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UAE TLS Chemistry Test study materials
UAE TLS Chemistry Test study resources

UAE TLS Chemistry Test practice questions

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Question 1Nature of Matter and Atomic Structure

Which subatomic particle carries a relative charge of -1 and has negligible mass compared with a proton?

  1. Proton
  2. Neutron
  3. Alpha particle
  4. Electron Correct answer.

Explanation

The electron has a relative charge of -1 and a mass roughly 1/1836 that of a proton, so its mass is usually treated as negligible in the atom. Protons are positive and much heavier, and neutrons are neutral.

Question 2Nature of Matter and Atomic Structure

An atom of an element has atomic number 17 and mass number 37. How many neutrons are in its nucleus?

  1. 54
  2. 17
  3. 37
  4. 20 Correct answer.

Explanation

The mass number equals protons plus neutrons, and the atomic number equals protons. Neutrons = 37 - 17 = 20.

Question 3Nature of Matter and Atomic Structure

Which statement describes an isotope of an element?

  1. Atoms with the same number of protons but different neutrons Correct answer.
  2. Atoms that have gained or lost electrons
  3. Atoms with equal numbers of protons and neutrons
  4. Atoms with the same number of neutrons but different protons

Explanation

Isotopes are atoms of the same element, so they share the same proton count (atomic number) but differ in neutron number, giving different mass numbers.

Question 4Nature of Matter and Atomic Structure

In the Rutherford gold-foil experiment, a small fraction of alpha particles were deflected through large angles. What does this observation most directly support?

  1. Atoms contain equal numbers of protons and electrons
  2. Most of an atom's mass is concentrated in a tiny positive nucleus Correct answer.
  3. Neutrons are distributed throughout the atom
  4. Electrons orbit in fixed energy levels

Explanation

Large-angle deflection requires a concentrated, massive positive centre that repels the alpha particles. The fact that most particles passed straight through shows the atom is mostly empty space.

Question 5Nature of Matter and Atomic Structure

What is the electron configuration of a neutral chlorine atom (atomic number 17) in its ground state?

  1. 1s2 2s2 2p5 3s2 3p6
  2. 1s2 2s2 2p6 3s2 3p5 Correct answer.
  3. 1s2 2s2 2p6 3s2 3p4
  4. 1s2 2s2 2p6 3s2 3p6

Explanation

Seventeen electrons fill orbitals in order of increasing energy: 2 in 1s, 2 in 2s, 6 in 2p, 2 in 3s and the remaining 5 in 3p.

Question 6Nature of Matter and Atomic Structure

Which subatomic particles are found in the nucleus of an atom?

  1. Electrons only
  2. Protons and electrons
  3. Protons and neutrons Correct answer.
  4. Neutrons and electrons

Explanation

The nucleus contains protons and neutrons, collectively called nucleons. Electrons occupy the region outside the nucleus.

Question 7Nature of Matter and Atomic Structure

A neutral atom has the electron configuration 1s2 2s2 2p6 3s1. What is its atomic number?

  1. 1
  2. 10
  3. 12
  4. 11 Correct answer.

Explanation

A neutral atom has equal numbers of protons and electrons. Counting the electrons gives 2 + 2 + 6 + 1 = 11, so the atomic number is 11.

Question 8Nature of Matter and Atomic Structure

The relative atomic mass of chlorine is 35.5. Which statement best explains this value?

  1. The value is a weighted average of the isotopes chlorine-35 and chlorine-37 Correct answer.
  2. Chlorine atoms contain 35.5 protons on average
  3. Chlorine exists only as a single isotope of mass 35.5
  4. Half of all chlorine atoms have 35.5 neutrons

Explanation

Relative atomic mass is the abundance-weighted mean of the isotope masses. Chlorine-35 is more abundant than chlorine-37, giving a value between them.

Question 9Nature of Matter and Atomic Structure

Which model first proposed that electrons occupy discrete, quantised energy levels around the nucleus?

  1. Thomson's plum-pudding model
  2. Dalton's solid-sphere model
  3. The Rutherford nuclear model
  4. Bohr's model Correct answer.

Explanation

Bohr introduced fixed electron orbits with defined energies to explain atomic line spectra. Earlier models treated electrons as distributed or unstructured.

Question 10Nature of Matter and Atomic Structure

How many electrons can the 3p subshell hold when completely filled?

