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. \(^{79} \mathrm{Br}\) b. \(^{81} \mathrm{Br}\) c. \(^{239} \mathrm{Pu}\) d. \(^{133} \mathrm{Cs}\) e. \(^{3} \mathrm{H}\) f. \(^{56} \mathrm{Fe}\)

Short answer

a. \(^{79}\mathrm{Br}\): 35 protons, 44 neutrons, 35 electrons b. \(^{81}\mathrm{Br}\): 35 protons, 46 neutrons, 35 electrons c. \(^{239}\mathrm{Pu}\): 94 protons, 145 neutrons, 94 electrons d. \(^{133}\mathrm{Cs}\): 55 protons, 78 neutrons, 55 electrons e. \(^{3}\mathrm{H}\): 1 proton, 2 neutrons, 1 electron f. \(^{56}\mathrm{Fe}\): 26 protons, 30 neutrons, 26 electrons

Step-by-step solution

a. Isotope \(^{79} \mathrm{Br}\)

To determine the number of protons, find the atomic number of Bromine (Br) using the periodic table, which is 35. Hence, the neutral atom of Bromine has 35 electrons. To find the number of neutrons, we subtract the atomic number from the mass number: 79 - 35 = 44 neutrons.

b. Isotope \(^{81}\mathrm{Br}\)

The atomic number of Bromine (Br) is still 35, so there are 35 protons and 35 electrons in a neutral atom. To find the number of neutrons, we subtract the atomic number from the mass number: 81 - 35 = 46 neutrons.

c. Isotope \(^{239}\mathrm{Pu}\)

To determine the number of protons, find the atomic number of Plutonium (Pu) using the periodic table, which is 94. Hence, the neutral atom of Plutonium has 94 electrons. To find the number of neutrons, we subtract the atomic number from the mass number: 239 - 94 = 145 neutrons.

d. Isotope \(^{133}\mathrm{Cs}\)

To determine the number of protons, find the atomic number of Cesium (Cs) using the periodic table, which is 55. Hence, the neutral atom of Cesium has 55 electrons. To find the number of neutrons, we subtract the atomic number from the mass number: 133 - 55 = 78 neutrons.

e. Isotope \(^{3}\mathrm{H}\)

To determine the number of protons, find the atomic number of Hydrogen (H) using the periodic table, which is 1. Hence, the neutral atom of Hydrogen has 1 electron. To find the number of neutrons, we subtract the atomic number from the mass number: 3 - 1 = 2 neutrons.

f. Isotope \(^{56}\mathrm{Fe}\)

To determine the number of protons, find the atomic number of Iron (Fe) using the periodic table, which is 26. Hence, the neutral atom of Iron has 26 electrons. To find the number of neutrons, we subtract the atomic number from the mass number: 56 - 26 = 30 neutrons.

Key concepts

Isotopes

Isotopes are different forms of the same element. They have the same number of protons but different numbers of neutrons. This means they belong to the same element but have different atomic masses. When you see notation like \(^{79} \mathrm{Br}\) or \(^{81} \mathrm{Br}\), these numbers are the mass numbers of isotopes of Bromine.

• The mass number is the sum of protons and neutrons.
• Isotopes of an element can have distinct physical properties but often behave similarly in chemical reactions.
• The difference in neutrons can affect properties like stability and radioactivity.

This concept explains why \(^{79} \mathrm{Br}\) and \(^{81} \mathrm{Br}\) have different neutron counts but the same protons.

Protons

Protons are positively charged particles located in the nucleus of an atom. The number of protons, known as the atomic number, defines the identity of an element. For instance, Bromine always has 35 protons regardless of the isotope. This consistency is what makes each element unique.

• The atomic number is usually found above the element symbol on the periodic table.
• Protons contribute significantly to the mass of an atom.
• The positive charge of protons attracts negative electrons, keeping them in orbit around the nucleus.

Understanding protons is essential for distinguishing between elements and predicting their behavior.

Neutrons

Neutrons are neutral particles, meaning they have no charge, found in the nucleus of an atom along with protons. The number of neutrons varies among isotopes, affecting the mass number but not the chemical properties of an element.

• To find the number of neutrons, subtract the atomic number from the mass number.
• Neutrons help to stabilize the nucleus and can influence the isotope's stability.
• They are similar in mass to protons but contribute no charge to the atom.

For example, in \(^{56} \mathrm{Fe}\), subtracting protons (26) from the mass number (56) gives 30 neutrons.

Periodic Table

The periodic table is a system for organizing all known elements based on their atomic number, electron configuration, and recurring chemical properties. It is divided into rows and columns, helping us quickly locate and understand an element's traits.

• Elements are arranged in order of increasing atomic number.
• The table helps predict properties of elements and their isotopes.
• Each element's symbol is accompanied by its atomic number and, often, its atomic mass.

By using the periodic table, one can easily find the number of protons in an element, aiding in the understanding of isotopic differences like those in Bromine, Cesium, and others.