Unit 1 Practice: Easy

Answer: 6.022 × 10²³ carbon atoms

Explanation: One mole of any substance contains Avogadro's number of particles:

6.022 × 10²³ particles/mol

Since the substance is carbon atoms, the particles are atoms.

Answer: 18.02 g/mol

Explanation: Add the atomic masses of every atom in the formula.

Hydrogen:

2(1.008) = 2.016

Oxygen:

1(16.00) = 16.00

Total:

2.016 + 16.00 = 18.016 ≈ 18.02 g/mol

Answer: 2.00 mol H₂O

Work:

moles = mass / molar mass

36.0 g / 18.02 g/mol ≈ 2.00 mol

Explanation: Molar mass acts as the conversion factor between grams and moles.

Answer: 117 g NaCl

Work:

Molar mass NaCl:

22.99 + 35.45 = 58.44 g/mol

Then:

2.00 mol × 58.44 g/mol = 116.88 g

With appropriate significant figures:

117 g

Answer: Isotopes differ in neutrons. Ions differ in electrons.

Explanation: Atoms of the same element always have the same number of protons.

Changing the number of neutrons creates a different isotope.

Changing the number of electrons creates an ion with an electrical charge.

Answer: −1

Explanation: Each proton contributes +1 charge and each electron contributes −1.

17 protons and 18 electrons means there is one extra negative charge.

Therefore:

charge = −1

This is a chloride ion, Cl⁻.

Answer: 8 neutrons

Explanation: Carbon has atomic number 6, so it has 6 protons.

Use:

mass number = protons + neutrons

Therefore:

14 = 6 + neutrons

neutrons = 8

Answer: 1s² 2s² 2p⁴

Explanation: Oxygen has 8 electrons.

Fill orbitals from lower to higher energy:

1s holds 2 2s holds 2 remaining 4 go into 2p.

So:

1s²2s²2p⁴

Answer: [Ne]3s¹

Explanation: Sodium has 11 electrons.

The first 10 electrons have the same configuration as neon.

The final electron occupies 3s.

Therefore:

[Ne]3s¹

Answer: 7

Explanation: Chlorine's outer configuration is:

3s²3p⁵

The highest occupied principal energy level is n=3.

2 + 5 = 7 valence electrons

Answer: Na

Explanation: Na and Cl are in the same period.

Moving left to right across a period, effective nuclear charge increases and pulls electrons closer to the nucleus.

Na is farther left, so it has the larger radius.

Answer: Mg

Explanation: Ionization energy generally increases across a period because nuclear attraction to the valence electrons becomes stronger.

Mg is to the right of Na, so its first electron is more difficult to remove.

Answer: Effective nuclear charge increases.

Explanation: More protons are added across the period, while electrons are added to the same main energy level.

Shielding does not increase enough to cancel the stronger nuclear charge.

The nucleus therefore pulls the electron cloud closer.

Answer: Mg²⁺

Explanation: Mg has two valence electrons:

[Ne]3s²

It can lose both to reach the stable neon electron configuration.

Answer: N³⁻

Explanation: Nitrogen has five valence electrons.

Gaining three electrons gives it eight valence electrons and a noble-gas-like configuration.

Answer: H₂O

Explanation: An empirical formula uses the simplest whole-number ratio of atoms.

The ratio is already:

2 H : 1 O

so the formula is H₂O.

Answer: About 88.8%

Work:

Molar mass H₂O ≈ 18.02 g/mol

O contributes 16.00 g.

% O = (16.00 / 18.02)(100)

≈ 88.8%

Answer: Their mass-to-charge ratios, which for singly charged isotopes correspond closely to isotope masses.

Explanation: Mass spectrometry separates ions according to m/z, meaning mass divided by charge.

If each isotope has a +1 charge, the numerical m/z values closely match their isotope mass numbers.

Answer: 4s electrons

Explanation: Neutral Fe is approximately:

[Ar]4s²3d⁶

When transition metals form ions, the electrons in the highest principal energy level are removed first.

So the two 4s electrons are removed:

Fe²⁺ = [Ar]3d⁶

Answer: 2

Explanation: According to the Pauli exclusion principle, an orbital can contain at most two electrons, and those electrons must have opposite spins.