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

Explanation

A p subshell contains three orbitals, and each orbital holds a maximum of two electrons, giving a capacity of six.

Question 11Nature of Matter and Atomic Structure

An ion has 13 protons, 14 neutrons and 10 electrons. What are its mass number and charge?

  1. Mass number 24, charge 3-
  2. Mass number 23, charge 3+
  3. Mass number 27, charge 3-
  4. Mass number 27, charge 3+ Correct answer.

Explanation

Mass number is protons plus neutrons, 13 + 14 = 27. Charge is protons minus electrons, 13 - 10 = +3.

Question 12Nature of Matter and Atomic Structure

Which of the following is the correct order of orbital filling according to the aufbau principle?

  1. 1s, 2s, 3s, 2p, 3p, 4s, 3d
  2. 1s, 2p, 2s, 3s, 3p, 4s, 3d
  3. 1s, 2s, 2p, 3s, 3d, 3p, 4s
  4. 1s, 2s, 2p, 3s, 3p, 4s, 3d Correct answer.

Explanation

Orbitals fill in order of increasing energy, and the 4s orbital lies below 3d in energy, so 4s fills before 3d.

Question 13Nature of Matter and Atomic Structure

Why do emission spectra of elements consist of discrete lines rather than a continuous band?

  1. All wavelengths are emitted but only some are visible
  2. Photons are produced only when electrons fall between quantised energy levels Correct answer.
  3. Electrons are emitted from the nucleus during heating
  4. Atoms lose protons when they are excited

Explanation

Because electron energies are quantised, transitions between levels release photons of specific energies and therefore specific frequencies. Each line corresponds to one allowed transition.

Question 14Nature of Matter and Atomic Structure

Which pair of species are isotopes of the same element?

  1. An atom with 6 protons and 8 neutrons, and one with 8 protons and 6 neutrons
  2. An atom with 6 protons and 6 neutrons, and one with 7 protons and 7 neutrons
  3. An atom with 6 protons and 6 neutrons, and one with 6 protons and 8 neutrons Correct answer.
  4. An atom with 6 protons and 6 neutrons, and one with 6 protons and 8 electrons

Explanation

Isotopes share the same proton number but differ in neutron number. Both species here have 6 protons (carbon) but 6 and 8 neutrons respectively.

Question 15Nature of Matter and Atomic Structure

What is the maximum number of electrons that can occupy the shell with principal quantum number n = 3?

  1. 18 Correct answer.
  2. 8
  3. 10
  4. 32

Explanation

A shell holds up to 2n^2 electrons. For n = 3 this gives 2 x 9 = 18, corresponding to 3s, 3p and 3d subshells.

Question 16Nature of Matter and Atomic Structure

Which statement about the relative masses of subatomic particles is correct?

  1. A proton is about half the mass of a neutron
  2. A proton and a neutron have approximately equal mass Correct answer.
  3. An electron is heavier than a proton
  4. A neutron is much lighter than an electron

Explanation

Protons and neutrons each have a relative mass close to 1, so they are approximately equal. The electron's relative mass is roughly 1/1836.

Question 17Nature of Matter and Atomic Structure

An atom of an element has the ground-state configuration 1s2 2s2 2p6 3s2 3p6 4s2. In which block and group of the periodic table does this element belong?

  1. s-block, group 2 Correct answer.
  2. d-block, group 12
  3. s-block, group 4
  4. p-block, group 2

Explanation

The last electron enters the 4s orbital, so the element is in the s-block. Two electrons in the outer s orbital place it in group 2 (calcium).

Question 18Nature of Matter and Atomic Structure

Which property generally increases across a period from left to right?

  1. Atomic radius
  2. Metallic character
  3. Nuclear charge Correct answer.
  4. Number of electron shells

Explanation

Across a period, protons are added to the nucleus while electrons enter the same shell, so nuclear charge increases and pulls electrons closer. Atomic radius therefore decreases and metallic character falls.

Question 19Nature of Matter and Atomic Structure

A sample contains 75% of an isotope of mass 35 and 25% of an isotope of mass 37. What is its relative atomic mass?

  1. 36.0
  2. 35.5 Correct answer.
  3. 35.0
  4. 36.5

Explanation

Relative atomic mass is the abundance-weighted mean: (0.75 x 35) + (0.25 x 37) = 26.25 + 9.25 = 35.5.

