The atomic number of an element is 35 and the mass number is 81. The number of electrons in the outer most shell is:

Answer

To solve the problem, we need to determine the number of electrons in the outermost shell of the element with atomic number 35 and mass number 81. Here’s a step-by-step solution:

### Step 1: Identify the Element
The atomic number of the element is given as 35. The atomic number represents the number of protons in the nucleus of an atom, and for neutral atoms, it also equals the number of electrons.

### Step 2: Determine the Electronic Configuration
To find the electronic configuration of the element, we need to distribute the 35 electrons according to the Aufbau principle, Hund's rule, and the Pauli exclusion principle.

1. The first two electrons fill the 1s orbital:
- 1s²
2. The next two electrons fill the 2s orbital:
- 2s²
3. The next six electrons fill the 2p orbital:
- 2p⁶
4. The next two electrons fill the 3s orbital:
- 3s²
5. The next six electrons fill the 3p orbital:
- 3p⁶
6. The next ten electrons fill the 3d orbital:
- 3d¹⁰
7. The remaining seven electrons will fill the 4s and 4p orbitals:
- 4s²
- 4p⁵

Putting this all together, the electronic configuration for the element with atomic number 35 is:
- **1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁵**

### Step 3: Identify the Outermost Shell
The outermost shell corresponds to the highest principal quantum number, which in this case is n=4. The orbitals in this shell are 4s and 4p.

### Step 4: Count the Electrons in the Outermost Shell
From the electronic configuration:
- 4s has 2 electrons
- 4p has 5 electrons

To find the total number of electrons in the outermost shell:
- Total = 2 (from 4s) + 5 (from 4p) = 7 electrons

### Conclusion
The number of electrons in the outermost shell of the element with atomic number 35 is **7**.

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