For the following atoms, give the number of protons,...

For the following atoms, give the number of protons, neutrons, and electrons: $(4.3)$
a. ${ }_{80}^{202} \mathrm{Hg}$
b. $\quad 127$
5 $\mathrm{I}$
c. ${ }_{35}^{75} \mathrm{Br}$
d. ${ }_{55}^{133} \mathrm{Cs}$
e. ${ }_{78}^{195} \mathrm{Pt}$

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Key Concepts

Atomic Number

The atomic number of an element represents the number of protons in the nucleus of an atom. It is usually written as the subscript to the left of the chemical symbol and defines the identity of the element. In a neutral atom, the atomic number also equals the number of electrons, which are responsible for the atom's chemical bonding.

Mass Number

The mass number is the sum of the protons and neutrons in the nucleus and is typically written as the superscript to the left of the chemical symbol. It gives an approximation of the atomic mass and is useful for identifying the isotope of an element. The number of neutrons in the nucleus can be calculated by subtracting the atomic number from the mass number.

Subatomic Particles

Atoms are composed of three main subatomic particles. Protons, which carry a positive charge, reside in the nucleus and determine the element's identity. Neutrons, which have no electrical charge, also reside in the nucleus and, together with protons, contribute to the mass of the atom. Electrons, which have a negative charge, orbit the nucleus and play a key role in chemical reactions and bonding. In a neutral atom, the number of electrons equals the number of protons.

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