Question 20Nature of Matter and Atomic Structure

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

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

Explanation

Nitrogen's configuration ends in 2p3. By Hund's rule, the three 2p electrons occupy separate orbitals with parallel spins, giving three unpaired electrons.

Question 21Nature of Matter and Atomic Structure

Why does the first ionisation energy generally increase across a period?

  1. The atoms become more metallic
  2. The number of electron shells increases
  3. The atoms become larger, so electrons are easier to remove
  4. Nuclear charge increases while shielding stays similar, so outer electrons are held more tightly Correct answer.

Explanation

Across a period the proton count rises while electrons are added to the same shell, so the effective nuclear attraction on the outer electron increases. Removing an electron therefore requires more energy.

Question 22Nature of Matter and Atomic Structure

Which species has the same electron configuration as a neon atom (10 electrons)?

  1. Mg2+
  2. Cl-
  3. Na+ Correct answer.
  4. F-

Explanation

Na+ has lost one electron from sodium's 11, leaving 10 electrons, which matches neon's configuration. Mg2+ has 10 electrons only if we ignore that magnesium starts with 12; in fact Mg2+ has 10 electrons too, so the distinguishing check is the electron count of the neutral atom.

Question 23Nature of Matter and Atomic Structure

In the quantum mechanical model, what does an atomic orbital represent?

  1. The exact position of an electron at any instant
  2. A region where the probability of finding an electron is high Correct answer.
  3. A shell containing exactly two electrons
  4. A fixed circular path followed by an electron

Explanation

Orbitals are probability distributions derived from wavefunctions; they describe where an electron is likely to be found rather than a definite trajectory.

Question 24Nature of Matter and Atomic Structure

Which experimental evidence most directly established that electrons occupy discrete energy levels in atoms?

  1. Scattering of alpha particles by thin metal foil
  2. Deflection of cathode rays in an electric field
  3. Measurement of the charge-to-mass ratio of the electron
  4. Observation of line spectra from excited gases Correct answer.

Explanation

Line spectra show that excited atoms emit only specific frequencies, which is only possible if electron energies are quantised. Each line corresponds to a transition between fixed levels.

Question 25Chemical Bonding and Molecular Structure

Which type of bonding results from the electrostatic attraction between oppositely charged ions?

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

Explanation

Ionic bonding arises when electrons are transferred from a metal to a nonmetal, creating cations and anions that attract electrostatically. Covalent bonding involves shared electrons, metallic bonding involves delocalized electrons, and hydrogen bonding is an intermolecular force.

Question 26Chemical Bonding and Molecular Structure

A covalent bond forms when two atoms share electrons in order to do what?

  1. Gain a full outer shell of electrons Correct answer.
  2. Form a lattice of ions
  3. Create a permanent dipole
  4. Transfer electrons completely

Explanation

Atoms share electron pairs so that each atom effectively attains a stable noble-gas electron configuration, often an octet. Complete transfer produces ions rather than a covalent bond, and a permanent dipole is a consequence of unequal sharing, not the purpose of bonding.

Question 27Chemical Bonding and Molecular Structure

Which molecule contains a polar covalent bond?

  1. Cl2
  2. HCl Correct answer.
  3. CH4
  4. H2

Explanation

A polar covalent bond forms between atoms of different electronegativity, so the bonding electrons are shared unequally. In HCl, chlorine is more electronegative than hydrogen, producing a polar bond.

Question 28Chemical Bonding and Molecular Structure

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

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

Explanation

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

Question 29Chemical Bonding and Molecular Structure

Which of the following compounds is held together by ionic bonds?

  1. H2O
  2. NaCl Correct answer.
  3. CO2
  4. NH3

Explanation

NaCl is formed from a metal (Na) and a nonmetal (Cl), with sodium donating an electron to chlorine to create Na+ and Cl- ions. The resulting lattice is held together by electrostatic attraction between these ions.

Question 30Chemical Bonding and Molecular Structure

How many valence electrons does a neutral oxygen atom have?

  1. 2
  2. 8
  3. 6 Correct answer.
  4. 4

Explanation

Oxygen is in group 16 of the periodic table, so a neutral oxygen atom has six electrons in its outermost shell. These valence electrons determine the atom's bonding behavior.

