the configuration shown. This is the same configuration as neon.

Question 50Inorganic Chemistry: Periodic Trends and Main-Group Elements

Which element is a metalloid?

  1. Na
  2. Cl
  3. Si Correct answer.
  4. S

Explanation

Silicon lies on the diagonal staircase separating metals from non-metals and shows intermediate properties, so it is classed as a metalloid. Sulfur and chlorine are non-metals and sodium is a metal.