Short answer: boron. Long answer: any element with 11 protons or less could theoretically have an atomic mass of 11, as the mass = Number of Protons + Neutrons, if you add/take a certain number of neutrons from the atom it will be able to have an atomic mass of 11, but some aren't possible and others are too unstable, and will decay instantly.
Boron is a non metal element. Atomic mass of it is 11.
Sodium is a meta element. Atomic mass of it is 23. Atomic number of it is 11.
The element with the lowest atomic number in period 3 is sodium (Na) with an atomic number of 11.
The second element in Period 3 is Magnesium, with an atomic number of 12.
Subtract the atomic # from the atomic mass. Atomic number = number of Protons. Atomic mass = total nucleons (Protons + Neutrons).Atomic # = 11 and Atomic Mass = 23. So 23-11 = 12 Neutrons.
Boron is a non metal element. Atomic mass of it is 11.
The approximate mass is 11. Technically you could express this in units of daltons as 11 Da (Daltons previously called amu, Atomic Mass unit, or as 11 (the dimensionless, relative atomic mass which chemists simply call atomic weight) .
Atomic mass numbers are not properties of elements overall, but only of particular isotopes of elements. The only stable element with an isotope with mass number 11 is boron. Beryllium and carbon also have isotopes with mass number 11, but these are radioactive.
Boron is the element that has two isotopes with atomic masses of approximately 10 amu and 11 amu. The most common isotope of Boron is Boron-11, which has an atomic mass of 11 amu, while the less common isotope Boron-10 has an atomic mass of 10 amu.
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Sodium, symbol is Na