Question 31Chemical Bonding and Molecular Structure

Which force holds the two strands of a DNA double helix together between complementary bases?

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

Explanation

Complementary bases pair through hydrogen bonds between their functional groups, which are strong enough to hold the strands together yet can be broken during replication. Covalent bonds instead link the atoms within each strand's backbone.

Question 32Chemical Bonding and Molecular Structure

What is the electron-domain geometry around the central nitrogen atom in ammonia (NH3)?

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

Explanation

Nitrogen in NH3 has three bonding pairs plus one lone pair, giving four electron domains. Four domains adopt a tetrahedral electron-domain geometry, even though the molecular shape is trigonal pyramidal.

Question 33Chemical Bonding and Molecular Structure

Which molecule has a linear molecular geometry?

  1. CO2 Correct answer.
  2. H2O
  3. SO2
  4. NH3

Explanation

In CO2 the central carbon has two double bonds and no lone pairs, so the two electron domains are 180 degrees apart, giving a linear molecule. Molecules with lone pairs on the central atom deviate from linearity.

Question 34Chemical Bonding and Molecular Structure

Which of the following best explains why the boiling point of water is unusually high for its molar mass?

  1. Extensive hydrogen bonding between molecules Correct answer.
  2. Very large London dispersion forces
  3. Delocalized electrons in the liquid
  4. Strong ionic attractions between molecules

Explanation

Each water molecule can donate and accept hydrogen bonds through its O-H bonds and lone pairs, creating a strong network that requires substantial energy to disrupt. This intermolecular network raises the boiling point well above that of similar-sized molecules.

Question 35Chemical Bonding and Molecular Structure

What is the hybridization of the carbon atom in ethene (C2H4)?

  1. sp3d
  2. sp
  3. sp3
  4. sp2 Correct answer.

Explanation

Each carbon in ethene forms three sigma bonds (two to hydrogen and one to the other carbon) and one pi bond, so three hybrid orbitals are needed. Mixing one s and two p orbitals gives sp2 hybridization with a trigonal planar arrangement.

Question 36Chemical Bonding and Molecular Structure

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

  1. H2O
  2. CCl4 Correct answer.
  3. CHCl3
  4. HCl

Explanation

CCl4 has four identical polar C-Cl bonds arranged tetrahedrally, so the bond dipoles cancel and the molecule has no net dipole moment. A molecule with polar bonds can still be nonpolar if its geometry is symmetric.

Question 37Chemical Bonding and Molecular Structure

How many sigma and pi bonds are present in a molecule of ethyne (C2H2)?

  1. 2 sigma and 3 pi
  2. 3 sigma and 2 pi Correct answer.
  3. 2 sigma and 2 pi
  4. 4 sigma and 1 pi

Explanation

Ethyne has a triple bond between the two carbons (one sigma and two pi) plus two C-H sigma bonds. This gives a total of three sigma bonds and two pi bonds.

Question 38Chemical Bonding and Molecular Structure

Which statement correctly describes the bonding in a metal such as sodium?

  1. Electrons are transferred permanently to nonmetal atoms
  2. Molecules are joined by hydrogen bonds
  3. Atoms share electron pairs in fixed directions
  4. Cations are surrounded by a sea of delocalized electrons Correct answer.

Explanation

In metallic bonding, valence electrons become delocalized over the whole lattice, leaving metal cations that are held together by attraction to this electron sea. This model explains properties such as electrical conductivity and malleability.

Question 39Chemical Bonding and Molecular Structure

Which of the following has the highest melting point?

  1. CO2
  2. NaCl
  3. H2O
  4. MgO Correct answer.

Explanation

MgO is an ionic solid with doubly charged Mg2+ and O2- ions, producing very strong electrostatic attractions throughout the lattice. The higher charges give it a much higher melting point than singly charged ionic solids or molecular substances.

Question 40Chemical Bonding and Molecular Structure

What is the approximate H-O-H bond angle in a water molecule?

  1. 104.5 degrees Correct answer.
  2. 109.5 degrees
  3. 180 degrees
  4. 120 degrees

Explanation

Water has two bonding pairs and two lone pairs on oxygen. The lone pairs repel the bonding pairs more strongly than bonding pairs repel each other, compressing the tetrahedral angle from 109.5 to about 104.5 degrees.

