Question

For the following atoms, give the number of protons, neutrons, and electrons: a. \(198 \mathrm{Au}\) b. \(127 \mathrm{I}\) C. \(35 \mathrm{Br}\) d. \(\frac{133}{55} \mathrm{Cs}\) e. \({ }^{195} \mathrm{Pt}\)

Short answer

a: 79p, 119n, 79e. b: 53p, 74n, 53e. c: 35p, 0n, 35e. d: 55p, 78n, 55e. e: 78p, 117n, 78e.

Step-by-step solution

- Identify atomic number and mass number

The atomic number (Z) is the number of protons in an atom and it is unique for each element. The mass number (A) is the total number of protons and neutrons in the atom's nucleus.

- Determine protons

The number of protons is equal to the atomic number. For each element provided, look up the atomic number on the Periodic Table.

- Calculate neutrons

Subtract the atomic number (Z) from the mass number (A) to find the number of neutrons: Neutrons = A - Z.

- Determine electrons

For neutral atoms, the number of electrons is equal to the number of protons.

- Apply steps to each atom

Now apply these steps to each given atom: a. - Atomic number (Z) of Au (Gold) is 79. - Protons = 79. - Neutrons = 198 - 79 = 119. - Electrons = 79. b. - Atomic number (Z) of I (Iodine) is 53. - Protons = 53. - Neutrons = 127 - 53 = 74. - Electrons = 53. c. - Atomic number (Z) of Br (Bromine) is 35. - Protons = 35. - Neutrons = 35 - 35 = 0. - Electrons = 35. d. - Atomic number (Z) of Cs (Cesium) is 55. - Protons = 55. - Neutrons = 133 - 55 = 78. - Electrons = 55. e. - Atomic number (Z) of Pt (Platinum) is 78. - Protons = 78. - Neutrons = 195 - 78 = 117. - Electrons = 78.

Key concepts

atomic number

The atomic number, often symbolized as Z, is a fundamental property of an element. It represents the number of protons in the nucleus of an atom. Since protons are positively charged, the atomic number also determines the positive charge of the nucleus.
The atomic number is unique for each element and serves as its identifier in the periodic table.

For example:
- Gold (Au) has an atomic number of 79.
- Iodine (I) has an atomic number of 53.
- Cesium (Cs) has an atomic number of 55.

Knowing the atomic number is essential in finding out other properties such as the number of electrons in a neutral atom, as these numbers will be equal.

mass number

The mass number, denoted as A, is defined as the total number of protons and neutrons in an atom’s nucleus.
This number gives an approximate mass of an atomic nucleus.

While the number of protons (atomic number Z) is fixed for each element, the number of neutrons can vary. These variations lead to different isotopes of the same element.

To calculate the number of neutrons in an atom, subtract the atomic number (Z) from the mass number (A).
For example:
- Gold (Au) with a mass number of 198 = 198 - 79 (atomic number) = 119 neutrons.
This property helps in understanding the structure and properties of isotopes.

protons

Protons are subatomic particles found in the nucleus of every atom.
They carry a positive charge and have a mass of approximately 1 atomic mass unit (amu).

The number of protons in an atom defines the element to which the atom belongs.

Some key points to remember:

• Protons determine the element’s identity.
• The number of protons in a neutral atom is equal to the number of electrons.
• Protons contribute to the mass number but not significantly to the volume.

For the elements listed in the exercise:
- Gold (Au) has 79 protons.
- Iodine (I) has 53 protons.
- Cesium (Cs) has 55 protons.

neutrons

Neutrons are electrically neutral particles found in the nucleus of an atom. They have a mass slightly larger than that of protons, about 1 atomic mass unit (amu).

Neutrons play a crucial role in providing stability to an atom's nucleus as they offset the repulsive forces between positively charged protons.
To find the number of neutrons in an atom, subtract the atomic number (Z) from the mass number (A):

• Neutrons = A - Z.

For example, for gold (Au) with a mass number of 198 and an atomic number of 79:
- The number of neutrons = 198 - 79 = 119.

Neutrons vary in number between isotopes of the same element, which affects atomic mass but not chemical properties.

electrons

Electrons are subatomic particles with a negative charge, found in the orbitals surrounding the nucleus.
They have a negligible mass compared to protons and neutrons.

In a neutral atom, the number of electrons equals the number of protons. Therefore, the atomic number also tells us the number of electrons in a neutral atom.
Electrons are crucial as they determine an atom's chemical behavior and reactions. For example:

• Electrons in Gold (Au) = 79.
• Electrons in Iodine (I) = 53.

Electrons are organized into energy levels or shells around the nucleus, which defines how atoms bond and interact with each other.