Nihonium (Nh) is a superheavy element with the atomic number 113. As of my last knowledge update in October 2023, nihonium has a few known isotopes, though due to its high instability and short half-lives, they are not found in nature and can only be produced artificially in laboratories.

Nickel has several isotopes, with the most notable ones being: 1. **Nickel-58 (¹⁵⁸Ni)**: This is the most abundant isotope of nickel, making up about 68% of natural nickel. It is stable. 2. **Nickel-60 (¹⁶⁰Ni)**: Another stable isotope, it accounts for about 26% of natural nickel.

Neutronium is a hypothetical substance that consists almost entirely of neutrons. It is often discussed in the context of astrophysics and is theorized to be found in the cores of neutron stars, where extreme gravitational pressures force neutrons together in massive quantities. Since neutronium is made up entirely of neutrons, it doesn't have isotopes in the traditional sense as isotopes refer to variants of a chemical element that have the same number of protons but different numbers of neutrons.

Neptunium (Np) has several isotopes, with the most notable ones being: 1. **Neptunium-237 (Np-237)**: This is the most stable and prominent isotope of neptunium, with a half-life of about 2.14 million years. It is produced in nuclear reactors and is of interest due to its potential use in nuclear waste management and as a source of plutonium-238.

Neon has three stable isotopes: 1. **Neon-20 (¹⁴Ne)**: This is the most abundant isotope, making up about 90.48% of natural neon. It has 10 protons and 10 neutrons. 2. **Neon-21 (¹⁵Ne)**: This isotope is much less common, accounting for about 0.27% of natural neon. It has 10 protons and 11 neutrons.

Neodymium (Nd) has several isotopes, with the most stable and significant ones being: 1. **Neodymium-144 (Nd-144)**: This isotope has a half-life of about 2.29 million years and is stable. 2. **Neodymium-145 (Nd-145)**: Another stable isotope with no significant radioactivity. 3. **Neodymium-146 (Nd-146)**: This isotope has a half-life of about 5.

Moscovium (Mc) is a synthetic element with the atomic number 115. As of my last knowledge update in October 2023, there are no stable isotopes of moscovium. The isotopes of moscovium that have been produced in laboratory settings are primarily radioactive and have very short half-lives.

Molybdenum (Mo) has several isotopes, which are variations of the element that contain different numbers of neutrons in their nuclei. The most stable and naturally occurring isotopes of molybdenum include: 1. **Molybdenum-92 (^92Mo)**: This isotope has 42 protons and 50 neutrons and is the most abundant isotope of molybdenum, making up about 14.8% of natural molybdenum.

Mercury has several isotopes, which are varieties of mercury atoms that have the same number of protons but different numbers of neutrons. The most stable and commonly occurring isotopes of mercury are: 1. **Mercury-196 (²⁰⁶Hg)**: This is the most abundant isotope, making up about 30.6% of naturally occurring mercury.

Mendelevium (Md) is a synthetic element with the atomic number 101, and it is a member of the actinide series in the periodic table. As of my last update in October 2023, mendelevium has no stable isotopes. The known isotopes of mendelevium are all radioactive, and they have relatively short half-lives.

Meitnerium (Mt) is a synthetic element with the atomic number 109 and is classified as a transactinide element in the periodic table. It is named in honor of physicist Lise Meitner. Due to its short half-life and the limited amount produced, there are only a few known isotopes of meitnerium.

Manganese has several isotopes, with the most common being ^55Mn, which is stable. In total, there are 26 known isotopes of manganese, ranging from ^46Mn to ^75Mn. Here are some key points regarding manganese isotopes: 1. **Stable Isotope**: - **^55Mn**: The only stable isotope of manganese, making up nearly all natural manganese found in the environment.

Magnesium has several isotopes, with the most notable being: 1. **Magnesium-24 (²⁴Mg)**: This is the most abundant isotope, making up about 79% of natural magnesium. It has 12 neutrons and is stable. 2. **Magnesium-25 (²⁵Mg)**: This isotope constitutes about 10% of natural magnesium. It has 13 neutrons and is also stable.

Lutetium (Lu) is a chemical element with the atomic number 71 and is part of the lanthanide series. It has several isotopes, but the most notable ones are as follows: 1. **Lutetium-175 (Lu-175)**: This is the most stable and abundant isotope of lutetium, making up about 97.4% of natural lutetium.

Livermorium (Lv) is a synthetic element with the atomic number 116. It belongs to the group of elements known as the post-transition metals. As of my last knowledge update in October 2021, livermorium has a limited number of known isotopes. The most stable and notable isotopes of livermorium are: 1. **Livermorium-293 (Lv-293)**: This isotope has been produced and has a half-life of approximately 60 milliseconds.

Lithium has several isotopes, but the three most notable ones are: 1. **Lithium-6 (\(^6Li\))**: This isotope has three protons and three neutrons. It makes up about 7.5% of naturally occurring lithium. \(^6Li\) is known for its applications in nuclear fusion and as a coolant in nuclear reactors.

Lead has four stable isotopes and several unstable (radioactive) isotopes. The four stable isotopes of lead are: 1. **Lead-204 (\(^{204}\)Pb)**: This isotope has 82 protons and 122 neutrons. It is the least abundant stable isotope of lead. 2. **Lead-206 (\(^{206}\)Pb)**: This isotope has 82 protons and 124 neutrons.

Lawrencium (Lr) is a synthetic element with the atomic number 103, and it is part of the actinide series. Due to its instability and short half-life, isotopes of lawrencium are not found naturally and have been produced in laboratories.

Lanthanum (La) has a few isotopes, but it has only one stable isotope: lanthanum-138 (¹³⁸La). This isotope accounts for nearly all naturally occurring lanthanum. In addition to the stable isotope, lanthanum has several radioactive isotopes. These isotopes include: 1. **Lanthanum-137 (¹³⁷La)**: A beta-emitting isotope with a half-life of about 6.

Krypton (Kr) is a noble gas with atomic number 36. It has several isotopes, which are variants of the element that have the same number of protons but different numbers of neutrons. The most notable isotopes of krypton include: 1. **Krypton-78 (Kr-78)**: This isotope has 42 neutrons and is stable. 2. **Krypton-80 (Kr-80)**: This stable isotope has 44 neutrons.