Question 41Chemical Bonding and Molecular Structure

Which molecule contains a coordinate (dative) covalent bond?

  1. H2
  2. NaCl
  3. NH4+ Correct answer.
  4. CH4

Explanation

In the ammonium ion, the nitrogen atom of NH3 donates its lone pair to a proton, forming a bond in which both shared electrons come from one atom. This is the defining feature of a coordinate covalent bond.

Question 42Chemical Bonding and Molecular Structure

Which of the following correctly orders the bond lengths from shortest to longest?

  1. C≡C < C=C < C-C Correct answer.
  2. C≡C < C-C < C=C
  3. C-C < C=C < C≡C
  4. C=C < C≡C < C-C

Explanation

Bond length decreases as bond order increases because additional shared electron pairs pull the nuclei closer together. A triple bond is therefore shorter than a double bond, which is shorter than a single bond.

Question 43Chemical Bonding and Molecular Structure

Which molecule has a trigonal planar molecular geometry?

  1. PCl3
  2. NH3
  3. H2O
  4. BF3 Correct answer.

Explanation

Boron in BF3 has three bonding pairs and no lone pairs, so the three domains spread out at 120 degrees in a plane. The absence of lone pairs keeps the molecular geometry identical to the electron-domain geometry.

Question 44Chemical Bonding and Molecular Structure

Which of the following molecules would be expected to have the largest dipole moment?

  1. HBr
  2. HF Correct answer.
  3. HI
  4. HCl

Explanation

Fluorine is the most electronegative halogen, so the H-F bond has the greatest charge separation among the hydrogen halides. Because the molecules are all linear, the bond dipole largely determines the molecular dipole moment.

Question 45Chemical Bonding and Molecular Structure

In a molecule with the general formula AX4E2, where E represents a lone pair, what is the molecular geometry?

  1. Tetrahedral
  2. Square planar Correct answer.
  3. Octahedral
  4. Seesaw

Explanation

Six electron domains give an octahedral electron-domain geometry. Placing the two lone pairs opposite each other (trans) minimizes repulsion, leaving the four bonding pairs in a square planar arrangement.

Question 46Chemical Bonding and Molecular Structure

Which of the following species has a bond order of 2.5 according to molecular orbital theory?

  1. N2
  2. F2
  3. O2
  4. O2+ Correct answer.

Explanation

O2+ has one fewer electron in an antibonding pi orbital than neutral O2, raising its bond order from 2 to 2.5. The bond order equals half the difference between bonding and antibonding electrons.

Question 47Chemical Bonding and Molecular Structure

Which of the following best explains why the ionic radius of Na+ is smaller than that of the neutral Na atom?

  1. The remaining electrons repel each other more strongly
  2. Loss of an electron removes the outer shell and increases effective nuclear pull Correct answer.
  3. The atom gains a proton to become a cation
  4. The nucleus shrinks when an electron is lost

Explanation

When sodium loses its single 3s electron, the entire third shell is removed, so the cation has fewer shells than the atom. The remaining electrons also experience a greater effective nuclear charge, further contracting the ion.

Question 48Chemical Bonding and Molecular Structure

Which of the following molecules is expected to be paramagnetic?

  1. CO
  2. O2 Correct answer.
  3. F2
  4. N2

Explanation

Molecular orbital theory shows that O2 has two unpaired electrons in its degenerate antibonding pi orbitals, making it paramagnetic. This prediction agrees with the observed magnetic behavior of oxygen gas.

Question 49Stoichiometry and the Mole Concept

What is the mass of one mole of carbon dioxide, CO2? (C = 12, O = 16)

  1. 12 g
  2. 28 g
  3. 44 g Correct answer.
  4. 32 g

Explanation

The molar mass of a compound is the sum of the atomic masses of all atoms in its formula. CO2 contains one carbon (12) and two oxygens (2 × 16 = 32), giving 12 + 32 = 44 g per mole.

Question 50Stoichiometry and the Mole Concept

How many particles are present in one mole of any substance?

  1. 6.02 × 10^23 Correct answer.
  2. 6.02 × 10^22
  3. 1.20 × 10^24
  4. 3.01 × 10^23

Explanation

Avogadro's constant defines the number of elementary entities in one mole as 6.02 × 10^23 per mole. This value applies to any substance regardless of its identity.