Question

How many protons and neutrons are in the nucleus of each of the following atoms? In a neutral atom of each element, how many electrons are present? a. \(^{7 y} B r\) d. \({ }^{133} \mathrm{Cs}\) b. \({ }^{\text {st }} \mathrm{Br}\) e. \({ }^{3} \mathrm{H}\) c. \({ }^{239} \mathrm{Pu}\). f. \({ }^{56} \mathrm{Fe}\)

Short answer

a. \(^{7} \mathrm{Br}\): Number of protons: 35 Number of neutrons: 0 Number of electrons: 35 d. \({ }^{133} \mathrm{Cs}\): Number of protons: 55 Number of neutrons: 78 Number of electrons: 55 b. \({ }^{81} \mathrm{Br}\): Number of protons: 35 Number of neutrons: 46 Number of electrons: 35 e. \({ }^{3} \mathrm{H}\): Number of protons: 1 Number of neutrons: 2 Number of electrons: 1 c. \({ }^{239} \mathrm{Pu}\): Number of protons: 94 Number of neutrons: 145 Number of electrons: 94 f. \({ }^{56} \mathrm{Fe}\): Number of protons: 26 Number of neutrons: 30 Number of electrons: 26

Step-by-step solution

Identify the element and its atomic number

First, we identify the given element, in this case, it's Bromine (Br). Look up the atomic number (number of protons) of Bromine, which is 35.

Calculate the number of neutrons

Now we calculate the number of neutrons using the given mass number. Since we know that there are 35 protons, and the mass number is 7, we can quickly see that there aren't any neutrons in this isotope. This isotope doesn't actually exist in reality, but we will try to solve it since it's in the question.

Calculate the number of electrons

Since the atom is neutral, the number of electrons will be equal to the number of protons. So, there are 35 electrons. #d. \({ }^{133} \mathrm{Cs}\)#

Identify the element and its atomic number

The given element is Cesium (Cs). Look up the atomic number of Cesium, which is 55.

Calculate the number of neutrons

Now, we have the mass number (133) and the number of protons (55). Therefore, the number of neutrons is 133 - 55 = 78.

Calculate the number of electrons

Since the atom is neutral, the number of electrons will be equal to the number of protons. So, there are 55 electrons. Proceed similarly with the remaining elements, given their mass number: #b. \({ }^{81} \mathrm{Br}\):# Number of protons: 35 Number of neutrons: 81 - 35 = 46 Number of electrons: 35 #e. \({ }^{3} \mathrm{H}\):# Number of protons: 1 Number of neutrons: 3 - 1 = 2 Number of electrons: 1 #c. \({ }^{239} \mathrm{Pu}\):# Number of protons: 94 Number of neutrons: 239 - 94 = 145 Number of electrons: 94 #f. \({ }^{56} \mathrm{Fe}\):# Number of protons: 26 Number of neutrons: 56 - 26 = 30 Number of electrons: 26

Key concepts

Atomic Number

The atomic number of an element is a fundamental property that defines each element on the periodic table. It is represented by the symbol 'Z' and indicates the number of protons found in the nucleus of an atom of that element. Protons are positively charged particles, and it's their number that distinguishes one element from another. For instance, hydrogen has an atomic number of 1, meaning it has one proton in its nucleus, while carbon has an atomic number of 6, having six protons.

When looking at the atomic number, it's crucial for students to recognize that all neutral atoms will also have the same number of electrons as protons, maintaining an electrical balance. If an atom gains or loses electrons, it becomes an ion, either negatively or positively charged. Understanding atomic numbers is not just helpful for identifying elements, it is also essential for predicting the chemical behavior of elements, because elements in the same group typically have similar properties and the same number of electrons in their outer shell.

Neutrons in Atoms

Neutrons are the neutral counterbalance to protons in an atom's nucleus. They have no electric charge, but their presence is crucial for atomic stability. Unlike protons, the number of neutrons is not dictated by the atom's identity, meaning isotopes can exist. Isotopes are atoms of the same element (and hence the same atomic number) that differ in the number of neutrons they possess.

To find the number of neutrons in an atom, you subtract the atomic number from the atom's mass number (represented by 'A'). The mass number is the sum of protons and neutrons in the nucleus. For example, Carbon-14, a radioactive isotope of carbon, has 6 protons (atomic number) and 14 mass number, so it has 14 - 6 = 8 neutrons. This concept is pivotal in fields like nuclear chemistry and physics, as the number of neutrons affects the atom's mass and stability. Some isotopes with a different number of neutrons can be unstable and radioactive, such as Uranium-235 and Carbon-14.

Electrons in Neutral Atoms

Electrons are subatomic particles with a negative charge, and they inhabit regions around the nucleus known as electron shells or orbitals. In a neutral atom, the number of electrons equals the number of protons, which is the atomic number. This equal number allows the positive and negative charges to balance out, resulting in a net zero charge.

Electrons play a tremendous role in chemical reactions as they can be shared (covalent bonding), transferred (ionic bonding), or influence magnetic properties through their spin. The arrangement of electrons in various orbitals, known as the electron configuration, determines how atoms will interact and bond with each other. For instance, in sodium (atomic number 11), which is a neutral atom, there will be 11 electrons orbiting the nucleus. These electrons are the primary factor in an element's chemical properties, with those in the outermost shell, called valence electrons, contributing most significantly to bonding behavior.