AP Chemistry - Elements and Atoms

1

Choose the answer with the correct electron configuration for a sodium atom.

$1s^{2}2s^{2}2p^{^{6}}3s^{1}$

$1s^{2}2s^{2}2p^{^{7}}$

$1s^{2}2s^{2}2p^{^{5}}3s^{2}$

$1s^{2}2s^{2}2p^{^{6}}$

$1s^{2}2s^{2}3s^{2}3p^{^{6}}$

Explanation

$1s^{2}2s^{2}2p^{^{6}}3s^{1}$ is the correct configuration for a neutral sodium atom.

A neutral sodium atom contains eleven electrons, so you can eliminate $1s^{2}2s^{2}3s^{2}3p^{^{6}}$ and $1s^{2}2s^{2}2p^{^{6}}$ which contain twelve and ten electrons, respectively.

Next, $1s^{2}2s^{2}2p^{^{5}}3s^{2}$ can be eliminated because of the Aufbau principal, which states that electrons are placed into orbitals from the lowest to highest energy. In this answer choice, the (lower energy) orbital is not completely filled, containing only five out of the possible six possible electron that it can hold. This orbital must be filled completely before electrons can be placed in the (higher energy) orbital.

$1s^{2}2s^{2}2p^{^{7}}$ is incorrect because there are seven electrons in the orbital. This is impossible, as p orbitals hold a maximum of six electrons.

2

Identify the number of protons, neutrons, and electrons in a charged cation of aluminum (mass number = 27).

Protons=13

Neutrons=14

Electrons=10

Protons=13

Neutrons=14

Electrons=13

Protons=10

Neutrons=14

Electrons=13

Protons=10

Neutrons=14

Electrons=16

Protons=16

Neutrons=14

Electrons=13

Explanation

The atomic number of aluminum is 13, which means it has 13 protons. All atoms with 13 protons are called aluminum. The mass number of aluminum is 27 and is the sum of neutrons and protons. We simply subtract the number of protons from the mass number to get the number of neutrons. Normally there are an equal number of electrons and protons. But since this aluminum exists as an ion with a +3 charge, it has lost three electrons and has only 10 electrons.

3

Identify the number of protons, neutrons, and electrons in a charged cation of aluminum (mass number = 27).

Protons=13

Neutrons=14

Electrons=10

Protons=13

Neutrons=14

Electrons=13

Protons=10

Neutrons=14

Electrons=13

Protons=10

Neutrons=14

Electrons=16

Protons=16

Neutrons=14

Electrons=13

Explanation

The atomic number of aluminum is 13, which means it has 13 protons. All atoms with 13 protons are called aluminum. The mass number of aluminum is 27 and is the sum of neutrons and protons. We simply subtract the number of protons from the mass number to get the number of neutrons. Normally there are an equal number of electrons and protons. But since this aluminum exists as an ion with a +3 charge, it has lost three electrons and has only 10 electrons.

4

Choose the answer with the correct electron configuration for a sodium atom.

$1s^{2}2s^{2}2p^{^{6}}3s^{1}$

$1s^{2}2s^{2}2p^{^{7}}$

$1s^{2}2s^{2}2p^{^{5}}3s^{2}$

$1s^{2}2s^{2}2p^{^{6}}$

$1s^{2}2s^{2}3s^{2}3p^{^{6}}$

Explanation

$1s^{2}2s^{2}2p^{^{6}}3s^{1}$ is the correct configuration for a neutral sodium atom.

A neutral sodium atom contains eleven electrons, so you can eliminate $1s^{2}2s^{2}3s^{2}3p^{^{6}}$ and $1s^{2}2s^{2}2p^{^{6}}$ which contain twelve and ten electrons, respectively.

Next, $1s^{2}2s^{2}2p^{^{5}}3s^{2}$ can be eliminated because of the Aufbau principal, which states that electrons are placed into orbitals from the lowest to highest energy. In this answer choice, the (lower energy) orbital is not completely filled, containing only five out of the possible six possible electron that it can hold. This orbital must be filled completely before electrons can be placed in the (higher energy) orbital.

$1s^{2}2s^{2}2p^{^{7}}$ is incorrect because there are seven electrons in the orbital. This is impossible, as p orbitals hold a maximum of six electrons.

5

What is the charge of a neutron?

Explanation

Neutrons have no charge. Electrons have a charge of and protons have a charge of .

6

What is the charge of a neutron?

Explanation

Neutrons have no charge. Electrons have a charge of and protons have a charge of .

7

Which of the following statements is true?

Electrons are smaller in size than nucleons, but have equal charge

The nucleus of an atom is electrically neutral

Electrons are smaller than protons, which means they have less charge than protons

Electrons are close in proximity to the atomic nucleus

Electrons are larger than protons, which means they must have less charge than protons

Explanation

Electrons are notably smaller than nucleons (protons and neutrons), however, they have an equal amount of charge to protons. This is why an atom with the same amount of protons as electrons is considered electrically neutral.

The nucleus itself is positively charged, as the electrons are located relatively far away from the atomic center.

8

Which of the following statements is true?

Electrons are smaller in size than nucleons, but have equal charge

The nucleus of an atom is electrically neutral

Electrons are smaller than protons, which means they have less charge than protons

Electrons are close in proximity to the atomic nucleus

Electrons are larger than protons, which means they must have less charge than protons

Explanation

Electrons are notably smaller than nucleons (protons and neutrons), however, they have an equal amount of charge to protons. This is why an atom with the same amount of protons as electrons is considered electrically neutral.

The nucleus itself is positively charged, as the electrons are located relatively far away from the atomic center.

9

If a neutron is added to the nucleus of an atom, which of the following has changed?

The mass number

The atomic number

The charge

The element identity

Explanation

Isotopes are variations of an element that differ by the number of neutrons in the nucleus. Neutrons are neutrally charged, so adding or removing them from a nucleus does not alter the charge of the atom. The atomic number is the number of protons in an atom, and the mass number is the sum of both protons and neutrons in the nucleus. As a result, the mass number will change by adding a neutron.

The number of protons is equal to the atomic number of the element. Adding or subtracting protons will change the element's identity. Ions can be created by changing the number of electrons and isotopes can be created by changing the number neutrons, but changing the number of protons changes the element.

10

Is it possible to form a compound from only one element?

Yes; diatomic gases are compounds that consist of only one element

Yes; benzene rings contain only carbon

Yes; water is a compound that consists of only one element

Yes; compounds and elements are synonyms

No; a compound must contain multiple elements

Explanation

Elements are defined by the number of protons in a given atom, and the atom is the smallest unit of an element. In contrast, a compound is defined by the identity and organization of multiple atoms, with the smallest unit of a compound being a molecule.

Some compounds do contain only one element, such as diatomic gases or graphite. Benzene rings are composed of carbon and hydrogen, while water molecules contain hydrogen and oxygen.

Elements and Atoms

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